diff --git a/CMakeLists.txt b/CMakeLists.txt
index aa3cfbfeac..6e21e7c7a3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -209,7 +209,7 @@ else()
     else()
       set(ZIG_LLD_COMPILE_FLAGS "-std=c++11 -fvisibility-inlines-hidden -fno-exceptions -fno-rtti -Wno-comment")
       if(MINGW)
-        set(ZIG_LLD_COMPILE_FLAGS "${ZIG_LLD_COMPILE_FLAGS} -D__STDC_FORMAT_MACROS -D__USE_MINGW_ANSI_STDIO -Wno-pedantic-ms-format")
+        set(ZIG_LLD_COMPILE_FLAGS "${ZIG_LLD_COMPILE_FLAGS} -D__STDC_FORMAT_MACROS -D__USE_MINGW_ANSI_STDIO")
       endif()
     endif()
     set_target_properties(embedded_lld_lib PROPERTIES
@@ -511,19 +511,23 @@ set(OPTIMIZED_C_FLAGS "-std=c99 -O3")
 
 set(EXE_LDFLAGS " ")
 if(MSVC)
-    set(EXE_LDFLAGS "/STACK:16777216")
+    set(EXE_LDFLAGS "${EXE_LDFLAGS} /STACK:16777216")
 elseif(MINGW) 
     set(EXE_LDFLAGS "${EXE_LDFLAGS} -Wl,--stack,16777216")
 endif()
 
 if(ZIG_STATIC)
     if(APPLE)
-        set(EXE_LDFLAGS "-static-libgcc -static-libstdc++")
+        set(EXE_LDFLAGS "${EXE_LDFLAGS} -static-libgcc -static-libstdc++")
     elseif(MINGW)
-        set(EXE_LDFLAGS "-static-libgcc -static-libstdc++ -Wl,-Bstatic,--whole-archive -lwinpthread -lz3 -lz -lgomp -Wl,--no-whole-archive")
-    else()
-        set(EXE_LDFLAGS "-static")
+        set(EXE_LDFLAGS "${EXE_LDFLAGS} -static-libgcc -static-libstdc++ -Wl,-Bstatic, -lwinpthread -lz3 -lz -lgomp")
+    elseif(NOT MSVC)
+        set(EXE_LDFLAGS "${EXE_LDFLAGS} -static")
     endif()
+else()
+    if(MINGW)
+        set(EXE_LDFLAGS "${EXE_LDFLAGS} -lz3")
+    endif() 
 endif()
 
 if(ZIG_TEST_COVERAGE)
@@ -559,11 +563,6 @@ if(NOT MSVC)
     target_link_libraries(compiler LINK_PUBLIC ${LIBXML2})
 endif()
 
-if(MINGW)
-    find_library(Z3_LIBRARIES NAMES z3 z3.dll)
-    target_link_libraries(compiler LINK_PUBLIC ${Z3_LIBRARIES})
-endif()
-
 if(ZIG_DIA_GUIDS_LIB)
     target_link_libraries(compiler LINK_PUBLIC ${ZIG_DIA_GUIDS_LIB})
 endif()
diff --git a/src/all_types.hpp b/src/all_types.hpp
index 9ab90b2785..079b8ded95 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -2529,6 +2529,7 @@ struct IrInstructionLoadPtrGen {
 struct IrInstructionStorePtr {
     IrInstruction base;
 
+    bool allow_write_through_const;
     IrInstruction *ptr;
     IrInstruction *value;
 };
@@ -3630,6 +3631,7 @@ enum ResultLocId {
 struct ResultLoc {
     ResultLocId id;
     bool written;
+    bool allow_write_through_const;
     IrInstruction *resolved_loc; // result ptr 
     IrInstruction *source_instruction;
     IrInstruction *gen_instruction; // value to store to the result loc
diff --git a/src/codegen.cpp b/src/codegen.cpp
index 00629bc568..f12880773b 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -4355,8 +4355,14 @@ static LLVMValueRef ir_render_cmpxchg(CodeGen *g, IrExecutable *executable, IrIn
         return LLVMBuildSelect(g->builder, success_bit, LLVMConstNull(get_llvm_type(g, child_type)), payload_val, "");
     }
 
+    // When the cmpxchg is discarded, the result location will have no bits.
+    if (!type_has_bits(instruction->result_loc->value.type)) {
+        return nullptr;
+    }
+
     LLVMValueRef result_loc = ir_llvm_value(g, instruction->result_loc);
-    assert(type_has_bits(child_type));
+    src_assert(result_loc != nullptr, instruction->base.source_node);
+    src_assert(type_has_bits(child_type), instruction->base.source_node);
 
     LLVMValueRef payload_val = LLVMBuildExtractValue(g->builder, result_val, 0, "");
     LLVMValueRef val_ptr = LLVMBuildStructGEP(g->builder, result_loc, maybe_child_index, "");
diff --git a/src/ir.cpp b/src/ir.cpp
index ecd2cd6f87..f140cfeabe 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -188,7 +188,8 @@ static IrInstruction *ir_analyze_bit_cast(IrAnalyze *ira, IrInstruction *source_
 static IrInstruction *ir_resolve_result_raw(IrAnalyze *ira, IrInstruction *suspend_source_instr,
         ResultLoc *result_loc, ZigType *value_type, IrInstruction *value, bool force_runtime, bool non_null_comptime);
 static IrInstruction *ir_resolve_result(IrAnalyze *ira, IrInstruction *suspend_source_instr,
-        ResultLoc *result_loc, ZigType *value_type, IrInstruction *value, bool force_runtime, bool non_null_comptime);
+        ResultLoc *result_loc, ZigType *value_type, IrInstruction *value, bool force_runtime,
+        bool non_null_comptime, bool allow_discard);
 static IrInstruction *ir_analyze_unwrap_optional_payload(IrAnalyze *ira, IrInstruction *source_instr,
         IrInstruction *base_ptr, bool safety_check_on, bool initializing);
 static IrInstruction *ir_analyze_unwrap_error_payload(IrAnalyze *ira, IrInstruction *source_instr,
@@ -196,7 +197,7 @@ static IrInstruction *ir_analyze_unwrap_error_payload(IrAnalyze *ira, IrInstruct
 static IrInstruction *ir_analyze_unwrap_err_code(IrAnalyze *ira, IrInstruction *source_instr,
         IrInstruction *base_ptr, bool initializing);
 static IrInstruction *ir_analyze_store_ptr(IrAnalyze *ira, IrInstruction *source_instr,
-        IrInstruction *ptr, IrInstruction *uncasted_value);
+        IrInstruction *ptr, IrInstruction *uncasted_value, bool allow_write_through_const);
 static IrInstruction *ir_gen_union_init_expr(IrBuilder *irb, Scope *scope, AstNode *source_node,
     IrInstruction *union_type, IrInstruction *field_name, AstNode *expr_node,
     LVal lval, ResultLoc *parent_result_loc);
@@ -1564,7 +1565,7 @@ static IrInstruction *ir_build_unreachable(IrBuilder *irb, Scope *scope, AstNode
     return &unreachable_instruction->base;
 }
 
-static IrInstruction *ir_build_store_ptr(IrBuilder *irb, Scope *scope, AstNode *source_node,
+static IrInstructionStorePtr *ir_build_store_ptr(IrBuilder *irb, Scope *scope, AstNode *source_node,
         IrInstruction *ptr, IrInstruction *value)
 {
     IrInstructionStorePtr *instruction = ir_build_instruction<IrInstructionStorePtr>(irb, scope, source_node);
@@ -1576,7 +1577,7 @@ static IrInstruction *ir_build_store_ptr(IrBuilder *irb, Scope *scope, AstNode *
     ir_ref_instruction(ptr, irb->current_basic_block);
     ir_ref_instruction(value, irb->current_basic_block);
 
-    return &instruction->base;
+    return instruction;
 }
 
 static IrInstruction *ir_build_var_decl_src(IrBuilder *irb, Scope *scope, AstNode *source_node,
@@ -3792,12 +3793,20 @@ static IrInstruction *ir_gen_bin_op_id(IrBuilder *irb, Scope *scope, AstNode *no
 
 static IrInstruction *ir_gen_assign(IrBuilder *irb, Scope *scope, AstNode *node) {
     IrInstruction *lvalue = ir_gen_node_extra(irb, node->data.bin_op_expr.op1, scope, LValPtr, nullptr);
-    IrInstruction *rvalue = ir_gen_node(irb, node->data.bin_op_expr.op2, scope);
-
-    if (lvalue == irb->codegen->invalid_instruction || rvalue == irb->codegen->invalid_instruction)
+    if (lvalue == irb->codegen->invalid_instruction)
+        return irb->codegen->invalid_instruction;
+
+    ResultLocInstruction *result_loc_inst = allocate<ResultLocInstruction>(1);
+    result_loc_inst->base.id = ResultLocIdInstruction;
+    result_loc_inst->base.source_instruction = lvalue;
+    ir_ref_instruction(lvalue, irb->current_basic_block);
+    ir_build_reset_result(irb, scope, node, &result_loc_inst->base);
+
+    IrInstruction *rvalue = ir_gen_node_extra(irb, node->data.bin_op_expr.op2, scope, LValNone,
+            &result_loc_inst->base);
+    if (rvalue == irb->codegen->invalid_instruction)
         return irb->codegen->invalid_instruction;
 
-    ir_build_store_ptr(irb, scope, node, lvalue, rvalue);
     return ir_build_const_void(irb, scope, node);
 }
 
@@ -5836,6 +5845,7 @@ static IrInstruction *ir_gen_container_init_expr(IrBuilder *irb, Scope *scope, A
                 ResultLocInstruction *result_loc_inst = allocate<ResultLocInstruction>(1);
                 result_loc_inst->base.id = ResultLocIdInstruction;
                 result_loc_inst->base.source_instruction = field_ptr;
+                result_loc_inst->base.allow_write_through_const = true;
                 ir_ref_instruction(field_ptr, irb->current_basic_block);
                 ir_build_reset_result(irb, scope, expr_node, &result_loc_inst->base);
 
@@ -5874,6 +5884,7 @@ static IrInstruction *ir_gen_container_init_expr(IrBuilder *irb, Scope *scope, A
                 ResultLocInstruction *result_loc_inst = allocate<ResultLocInstruction>(1);
                 result_loc_inst->base.id = ResultLocIdInstruction;
                 result_loc_inst->base.source_instruction = elem_ptr;
+                result_loc_inst->base.allow_write_through_const = true;
                 ir_ref_instruction(elem_ptr, irb->current_basic_block);
                 ir_build_reset_result(irb, scope, expr_node, &result_loc_inst->base);
 
@@ -6431,7 +6442,7 @@ static IrInstruction *ir_gen_for_expr(IrBuilder *irb, Scope *parent_scope, AstNo
 
     ir_set_cursor_at_end_and_append_block(irb, continue_block);
     IrInstruction *new_index_val = ir_build_bin_op(irb, child_scope, node, IrBinOpAdd, index_val, one, false);
-    ir_mark_gen(ir_build_store_ptr(irb, child_scope, node, index_ptr, new_index_val));
+    ir_build_store_ptr(irb, child_scope, node, index_ptr, new_index_val)->allow_write_through_const = true;
     ir_build_br(irb, child_scope, node, cond_block, is_comptime);
 
     IrInstruction *else_result = nullptr;
@@ -10344,7 +10355,8 @@ static IrInstruction *ir_resolve_ptr_of_array_to_slice(IrAnalyze *ira, IrInstruc
     }
 
     if (result_loc == nullptr) result_loc = no_result_loc();
-    IrInstruction *result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr, true, false);
+    IrInstruction *result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr, true,
+            false, true);
     if (type_is_invalid(result_loc_inst->value.type) || instr_is_unreachable(result_loc_inst)) {
         return result_loc_inst;
     }
@@ -10804,7 +10816,7 @@ static IrInstruction *ir_analyze_optional_wrap(IrAnalyze *ira, IrInstruction *so
     }
     IrInstruction *result_loc_inst = nullptr;
     if (result_loc != nullptr) {
-        result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr, true, false);
+        result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr, true, false, true);
         if (type_is_invalid(result_loc_inst->value.type) || instr_is_unreachable(result_loc_inst)) {
             return result_loc_inst;
         }
@@ -10847,7 +10859,7 @@ static IrInstruction *ir_analyze_err_wrap_payload(IrAnalyze *ira, IrInstruction
     IrInstruction *result_loc_inst;
     if (handle_is_ptr(wanted_type)) {
         if (result_loc == nullptr) result_loc = no_result_loc();
-        result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr, true, false);
+        result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr, true, false, true);
         if (type_is_invalid(result_loc_inst->value.type) || instr_is_unreachable(result_loc_inst)) {
             return result_loc_inst;
         }
@@ -10959,7 +10971,7 @@ static IrInstruction *ir_analyze_err_wrap_code(IrAnalyze *ira, IrInstruction *so
     IrInstruction *result_loc_inst;
     if (handle_is_ptr(wanted_type)) {
         if (result_loc == nullptr) result_loc = no_result_loc();
-        result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr, true, false);
+        result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr, true, false, true);
         if (type_is_invalid(result_loc_inst->value.type) || instr_is_unreachable(result_loc_inst)) {
             return result_loc_inst;
         }
@@ -11032,7 +11044,8 @@ static IrInstruction *ir_get_ref(IrAnalyze *ira, IrInstruction *source_instructi
 
     IrInstruction *result_loc;
     if (type_has_bits(ptr_type) && !handle_is_ptr(value->value.type)) {
-        result_loc = ir_resolve_result(ira, source_instruction, no_result_loc(), value->value.type, nullptr, true, false);
+        result_loc = ir_resolve_result(ira, source_instruction, no_result_loc(), value->value.type, nullptr, true,
+                false, true);
     } else {
         result_loc = nullptr;
     }
@@ -11076,7 +11089,8 @@ static IrInstruction *ir_analyze_array_to_slice(IrAnalyze *ira, IrInstruction *s
     if (!array_ptr) array_ptr = ir_get_ref(ira, source_instr, array, true, false);
 
     if (result_loc == nullptr) result_loc = no_result_loc();
-    IrInstruction *result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr, true, false);
+    IrInstruction *result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, wanted_type, nullptr,
+            true, false, true);
     if (type_is_invalid(result_loc_inst->value.type) || instr_is_unreachable(result_loc_inst)) {
         return result_loc_inst;
     }
@@ -11732,7 +11746,8 @@ static IrInstruction *ir_analyze_vector_to_array(IrAnalyze *ira, IrInstruction *
     if (result_loc == nullptr) {
         result_loc = no_result_loc();
     }
-    IrInstruction *result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, array_type, nullptr, true, false);
+    IrInstruction *result_loc_inst = ir_resolve_result(ira, source_instr, result_loc, array_type, nullptr,
+            true, false, true);
     if (type_is_invalid(result_loc_inst->value.type) || instr_is_unreachable(result_loc_inst)) {
         return result_loc_inst;
     }
@@ -12334,7 +12349,8 @@ static IrInstruction *ir_get_deref(IrAnalyze *ira, IrInstruction *source_instruc
     IrInstruction *result_loc_inst;
     if (type_entry->data.pointer.host_int_bytes != 0 && handle_is_ptr(child_type)) {
         if (result_loc == nullptr) result_loc = no_result_loc();
-        result_loc_inst = ir_resolve_result(ira, source_instruction, result_loc, child_type, nullptr, true, false);
+        result_loc_inst = ir_resolve_result(ira, source_instruction, result_loc, child_type, nullptr,
+                true, false, true);
         if (type_is_invalid(result_loc_inst->value.type) || instr_is_unreachable(result_loc_inst)) {
             return result_loc_inst;
         }
@@ -14072,7 +14088,7 @@ static IrInstruction *ir_analyze_instruction_decl_var(IrAnalyze *ira,
             // instruction.
             assert(deref->value.special != ConstValSpecialRuntime);
             var_ptr->value.special = ConstValSpecialRuntime;
-            ir_analyze_store_ptr(ira, var_ptr, var_ptr, deref);
+            ir_analyze_store_ptr(ira, var_ptr, var_ptr, deref, false);
         }
 
         if (var_ptr->value.special == ConstValSpecialStatic && var->mem_slot_index != SIZE_MAX) {
@@ -14556,7 +14572,7 @@ static IrInstruction *ir_resolve_result_raw(IrAnalyze *ira, IrInstruction *suspe
 
             if (peer_parent->peers.length == 1) {
                 IrInstruction *parent_result_loc = ir_resolve_result(ira, suspend_source_instr, peer_parent->parent,
-                        value_type, value, force_runtime, non_null_comptime);
+                        value_type, value, force_runtime, non_null_comptime, true);
                 result_peer->suspend_pos.basic_block_index = SIZE_MAX;
                 result_peer->suspend_pos.instruction_index = SIZE_MAX;
                 if (parent_result_loc == nullptr || type_is_invalid(parent_result_loc->value.type) ||
@@ -14576,7 +14592,7 @@ static IrInstruction *ir_resolve_result_raw(IrAnalyze *ira, IrInstruction *suspe
             if (peer_parent->skipped) {
                 if (non_null_comptime) {
                     return ir_resolve_result(ira, suspend_source_instr, peer_parent->parent,
-                            value_type, value, force_runtime, non_null_comptime);
+                            value_type, value, force_runtime, non_null_comptime, true);
                 }
                 return nullptr;
             }
@@ -14594,7 +14610,7 @@ static IrInstruction *ir_resolve_result_raw(IrAnalyze *ira, IrInstruction *suspe
             }
 
             IrInstruction *parent_result_loc = ir_resolve_result(ira, suspend_source_instr, peer_parent->parent,
-                    peer_parent->resolved_type, nullptr, force_runtime, non_null_comptime);
+                    peer_parent->resolved_type, nullptr, force_runtime, non_null_comptime, true);
             if (parent_result_loc == nullptr || type_is_invalid(parent_result_loc->value.type) ||
                 parent_result_loc->value.type->id == ZigTypeIdUnreachable)
             {
@@ -14644,7 +14660,7 @@ static IrInstruction *ir_resolve_result_raw(IrAnalyze *ira, IrInstruction *suspe
             }
 
             IrInstruction *parent_result_loc = ir_resolve_result(ira, suspend_source_instr, result_bit_cast->parent,
-                    dest_type, bitcasted_value, force_runtime, non_null_comptime);
+                    dest_type, bitcasted_value, force_runtime, non_null_comptime, true);
             if (parent_result_loc == nullptr || type_is_invalid(parent_result_loc->value.type) ||
                 parent_result_loc->value.type->id == ZigTypeIdUnreachable)
             {
@@ -14673,8 +14689,15 @@ static IrInstruction *ir_resolve_result_raw(IrAnalyze *ira, IrInstruction *suspe
 
 static IrInstruction *ir_resolve_result(IrAnalyze *ira, IrInstruction *suspend_source_instr,
         ResultLoc *result_loc_pass1, ZigType *value_type, IrInstruction *value, bool force_runtime,
-        bool non_null_comptime)
+        bool non_null_comptime, bool allow_discard)
 {
+    if (!allow_discard && result_loc_pass1->id == ResultLocIdInstruction &&
+        instr_is_comptime(result_loc_pass1->source_instruction) &&
+        result_loc_pass1->source_instruction->value.type->id == ZigTypeIdPointer &&
+        result_loc_pass1->source_instruction->value.data.x_ptr.special == ConstPtrSpecialDiscard)
+    {
+        result_loc_pass1 = no_result_loc();
+    }
     IrInstruction *result_loc = ir_resolve_result_raw(ira, suspend_source_instr, result_loc_pass1, value_type,
             value, force_runtime, non_null_comptime);
     if (result_loc == nullptr || (instr_is_unreachable(result_loc) || type_is_invalid(result_loc->value.type)))
@@ -14729,7 +14752,7 @@ static IrInstruction *ir_analyze_instruction_resolve_result(IrAnalyze *ira, IrIn
     if (type_is_invalid(implicit_elem_type))
         return ira->codegen->invalid_instruction;
     IrInstruction *result_loc = ir_resolve_result(ira, &instruction->base, instruction->result_loc,
-            implicit_elem_type, nullptr, false, true);
+            implicit_elem_type, nullptr, false, true, true);
     if (result_loc != nullptr)
         return result_loc;
 
@@ -14738,7 +14761,7 @@ static IrInstruction *ir_analyze_instruction_resolve_result(IrAnalyze *ira, IrIn
             instruction->result_loc->id == ResultLocIdReturn)
     {
         result_loc = ir_resolve_result(ira, &instruction->base, no_result_loc(),
-                implicit_elem_type, nullptr, false, true);
+                implicit_elem_type, nullptr, false, true, true);
         if (result_loc != nullptr &&
                 (type_is_invalid(result_loc->value.type) || instr_is_unreachable(result_loc)))
         {
@@ -14800,7 +14823,7 @@ static IrInstruction *ir_analyze_async_call(IrAnalyze *ira, IrInstructionCallSrc
 
     ZigType *frame_type = get_coro_frame_type(ira->codegen, fn_entry);
     IrInstruction *result_loc = ir_resolve_result(ira, &call_instruction->base, call_instruction->result_loc,
-            frame_type, nullptr, true, true);
+            frame_type, nullptr, true, true, false);
     if (result_loc != nullptr && (type_is_invalid(result_loc->value.type) || instr_is_unreachable(result_loc))) {
         return result_loc;
     }
@@ -15015,7 +15038,7 @@ no_mem_slot:
 }
 
 static IrInstruction *ir_analyze_store_ptr(IrAnalyze *ira, IrInstruction *source_instr,
-        IrInstruction *ptr, IrInstruction *uncasted_value)
+        IrInstruction *ptr, IrInstruction *uncasted_value, bool allow_write_through_const)
 {
     assert(ptr->value.type->id == ZigTypeIdPointer);
 
@@ -15031,7 +15054,7 @@ static IrInstruction *ir_analyze_store_ptr(IrAnalyze *ira, IrInstruction *source
 
     ZigType *child_type = ptr->value.type->data.pointer.child_type;
 
-    if (ptr->value.type->data.pointer.is_const && !source_instr->is_gen) {
+    if (ptr->value.type->data.pointer.is_const && !allow_write_through_const) {
         ir_add_error(ira, source_instr, buf_sprintf("cannot assign to constant"));
         return ira->codegen->invalid_instruction;
     }
@@ -15110,10 +15133,9 @@ static IrInstruction *ir_analyze_store_ptr(IrAnalyze *ira, IrInstruction *source
             break;
     }
 
-    IrInstruction *result = ir_build_store_ptr(&ira->new_irb, source_instr->scope, source_instr->source_node,
-        ptr, value);
-    result->value.type = ira->codegen->builtin_types.entry_void;
-    return result;
+    IrInstructionStorePtr *store_ptr = ir_build_store_ptr(&ira->new_irb, source_instr->scope,
+            source_instr->source_node, ptr, value);
+    return &store_ptr->base;
 }
 
 static IrInstruction *ir_analyze_fn_call(IrAnalyze *ira, IrInstructionCallSrc *call_instruction,
@@ -15518,7 +15540,7 @@ static IrInstruction *ir_analyze_fn_call(IrAnalyze *ira, IrInstructionCallSrc *c
         IrInstruction *result_loc;
         if (handle_is_ptr(impl_fn_type_id->return_type)) {
             result_loc = ir_resolve_result(ira, &call_instruction->base, call_instruction->result_loc,
-                    impl_fn_type_id->return_type, nullptr, true, true);
+                    impl_fn_type_id->return_type, nullptr, true, true, false);
             if (result_loc != nullptr && (type_is_invalid(result_loc->value.type) ||
                         instr_is_unreachable(result_loc)))
             {
@@ -15635,7 +15657,7 @@ static IrInstruction *ir_analyze_fn_call(IrAnalyze *ira, IrInstructionCallSrc *c
     IrInstruction *result_loc;
     if (handle_is_ptr(return_type)) {
         result_loc = ir_resolve_result(ira, &call_instruction->base, call_instruction->result_loc,
-                return_type, nullptr, true, true);
+                return_type, nullptr, true, true, false);
         if (result_loc != nullptr && (type_is_invalid(result_loc->value.type) || instr_is_unreachable(result_loc))) {
             return result_loc;
         }
@@ -16154,7 +16176,7 @@ static IrInstruction *ir_analyze_instruction_phi(IrAnalyze *ira, IrInstructionPh
 
             // In case resolving the parent activates a suspend, do it now
             IrInstruction *parent_result_loc = ir_resolve_result(ira, &phi_instruction->base, peer_parent->parent,
-                    peer_parent->resolved_type, nullptr, false, false);
+                    peer_parent->resolved_type, nullptr, false, false, true);
             if (parent_result_loc != nullptr &&
                 (type_is_invalid(parent_result_loc->value.type) || instr_is_unreachable(parent_result_loc)))
             {
@@ -16611,6 +16633,7 @@ static IrInstruction *ir_analyze_instruction_elem_ptr(IrAnalyze *ira, IrInstruct
                     return result;
                 } else if (is_slice(array_type)) {
                     ConstExprValue *ptr_field = &array_ptr_val->data.x_struct.fields[slice_ptr_index];
+                    ir_assert(ptr_field != nullptr, &elem_ptr_instruction->base);
                     if (ptr_field->data.x_ptr.special == ConstPtrSpecialHardCodedAddr) {
                         IrInstruction *result = ir_build_elem_ptr(&ira->new_irb, elem_ptr_instruction->base.scope,
                                 elem_ptr_instruction->base.source_node, array_ptr, casted_elem_index, false,
@@ -16797,7 +16820,7 @@ static IrInstruction *ir_analyze_struct_field_ptr(IrAnalyze *ira, IrInstruction
                 return ira->codegen->invalid_instruction;
             if (type_is_invalid(struct_val->type))
                 return ira->codegen->invalid_instruction;
-            if (struct_val->special == ConstValSpecialUndef && initializing) {
+            if (initializing && struct_val->special == ConstValSpecialUndef) {
                 struct_val->data.x_struct.fields = create_const_vals(struct_type->data.structure.src_field_count);
                 struct_val->special = ConstValSpecialStatic;
                 for (size_t i = 0; i < struct_type->data.structure.src_field_count; i += 1) {
@@ -17395,7 +17418,7 @@ static IrInstruction *ir_analyze_instruction_store_ptr(IrAnalyze *ira, IrInstruc
     if (type_is_invalid(value->value.type))
         return ira->codegen->invalid_instruction;
 
-    return ir_analyze_store_ptr(ira, &instruction->base, ptr, value);
+    return ir_analyze_store_ptr(ira, &instruction->base, ptr, value, instruction->allow_write_through_const);
 }
 
 static IrInstruction *ir_analyze_instruction_load_ptr(IrAnalyze *ira, IrInstructionLoadPtr *instruction) {
@@ -17899,7 +17922,7 @@ static IrInstruction *ir_analyze_unwrap_optional_payload(IrAnalyze *ira, IrInstr
             if (optional_val == nullptr)
                 return ira->codegen->invalid_instruction;
 
-            if (initializing && optional_val->special == ConstValSpecialUndef) {
+            if (initializing) {
                 switch (type_has_one_possible_value(ira->codegen, child_type)) {
                     case OnePossibleValueInvalid:
                         return ira->codegen->invalid_instruction;
@@ -18805,7 +18828,7 @@ static IrInstruction *ir_analyze_container_init_fields(IrAnalyze *ira, IrInstruc
 
         IrInstruction *field_ptr = ir_analyze_struct_field_ptr(ira, instruction, field, result_loc,
                 container_type, true);
-        ir_analyze_store_ptr(ira, instruction, field_ptr, runtime_inst);
+        ir_analyze_store_ptr(ira, instruction, field_ptr, runtime_inst, false);
         if (instr_is_comptime(field_ptr) && field_ptr->value.data.x_ptr.mut != ConstPtrMutRuntimeVar) {
             const_ptrs.append(field_ptr);
         } else {
@@ -18822,7 +18845,7 @@ static IrInstruction *ir_analyze_container_init_fields(IrAnalyze *ira, IrInstruc
                 IrInstruction *field_result_loc = const_ptrs.at(i);
                 IrInstruction *deref = ir_get_deref(ira, field_result_loc, field_result_loc, nullptr);
                 field_result_loc->value.special = ConstValSpecialRuntime;
-                ir_analyze_store_ptr(ira, field_result_loc, field_result_loc, deref);
+                ir_analyze_store_ptr(ira, field_result_loc, field_result_loc, deref, false);
             }
         }
     }
@@ -18949,7 +18972,7 @@ static IrInstruction *ir_analyze_instruction_container_init_list(IrAnalyze *ira,
                 assert(elem_result_loc->value.special == ConstValSpecialStatic);
                 IrInstruction *deref = ir_get_deref(ira, elem_result_loc, elem_result_loc, nullptr);
                 elem_result_loc->value.special = ConstValSpecialRuntime;
-                ir_analyze_store_ptr(ira, elem_result_loc, elem_result_loc, deref);
+                ir_analyze_store_ptr(ira, elem_result_loc, elem_result_loc, deref, false);
             }
         }
     }
@@ -20646,7 +20669,7 @@ static IrInstruction *ir_analyze_instruction_cmpxchg(IrAnalyze *ira, IrInstructi
     IrInstruction *result_loc;
     if (handle_is_ptr(result_type)) {
         result_loc = ir_resolve_result(ira, &instruction->base, instruction->result_loc,
-                result_type, nullptr, true, false);
+                result_type, nullptr, true, false, true);
         if (type_is_invalid(result_loc->value.type) || instr_is_unreachable(result_loc)) {
             return result_loc;
         }
@@ -20903,7 +20926,7 @@ static IrInstruction *ir_analyze_instruction_from_bytes(IrAnalyze *ira, IrInstru
     }
 
     IrInstruction *result_loc = ir_resolve_result(ira, &instruction->base, instruction->result_loc,
-            dest_slice_type, nullptr, true, false);
+            dest_slice_type, nullptr, true, false, true);
     if (result_loc != nullptr && (type_is_invalid(result_loc->value.type) || instr_is_unreachable(result_loc))) {
         return result_loc;
     }
@@ -20980,7 +21003,7 @@ static IrInstruction *ir_analyze_instruction_to_bytes(IrAnalyze *ira, IrInstruct
     }
 
     IrInstruction *result_loc = ir_resolve_result(ira, &instruction->base, instruction->result_loc,
-            dest_slice_type, nullptr, true, false);
+            dest_slice_type, nullptr, true, false, true);
     if (type_is_invalid(result_loc->value.type) || instr_is_unreachable(result_loc)) {
         return result_loc;
     }
@@ -21722,7 +21745,7 @@ static IrInstruction *ir_analyze_instruction_slice(IrAnalyze *ira, IrInstruction
     }
 
     IrInstruction *result_loc = ir_resolve_result(ira, &instruction->base, instruction->result_loc,
-            return_type, nullptr, true, false);
+            return_type, nullptr, true, false, true);
     if (type_is_invalid(result_loc->value.type) || instr_is_unreachable(result_loc)) {
         return result_loc;
     }
@@ -22405,7 +22428,7 @@ static IrInstruction *ir_analyze_unwrap_error_payload(IrAnalyze *ira, IrInstruct
             ConstExprValue *err_union_val = const_ptr_pointee(ira, ira->codegen, ptr_val, source_instr->source_node);
             if (err_union_val == nullptr)
                 return ira->codegen->invalid_instruction;
-            if (err_union_val->special == ConstValSpecialUndef && initializing) {
+            if (initializing && err_union_val->special == ConstValSpecialUndef) {
                 ConstExprValue *vals = create_const_vals(2);
                 ConstExprValue *err_set_val = &vals[0];
                 ConstExprValue *payload_val = &vals[1];
@@ -23700,10 +23723,11 @@ static ZigType *ir_resolve_atomic_operand_type(IrAnalyze *ira, IrInstruction *op
                     operand_type->data.integral.bit_count));
             return ira->codegen->builtin_types.entry_invalid;
         }
-        if (operand_type->data.integral.bit_count > ira->codegen->pointer_size_bytes * 8) {
+        uint32_t max_atomic_bits = target_arch_largest_atomic_bits(ira->codegen->zig_target->arch);
+        if (operand_type->data.integral.bit_count > max_atomic_bits) {
             ir_add_error(ira, op,
-                buf_sprintf("expected integer type pointer size or smaller, found %" PRIu32 "-bit integer type",
-                    operand_type->data.integral.bit_count));
+                buf_sprintf("expected %" PRIu32 "-bit integer type or smaller, found %" PRIu32 "-bit integer type",
+                    max_atomic_bits, operand_type->data.integral.bit_count));
             return ira->codegen->builtin_types.entry_invalid;
         }
         if (!is_power_of_2(operand_type->data.integral.bit_count)) {
@@ -24293,7 +24317,7 @@ static IrInstruction *ir_analyze_instruction_end_expr(IrAnalyze *ira, IrInstruct
 
     bool was_written = instruction->result_loc->written;
     IrInstruction *result_loc = ir_resolve_result(ira, &instruction->base, instruction->result_loc,
-            value->value.type, value, false, false);
+            value->value.type, value, false, false, true);
     if (result_loc != nullptr) {
         if (type_is_invalid(result_loc->value.type))
             return ira->codegen->invalid_instruction;
@@ -24301,7 +24325,8 @@ static IrInstruction *ir_analyze_instruction_end_expr(IrAnalyze *ira, IrInstruct
             return result_loc;
 
         if (!was_written) {
-            IrInstruction *store_ptr = ir_analyze_store_ptr(ira, &instruction->base, result_loc, value);
+            IrInstruction *store_ptr = ir_analyze_store_ptr(ira, &instruction->base, result_loc, value,
+                    instruction->result_loc->allow_write_through_const);
             if (type_is_invalid(store_ptr->value.type)) {
                 return ira->codegen->invalid_instruction;
             }
@@ -24325,7 +24350,7 @@ static IrInstruction *ir_analyze_instruction_bit_cast_src(IrAnalyze *ira, IrInst
         return operand;
 
     IrInstruction *result_loc = ir_resolve_result(ira, &instruction->base,
-            &instruction->result_loc_bit_cast->base, operand->value.type, operand, false, false);
+            &instruction->result_loc_bit_cast->base, operand->value.type, operand, false, false, true);
     if (result_loc != nullptr && (type_is_invalid(result_loc->value.type) || instr_is_unreachable(result_loc)))
         return result_loc;
 
diff --git a/src/target.cpp b/src/target.cpp
index 6a949270ae..7bb248a35f 100644
--- a/src/target.cpp
+++ b/src/target.cpp
@@ -863,6 +863,71 @@ uint32_t target_arch_pointer_bit_width(ZigLLVM_ArchType arch) {
     zig_unreachable();
 }
 
+uint32_t target_arch_largest_atomic_bits(ZigLLVM_ArchType arch) {
+    switch (arch) {
+        case ZigLLVM_UnknownArch:
+            zig_unreachable();
+
+        case ZigLLVM_avr:
+        case ZigLLVM_msp430:
+            return 16;
+
+        case ZigLLVM_arc:
+        case ZigLLVM_arm:
+        case ZigLLVM_armeb:
+        case ZigLLVM_hexagon:
+        case ZigLLVM_le32:
+        case ZigLLVM_mips:
+        case ZigLLVM_mipsel:
+        case ZigLLVM_nvptx:
+        case ZigLLVM_ppc:
+        case ZigLLVM_r600:
+        case ZigLLVM_riscv32:
+        case ZigLLVM_sparc:
+        case ZigLLVM_sparcel:
+        case ZigLLVM_tce:
+        case ZigLLVM_tcele:
+        case ZigLLVM_thumb:
+        case ZigLLVM_thumbeb:
+        case ZigLLVM_x86:
+        case ZigLLVM_xcore:
+        case ZigLLVM_amdil:
+        case ZigLLVM_hsail:
+        case ZigLLVM_spir:
+        case ZigLLVM_kalimba:
+        case ZigLLVM_lanai:
+        case ZigLLVM_shave:
+        case ZigLLVM_wasm32:
+        case ZigLLVM_renderscript32:
+            return 32;
+
+        case ZigLLVM_aarch64:
+        case ZigLLVM_aarch64_be:
+        case ZigLLVM_amdgcn:
+        case ZigLLVM_bpfel:
+        case ZigLLVM_bpfeb:
+        case ZigLLVM_le64:
+        case ZigLLVM_mips64:
+        case ZigLLVM_mips64el:
+        case ZigLLVM_nvptx64:
+        case ZigLLVM_ppc64:
+        case ZigLLVM_ppc64le:
+        case ZigLLVM_riscv64:
+        case ZigLLVM_sparcv9:
+        case ZigLLVM_systemz:
+        case ZigLLVM_amdil64:
+        case ZigLLVM_hsail64:
+        case ZigLLVM_spir64:
+        case ZigLLVM_wasm64:
+        case ZigLLVM_renderscript64:
+            return 64;
+
+        case ZigLLVM_x86_64:
+            return 128;
+    }
+    zig_unreachable();
+}
+
 uint32_t target_c_type_size_in_bits(const ZigTarget *target, CIntType id) {
     switch (target->os) {
         case OsFreestanding:
@@ -1693,3 +1758,4 @@ bool target_supports_libunwind(const ZigTarget *target) {
     }
     return true;
 }
+
diff --git a/src/target.hpp b/src/target.hpp
index fcda9955b9..985a4c11b4 100644
--- a/src/target.hpp
+++ b/src/target.hpp
@@ -192,6 +192,7 @@ const char *target_arch_musl_name(ZigLLVM_ArchType arch);
 bool target_supports_libunwind(const ZigTarget *target);
 
 uint32_t target_arch_pointer_bit_width(ZigLLVM_ArchType arch);
+uint32_t target_arch_largest_atomic_bits(ZigLLVM_ArchType arch);
 
 size_t target_libc_count(void);
 void target_libc_enum(size_t index, ZigTarget *out_target);
diff --git a/std/build.zig b/std/build.zig
index 3a0c34c8d1..6be88ae7ec 100644
--- a/std/build.zig
+++ b/std/build.zig
@@ -1802,7 +1802,7 @@ pub const LibExeObjStep = struct {
             try zig_args.append("--bundle-compiler-rt");
         }
         if (self.disable_stack_probing) {
-            try zig_args.append("--disable-stack-probing");
+            try zig_args.append("-fno-stack-check");
         }
 
         switch (self.target) {
diff --git a/std/coff.zig b/std/coff.zig
index 9fdc368878..3890151d09 100644
--- a/std/coff.zig
+++ b/std/coff.zig
@@ -19,6 +19,7 @@ const IMAGE_NT_OPTIONAL_HDR32_MAGIC = 0x10b;
 const IMAGE_NT_OPTIONAL_HDR64_MAGIC = 0x20b;
 
 const IMAGE_NUMBEROF_DIRECTORY_ENTRIES = 16;
+const IMAGE_DEBUG_TYPE_CODEVIEW = 2;
 const DEBUG_DIRECTORY = 6;
 
 pub const CoffError = error{
@@ -28,6 +29,7 @@ pub const CoffError = error{
     MissingCoffSection,
 };
 
+// Official documentation of the format: https://docs.microsoft.com/en-us/windows/win32/debug/pe-format
 pub const Coff = struct {
     in_file: File,
     allocator: *mem.Allocator,
@@ -120,16 +122,43 @@ pub const Coff = struct {
 
     pub fn getPdbPath(self: *Coff, buffer: []u8) !usize {
         try self.loadSections();
-        const header = (self.getSection(".rdata") orelse return error.MissingCoffSection).header;
 
-        // The linker puts a chunk that contains the .pdb path right after the
-        // debug_directory.
+        const header = blk: {
+            if (self.getSection(".buildid")) |section| {
+                break :blk section.header;
+            } else if (self.getSection(".rdata")) |section| {
+                break :blk section.header;
+            } else {
+                return error.MissingCoffSection;
+            }
+        };
+
         const debug_dir = &self.pe_header.data_directory[DEBUG_DIRECTORY];
         const file_offset = debug_dir.virtual_address - header.virtual_address + header.pointer_to_raw_data;
-        try self.in_file.seekTo(file_offset + debug_dir.size);
 
         var file_stream = self.in_file.inStream();
         const in = &file_stream.stream;
+        try self.in_file.seekTo(file_offset);
+
+        // Find the correct DebugDirectoryEntry, and where its data is stored.
+        // It can be in any section.
+        const debug_dir_entry_count = debug_dir.size / @sizeOf(DebugDirectoryEntry);
+        var i: u32 = 0;
+        blk: while (i < debug_dir_entry_count) : (i += 1) {
+            const debug_dir_entry = try in.readStruct(DebugDirectoryEntry);
+            if (debug_dir_entry.type == IMAGE_DEBUG_TYPE_CODEVIEW) {
+                for (self.sections.toSlice()) |*section| {
+                    const section_start = section.header.virtual_address;
+                    const section_size = section.header.misc.virtual_size;
+                    const rva = debug_dir_entry.address_of_raw_data;
+                    const offset = rva - section_start;
+                    if (section_start <= rva and offset < section_size and debug_dir_entry.size_of_data <= section_size - offset) {
+                        try self.in_file.seekTo(section.header.pointer_to_raw_data + offset);
+                        break :blk;
+                    }
+                }
+            }
+        }
 
         var cv_signature: [4]u8 = undefined; // CodeView signature
         try in.readNoEof(cv_signature[0..]);
@@ -141,7 +170,7 @@ pub const Coff = struct {
 
         // Finally read the null-terminated string.
         var byte = try in.readByte();
-        var i: usize = 0;
+        i = 0;
         while (byte != 0 and i < buffer.len) : (i += 1) {
             buffer[i] = byte;
             byte = try in.readByte();
@@ -170,7 +199,7 @@ pub const Coff = struct {
             try self.sections.append(Section{
                 .header = SectionHeader{
                     .name = name,
-                    .misc = SectionHeader.Misc{ .physical_address = try in.readIntLittle(u32) },
+                    .misc = SectionHeader.Misc{ .virtual_size = try in.readIntLittle(u32) },
                     .virtual_address = try in.readIntLittle(u32),
                     .size_of_raw_data = try in.readIntLittle(u32),
                     .pointer_to_raw_data = try in.readIntLittle(u32),
@@ -214,6 +243,17 @@ const OptionalHeader = struct {
     data_directory: [IMAGE_NUMBEROF_DIRECTORY_ENTRIES]DataDirectory,
 };
 
+const DebugDirectoryEntry = packed struct {
+    characteristiccs: u32,
+    time_date_stamp: u32,
+    major_version: u16,
+    minor_version: u16,
+    @"type": u32,
+    size_of_data: u32,
+    address_of_raw_data: u32,
+    pointer_to_raw_data: u32,
+};
+
 pub const Section = struct {
     header: SectionHeader,
 };
diff --git a/std/debug.zig b/std/debug.zig
index 7d1fd6ce47..32f96d3e15 100644
--- a/std/debug.zig
+++ b/std/debug.zig
@@ -375,7 +375,7 @@ fn printSourceAtAddressWindows(di: *DebugInfo, out_stream: var, relocated_addres
     const obj_basename = fs.path.basename(mod.obj_file_name);
 
     var symbol_i: usize = 0;
-    const symbol_name = while (symbol_i != mod.symbols.len) {
+    const symbol_name = if (!mod.populated) "???" else while (symbol_i != mod.symbols.len) {
         const prefix = @ptrCast(*pdb.RecordPrefix, &mod.symbols[symbol_i]);
         if (prefix.RecordLen < 2)
             return error.InvalidDebugInfo;
@@ -858,8 +858,10 @@ fn openSelfDebugInfoWindows(allocator: *mem.Allocator) !DebugInfo {
     const age = try pdb_stream.stream.readIntLittle(u32);
     var guid: [16]u8 = undefined;
     try pdb_stream.stream.readNoEof(guid[0..]);
+    if (version != 20000404) // VC70, only value observed by LLVM team
+        return error.UnknownPDBVersion;
     if (!mem.eql(u8, di.coff.guid, guid) or di.coff.age != age)
-        return error.InvalidDebugInfo;
+        return error.PDBMismatch;
     // We validated the executable and pdb match.
 
     const string_table_index = str_tab_index: {
@@ -903,13 +905,18 @@ fn openSelfDebugInfoWindows(allocator: *mem.Allocator) !DebugInfo {
         return error.MissingDebugInfo;
     };
 
-    di.pdb.string_table = di.pdb.getStreamById(string_table_index) orelse return error.InvalidDebugInfo;
+    di.pdb.string_table = di.pdb.getStreamById(string_table_index) orelse return error.MissingDebugInfo;
     di.pdb.dbi = di.pdb.getStream(pdb.StreamType.Dbi) orelse return error.MissingDebugInfo;
 
     const dbi = di.pdb.dbi;
 
     // Dbi Header
     const dbi_stream_header = try dbi.stream.readStruct(pdb.DbiStreamHeader);
+    if (dbi_stream_header.VersionHeader != 19990903) // V70, only value observed by LLVM team
+        return error.UnknownPDBVersion;
+    if (dbi_stream_header.Age != age)
+        return error.UnmatchingPDB;
+
     const mod_info_size = dbi_stream_header.ModInfoSize;
     const section_contrib_size = dbi_stream_header.SectionContributionSize;
 
diff --git a/std/fmt.zig b/std/fmt.zig
index 961c7279a2..8c4995cb91 100644
--- a/std/fmt.zig
+++ b/std/fmt.zig
@@ -371,9 +371,10 @@ pub fn formatType(
                 return output(context, "{ ... }");
             }
             comptime var field_i = 0;
+            try output(context, "{");
             inline while (field_i < @memberCount(T)) : (field_i += 1) {
                 if (field_i == 0) {
-                    try output(context, "{ .");
+                    try output(context, " .");
                 } else {
                     try output(context, ", .");
                 }
@@ -422,6 +423,9 @@ pub fn formatType(
             if (info.child == u8) {
                 return formatText(value, fmt, options, context, Errors, output);
             }
+            if (value.len == 0) {
+                return format(context, Errors, output, "[0]{}", @typeName(T.Child));
+            }
             return format(context, Errors, output, "{}@{x}", @typeName(T.Child), @ptrToInt(&value));
         },
         .Fn => {
@@ -1436,6 +1440,21 @@ test "struct.self-referential" {
     try testFmt("S{ .a = S{ .a = S{ .a = S{ ... } } } }", "{}", inst);
 }
 
+test "struct.zero-size" {
+    const A = struct {
+        fn foo() void {}
+    };
+    const B = struct {
+        a: A,
+        c: i32,
+    };
+
+    const a = A{};
+    const b = B{ .a = a, .c = 0 };
+
+    try testFmt("B{ .a = A{ }, .c = 0 }", "{}", b);
+}
+
 test "bytes.hex" {
     const some_bytes = "\xCA\xFE\xBA\xBE";
     try testFmt("lowercase: cafebabe\n", "lowercase: {x}\n", some_bytes);
diff --git a/std/os.zig b/std/os.zig
index 190f02101e..9ff2e8f87f 100644
--- a/std/os.zig
+++ b/std/os.zig
@@ -2053,6 +2053,22 @@ pub fn accessC(path: [*]const u8, mode: u32) AccessError!void {
     }
 }
 
+/// Call from Windows-specific code if you already have a UTF-16LE encoded, null terminated string.
+/// Otherwise use `access` or `accessC`.
+/// TODO currently this ignores `mode`.
+pub fn accessW(path: [*]const u16, mode: u32) windows.GetFileAttributesError!void {
+    const ret = try windows.GetFileAttributesW(path);
+    if (ret != windows.INVALID_FILE_ATTRIBUTES) {
+        return;
+    }
+    switch (windows.kernel32.GetLastError()) {
+        windows.ERROR.FILE_NOT_FOUND => return error.FileNotFound,
+        windows.ERROR.PATH_NOT_FOUND => return error.FileNotFound,
+        windows.ERROR.ACCESS_DENIED => return error.PermissionDenied,
+        else => |err| return windows.unexpectedError(err),
+    }
+}
+
 pub const PipeError = error{
     SystemFdQuotaExceeded,
     ProcessFdQuotaExceeded,
diff --git a/std/pdb.zig b/std/pdb.zig
index ffe2120296..7a2b2c6b6b 100644
--- a/std/pdb.zig
+++ b/std/pdb.zig
@@ -499,45 +499,78 @@ const Msf = struct {
 
         const superblock = try in.readStruct(SuperBlock);
 
+        // Sanity checks
         if (!mem.eql(u8, superblock.FileMagic, SuperBlock.file_magic))
             return error.InvalidDebugInfo;
-
+        if (superblock.FreeBlockMapBlock != 1 and superblock.FreeBlockMapBlock != 2)
+            return error.InvalidDebugInfo;
+        if (superblock.NumBlocks * superblock.BlockSize != try file.getEndPos())
+            return error.InvalidDebugInfo;
         switch (superblock.BlockSize) {
             // llvm only supports 4096 but we can handle any of these values
             512, 1024, 2048, 4096 => {},
             else => return error.InvalidDebugInfo,
         }
 
-        if (superblock.NumBlocks * superblock.BlockSize != try file.getEndPos())
-            return error.InvalidDebugInfo;
+        const dir_block_count = blockCountFromSize(superblock.NumDirectoryBytes, superblock.BlockSize);
+        if (dir_block_count > superblock.BlockSize / @sizeOf(u32))
+            return error.UnhandledBigDirectoryStream; // cf. BlockMapAddr comment.
 
-        self.directory = try MsfStream.init(
+        try file.seekTo(superblock.BlockSize * superblock.BlockMapAddr);
+        var dir_blocks = try allocator.alloc(u32, dir_block_count);
+        for (dir_blocks) |*b| {
+            b.* = try in.readIntLittle(u32);
+        }
+        self.directory = MsfStream.init(
             superblock.BlockSize,
-            blockCountFromSize(superblock.NumDirectoryBytes, superblock.BlockSize),
-            superblock.BlockSize * superblock.BlockMapAddr,
             file,
-            allocator,
+            dir_blocks,
         );
 
+        const begin = self.directory.pos;
         const stream_count = try self.directory.stream.readIntLittle(u32);
-
         const stream_sizes = try allocator.alloc(u32, stream_count);
-        for (stream_sizes) |*s| {
+        defer allocator.free(stream_sizes);
+
+        // Microsoft's implementation uses u32(-1) for inexistant streams.
+        // These streams are not used, but still participate in the file
+        // and must be taken into account when resolving stream indices.
+        const Nil = 0xFFFFFFFF;
+        for (stream_sizes) |*s, i| {
             const size = try self.directory.stream.readIntLittle(u32);
-            s.* = blockCountFromSize(size, superblock.BlockSize);
+            s.* = if (size == Nil) 0 else blockCountFromSize(size, superblock.BlockSize);
         }
 
         self.streams = try allocator.alloc(MsfStream, stream_count);
         for (self.streams) |*stream, i| {
-            stream.* = try MsfStream.init(
-                superblock.BlockSize,
-                stream_sizes[i],
-                // MsfStream.init expects the file to be at the part where it reads [N]u32
-                try file.getPos(),
-                file,
-                allocator,
-            );
+            const size = stream_sizes[i];
+            if (size == 0) {
+                stream.* = MsfStream{
+                    .blocks = [_]u32{},
+                };
+            } else {
+                var blocks = try allocator.alloc(u32, size);
+                var j: u32 = 0;
+                while (j < size) : (j += 1) {
+                    const block_id = try self.directory.stream.readIntLittle(u32);
+                    const n = (block_id % superblock.BlockSize);
+                    // 0 is for SuperBlock, 1 and 2 for FPMs.
+                    if (block_id == 0 or n == 1 or n == 2 or block_id * superblock.BlockSize > try file.getEndPos())
+                        return error.InvalidBlockIndex;
+                    blocks[j] = block_id;
+                }
+
+                stream.* = MsfStream.init(
+                    superblock.BlockSize,
+                    file,
+                    blocks,
+                );
+            }
         }
+
+        const end = self.directory.pos;
+        if (end - begin != superblock.NumDirectoryBytes)
+            return error.InvalidStreamDirectory;
     }
 };
 
@@ -574,7 +607,6 @@ const SuperBlock = packed struct {
     NumDirectoryBytes: u32,
 
     Unknown: u32,
-
     /// The index of a block within the MSF file. At this block is an array of
     /// ulittle32_t’s listing the blocks that the stream directory resides on.
     /// For large MSF files, the stream directory (which describes the block
@@ -584,45 +616,41 @@ const SuperBlock = packed struct {
     /// and the stream directory itself can be stitched together accordingly.
     /// The number of ulittle32_t’s in this array is given by
     /// ceil(NumDirectoryBytes / BlockSize).
+    // Note: microsoft-pdb code actually suggests this is a variable-length
+    // array. If the indices of blocks occupied by the Stream Directory didn't
+    // fit in one page, there would be other u32 following it.
+    // This would mean the Stream Directory is bigger than BlockSize / sizeof(u32)
+    // blocks. We're not even close to this with a 1GB pdb file, and LLVM didn't
+    // implement it so we're kind of safe making this assumption for now.
     BlockMapAddr: u32,
 };
 
 const MsfStream = struct {
-    in_file: File,
-    pos: u64,
-    blocks: []u32,
-    block_size: u32,
+    in_file: File = undefined,
+    pos: u64 = undefined,
+    blocks: []u32 = undefined,
+    block_size: u32 = undefined,
 
     /// Implementation of InStream trait for Pdb.MsfStream
-    stream: Stream,
+    stream: Stream = undefined,
 
     pub const Error = @typeOf(read).ReturnType.ErrorSet;
     pub const Stream = io.InStream(Error);
 
-    fn init(block_size: u32, block_count: u32, pos: u64, file: File, allocator: *mem.Allocator) !MsfStream {
-        var stream = MsfStream{
+    fn init(block_size: u32, file: File, blocks: []u32) MsfStream {
+        const stream = MsfStream{
             .in_file = file,
             .pos = 0,
-            .blocks = try allocator.alloc(u32, block_count),
+            .blocks = blocks,
             .block_size = block_size,
             .stream = Stream{ .readFn = readFn },
         };
 
-        var file_stream = file.inStream();
-        const in = &file_stream.stream;
-        try file.seekTo(pos);
-
-        var i: u32 = 0;
-        while (i < block_count) : (i += 1) {
-            stream.blocks[i] = try in.readIntLittle(u32);
-        }
-
         return stream;
     }
 
     fn readNullTermString(self: *MsfStream, allocator: *mem.Allocator) ![]u8 {
         var list = ArrayList(u8).init(allocator);
-        defer list.deinit();
         while (true) {
             const byte = try self.stream.readByte();
             if (byte == 0) {
@@ -642,11 +670,12 @@ const MsfStream = struct {
         const in = &file_stream.stream;
 
         var size: usize = 0;
-        for (buffer) |*byte| {
-            byte.* = try in.readByte();
-
-            offset += 1;
-            size += 1;
+        var rem_buffer = buffer;
+        while (size < buffer.len) {
+            const size_to_read = math.min(self.block_size - offset, rem_buffer.len);
+            size += try in.read(rem_buffer[0..size_to_read]);
+            rem_buffer = buffer[size..];
+            offset += size_to_read;
 
             // If we're at the end of a block, go to the next one.
             if (offset == self.block_size) {
@@ -657,8 +686,8 @@ const MsfStream = struct {
             }
         }
 
-        self.pos += size;
-        return size;
+        self.pos += buffer.len;
+        return buffer.len;
     }
 
     fn seekBy(self: *MsfStream, len: i64) !void {
diff --git a/std/rb.zig b/std/rb.zig
index 0b28550a13..b5935a2eac 100644
--- a/std/rb.zig
+++ b/std/rb.zig
@@ -93,7 +93,8 @@ pub const Node = struct {
         comptime {
             assert(@alignOf(*Node) >= 2);
         }
-        return @intToPtr(*Node, node.parent_and_color & ~mask);
+        const maybe_ptr = node.parent_and_color & ~mask;
+        return if (maybe_ptr == 0) null else @intToPtr(*Node, maybe_ptr);
     }
 
     fn setColor(node: *Node, color: Color) void {
diff --git a/std/segmented_list.zig b/std/segmented_list.zig
index e0b84d5c0d..3bbbde782e 100644
--- a/std/segmented_list.zig
+++ b/std/segmented_list.zig
@@ -77,16 +77,20 @@ const Allocator = std.mem.Allocator;
 pub fn SegmentedList(comptime T: type, comptime prealloc_item_count: usize) type {
     return struct {
         const Self = @This();
-        const prealloc_exp = blk: {
-            // we don't use the prealloc_exp constant when prealloc_item_count is 0.
-            assert(prealloc_item_count != 0);
-            assert(std.math.isPowerOfTwo(prealloc_item_count));
-
-            const value = std.math.log2_int(usize, prealloc_item_count);
-            break :blk @typeOf(1)(value);
-        };
         const ShelfIndex = std.math.Log2Int(usize);
 
+        const prealloc_exp: ShelfIndex = blk: {
+            // we don't use the prealloc_exp constant when prealloc_item_count is 0
+            // but lazy-init may still be triggered by other code so supply a value
+            if (prealloc_item_count == 0) {
+                break :blk 0;
+            } else {
+                assert(std.math.isPowerOfTwo(prealloc_item_count));
+                const value = std.math.log2_int(usize, prealloc_item_count);
+                break :blk value;
+            }
+        };
+
         prealloc_segment: [prealloc_item_count]T,
         dynamic_segments: [][*]T,
         allocator: *Allocator,
@@ -157,11 +161,12 @@ pub fn SegmentedList(comptime T: type, comptime prealloc_item_count: usize) type
 
         /// Grows or shrinks capacity to match usage.
         pub fn setCapacity(self: *Self, new_capacity: usize) !void {
-            if (new_capacity <= usize(1) << (prealloc_exp + self.dynamic_segments.len)) {
-                return self.shrinkCapacity(new_capacity);
-            } else {
-                return self.growCapacity(new_capacity);
+            if (prealloc_item_count != 0) {
+                if (new_capacity <= usize(1) << (prealloc_exp + @intCast(ShelfIndex, self.dynamic_segments.len))) {
+                    return self.shrinkCapacity(new_capacity);
+                }
             }
+            return self.growCapacity(new_capacity);
         }
 
         /// Only grows capacity, or retains current capacity
@@ -399,4 +404,6 @@ fn testSegmentedList(comptime prealloc: usize, allocator: *Allocator) !void {
         testing.expect(item == i);
         list.shrinkCapacity(list.len);
     }
+
+    try list.setCapacity(0);
 }
diff --git a/std/special/start.zig b/std/special/start.zig
index f8a018e0ae..3427ff422d 100644
--- a/std/special/start.zig
+++ b/std/special/start.zig
@@ -35,13 +35,13 @@ nakedcc fn _start() noreturn {
 
     switch (builtin.arch) {
         .x86_64 => {
-            argc_ptr = asm ("lea (%%rsp), %[argc]"
-                : [argc] "=r" (-> [*]usize)
+            argc_ptr = asm (""
+                : [argc] "={rsp}" (-> [*]usize)
             );
         },
         .i386 => {
-            argc_ptr = asm ("lea (%%esp), %[argc]"
-                : [argc] "=r" (-> [*]usize)
+            argc_ptr = asm (""
+                : [argc] "={esp}" (-> [*]usize)
             );
         },
         .aarch64, .aarch64_be => {
diff --git a/std/std.zig b/std/std.zig
index 350f0fb437..e48586d873 100644
--- a/std/std.zig
+++ b/std/std.zig
@@ -105,6 +105,7 @@ test "std" {
     _ = @import("packed_int_array.zig");
     _ = @import("priority_queue.zig");
     _ = @import("rand.zig");
+    _ = @import("rb.zig");
     _ = @import("sort.zig");
     _ = @import("testing.zig");
     _ = @import("thread.zig");
diff --git a/test/compile_errors.zig b/test/compile_errors.zig
index c4948135a0..272d99c930 100644
--- a/test/compile_errors.zig
+++ b/test/compile_errors.zig
@@ -219,7 +219,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\    return error.OutOfMemory;
         \\}
     ,
-        "tmp.zig:2:7: error: error is discarded",
+        "tmp.zig:2:12: error: error is discarded",
     );
 
     cases.add(
@@ -2758,7 +2758,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\    3 = 3;
         \\}
     ,
-        "tmp.zig:2:7: error: cannot assign to constant",
+        "tmp.zig:2:9: error: cannot assign to constant",
     );
 
     cases.add(
@@ -2768,7 +2768,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\    a = 4;
         \\}
     ,
-        "tmp.zig:3:7: error: cannot assign to constant",
+        "tmp.zig:3:9: error: cannot assign to constant",
     );
 
     cases.add(
@@ -2838,7 +2838,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\}
         \\export fn entry() void { f(); }
     ,
-        "tmp.zig:3:7: error: cannot assign to constant",
+        "tmp.zig:3:9: error: cannot assign to constant",
     );
 
     cases.add(
@@ -3901,7 +3901,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\
         \\export fn entry() usize { return @sizeOf(@typeOf(a)); }
     ,
-        "tmp.zig:6:24: error: unable to evaluate constant expression",
+        "tmp.zig:6:26: error: unable to evaluate constant expression",
         "tmp.zig:4:17: note: called from here",
     );
 
@@ -4151,7 +4151,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\  cstr[0] = 'W';
         \\}
     ,
-        "tmp.zig:3:11: error: cannot assign to constant",
+        "tmp.zig:3:13: error: cannot assign to constant",
     );
 
     cases.add(
@@ -4161,7 +4161,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\  cstr[0] = 'W';
         \\}
     ,
-        "tmp.zig:3:11: error: cannot assign to constant",
+        "tmp.zig:3:13: error: cannot assign to constant",
     );
 
     cases.add(
@@ -4309,7 +4309,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\    f.field = 0;
         \\}
     ,
-        "tmp.zig:6:13: error: cannot assign to constant",
+        "tmp.zig:6:15: error: cannot assign to constant",
     );
 
     cases.add(
diff --git a/test/stage1/behavior/atomics.zig b/test/stage1/behavior/atomics.zig
index daa463fd45..1a941cf21c 100644
--- a/test/stage1/behavior/atomics.zig
+++ b/test/stage1/behavior/atomics.zig
@@ -1,5 +1,6 @@
 const std = @import("std");
 const expect = std.testing.expect;
+const expectEqual = std.testing.expectEqual;
 const builtin = @import("builtin");
 const AtomicRmwOp = builtin.AtomicRmwOp;
 const AtomicOrder = builtin.AtomicOrder;
@@ -69,3 +70,33 @@ test "cmpxchg with ptr" {
     expect(@cmpxchgStrong(*i32, &x, &data3, &data2, AtomicOrder.SeqCst, AtomicOrder.SeqCst) == null);
     expect(x == &data2);
 }
+
+// TODO this test is disabled until this issue is resolved:
+// https://github.com/ziglang/zig/issues/2883
+// otherwise cross compiling will result in:
+// lld: error: undefined symbol: __sync_val_compare_and_swap_16
+//test "128-bit cmpxchg" {
+//    var x: u128 align(16) = 1234; // TODO: https://github.com/ziglang/zig/issues/2987
+//    if (@cmpxchgWeak(u128, &x, 99, 5678, .SeqCst, .SeqCst)) |x1| {
+//        expect(x1 == 1234);
+//    } else {
+//        @panic("cmpxchg should have failed");
+//    }
+//
+//    while (@cmpxchgWeak(u128, &x, 1234, 5678, .SeqCst, .SeqCst)) |x1| {
+//        expect(x1 == 1234);
+//    }
+//    expect(x == 5678);
+//
+//    expect(@cmpxchgStrong(u128, &x, 5678, 42, .SeqCst, .SeqCst) == null);
+//    expect(x == 42);
+//}
+
+test "cmpxchg with ignored result" {
+    var x: i32 = 1234;
+    var ptr = &x;
+
+    _ = @cmpxchgStrong(i32, &x, 1234, 5678, .Monotonic, .Monotonic);
+
+    expectEqual(i32(5678), x);
+}
diff --git a/test/stage1/behavior/eval.zig b/test/stage1/behavior/eval.zig
index 97d3a269cc..58d662d768 100644
--- a/test/stage1/behavior/eval.zig
+++ b/test/stage1/behavior/eval.zig
@@ -1,5 +1,6 @@
 const std = @import("std");
 const expect = std.testing.expect;
+const expectEqual = std.testing.expectEqual;
 const builtin = @import("builtin");
 
 test "compile time recursion" {
@@ -794,3 +795,12 @@ test "no undeclared identifier error in unanalyzed branches" {
         lol_this_doesnt_exist = nonsense;
     }
 }
+
+test "comptime assign int to optional int" {
+    comptime {
+        var x: ?i32 = null;
+        x = 2;
+        x.? *= 10;
+        expectEqual(20, x.?);
+    }
+}
diff --git a/test/stage1/behavior/fn.zig b/test/stage1/behavior/fn.zig
index d6d670b09b..6b9c8b8fe7 100644
--- a/test/stage1/behavior/fn.zig
+++ b/test/stage1/behavior/fn.zig
@@ -228,3 +228,22 @@ test "implicit cast fn call result to optional in field result" {
     S.entry();
     comptime S.entry();
 }
+
+test "discard the result of a function that returns a struct" {
+    const S = struct {
+        fn entry() void {
+            _ = func();
+        }
+
+        fn func() Foo {
+            return undefined;
+        }
+
+        const Foo = struct {
+            a: u64,
+            b: u64,
+        };
+    };
+    S.entry();
+    comptime S.entry();
+}