no-copy semantics for if expr

```zig
export fn entry() void {
    var c = true;
    var x = if (c) u8(4) else u32(10);
}
```

```llvm
define void @entry() #2 !dbg !35 {
Entry:
  %c = alloca i1, align 1
  %x = alloca i32, align 4
  store i1 true, i1* %c, align 1, !dbg !44
  call void @llvm.dbg.declare(metadata i1* %c, metadata !39, metadata !DIExpression()), !dbg !45
  %0 = load i1, i1* %c, align 1, !dbg !46
  br i1 %0, label %Then, label %Else, !dbg !46

Then:                                             ; preds = %Entry
  br label %EndIf, !dbg !47

Else:                                             ; preds = %Entry
  br label %EndIf, !dbg !47

EndIf:                                            ; preds = %Else, %Then
  %1 = phi i32 [ 4, %Then ], [ 10, %Else ], !dbg !47
  store i32 %1, i32* %x, align 4, !dbg !47
  call void @llvm.dbg.declare(metadata i32* %x, metadata !42, metadata !DIExpression()), !dbg !48
  ret void, !dbg !49
}
```

BRANCH_TODO

@@ -8,3 +8,21 @@ migrate all the alloca_list to alloca_gen_list
 migrate ir_build_var_decl_src to use ir_build_alloca_src and explicitly initialize
 
 inferred comptime
+
+
+    if (lval == LValNone) {
+        if (result_loc->id == ResultLocIdNone)
+            return value;
+    }
+
+    assert(lval == LValPtr);
+
+    // We needed a pointer to a value, but we got a value. So we create
+    // an instruction which just makes a pointer of it.
+    return ir_build_ref(irb, scope, value->source_node, value, false, false);
+
+handle if with no else
+
+handle comptime if condition
+
+handle mixed runtime and comptime peers

src/all_types.hpp

@@ -42,6 +42,7 @@ struct Tld;
 struct TldExport;
 struct IrAnalyze;
 struct ResultLoc;
+struct ResultLocPeer;
 
 enum X64CABIClass {
     X64CABIClass_Unknown,
@@ -2127,6 +2128,8 @@ struct IrBasicBlock {
     const char *name_hint;
     size_t debug_id;
     size_t ref_count;
+    // index into the basic block list
+    size_t index;
     LLVMBasicBlockRef llvm_block;
     LLVMBasicBlockRef llvm_exit_block;
     // The instruction that referenced this basic block and caused us to
@@ -2315,10 +2318,14 @@ enum IrInstructionId {
     IrInstructionIdUndeclaredIdent,
     IrInstructionIdAllocaSrc,
     IrInstructionIdAllocaGen,
+    IrInstructionIdEndExpr,
 };
 
 struct IrInstruction {
     IrInstructionId id;
+    // true if this instruction was generated by zig and not from user code
+    bool is_gen;
+
     Scope *scope;
     AstNode *source_node;
     ConstExprValue value;
@@ -2332,8 +2339,6 @@ struct IrInstruction {
     // with this child field.
     IrInstruction *child;
     IrBasicBlock *owner_bb;
-    // true if this instruction was generated by zig and not from user code
-    bool is_gen;
 };
 
 struct IrInstructionDeclVarSrc {
@@ -3571,16 +3576,28 @@ struct IrInstructionAllocaGen {
     const char *name_hint;
 };
 
+struct IrInstructionEndExpr {
+    IrInstruction base;
+
+    IrInstruction *value;
+    ResultLoc *result_loc;
+    LVal lval;
+};
+
 enum ResultLocId {
     ResultLocIdInvalid,
     ResultLocIdNone,
     ResultLocIdVar,
     ResultLocIdReturn,
+    ResultLocIdPeer,
+    ResultLocIdPeerParent,
 };
 
 struct ResultLoc {
     ResultLocId id;
     IrInstruction *source_instruction;
+    IrInstruction *gen_instruction;
+    ZigType *implicit_elem_type;
 };
 
 struct ResultLocNone {
@@ -3597,6 +3614,28 @@ struct ResultLocReturn {
     ResultLoc base;
 };
 
+struct ResultLocPeerParent {
+    ResultLoc base;
+
+    ResultLoc *parent;
+    ResultLocPeer *peers;
+    size_t peer_count;
+    ZigType *resolved_type;
+};
+
+struct IrSuspendPosition {
+    size_t basic_block_index;
+    size_t instruction_index;
+};
+
+struct ResultLocPeer {
+    ResultLoc base;
+
+    ResultLocPeerParent *parent;
+    IrBasicBlock *next_bb;
+    IrSuspendPosition suspend_pos;
+};
+
 static const size_t slice_ptr_index = 0;
 static const size_t slice_len_index = 1;
 

src/codegen.cpp

@@ -5606,10 +5606,9 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
         case IrInstructionIdUndeclaredIdent:
         case IrInstructionIdCallSrc:
         case IrInstructionIdAllocaSrc:
-            zig_unreachable();
-
+        case IrInstructionIdEndExpr:
         case IrInstructionIdAllocaGen:
-            return nullptr;
+            zig_unreachable();
 
         case IrInstructionIdDeclVarGen:
             return ir_render_decl_var(g, executable, (IrInstructionDeclVarGen *)instruction);

src/ir_print.cpp

@@ -158,6 +158,16 @@ static const char *ir_un_op_id_str(IrUnOp op_id) {
     zig_unreachable();
 }
 
+static const char *ir_lval_str(LVal lval) {
+    switch (lval) {
+        case LValNone:
+            return "None";
+        case LValPtr:
+            return "Ptr";
+    }
+    zig_unreachable();
+}
+
 static void ir_print_un_op(IrPrint *irp, IrInstructionUnOp *un_op_instruction) {
     fprintf(irp->f, "%s ", ir_un_op_id_str(un_op_instruction->op_id));
     ir_print_other_instruction(irp, un_op_instruction->value);
@@ -219,6 +229,12 @@ static void ir_print_result_loc(IrPrint *irp, ResultLoc *result_loc) {
             return;
         case ResultLocIdVar:
             return ir_print_result_loc_var(irp, (ResultLocVar *)result_loc);
+        case ResultLocIdPeer:
+            fprintf(irp->f, "peer");
+            return;
+        case ResultLocIdPeerParent:
+            fprintf(irp->f, "peer_parent");
+            return;
     }
     zig_unreachable();
 }
@@ -1128,6 +1144,14 @@ static void ir_print_alloca_gen(IrPrint *irp, IrInstructionAllocaGen *instructio
     fprintf(irp->f, "Alloca(align=%" PRIu32 ",name=%s)", instruction->align, instruction->name_hint);
 }
 
+static void ir_print_end_expr(IrPrint *irp, IrInstructionEndExpr *instruction) {
+    fprintf(irp->f, "EndExpr(result=");
+    ir_print_result_loc(irp, instruction->result_loc);
+    fprintf(irp->f, ",value=");
+    ir_print_other_instruction(irp, instruction->value);
+    fprintf(irp->f, ",lval=%s)", ir_lval_str(instruction->lval));
+}
+
 static void ir_print_int_to_err(IrPrint *irp, IrInstructionIntToErr *instruction) {
     fprintf(irp->f, "inttoerr ");
     ir_print_other_instruction(irp, instruction->target);
@@ -2014,6 +2038,9 @@ static void ir_print_instruction(IrPrint *irp, IrInstruction *instruction) {
         case IrInstructionIdAllocaGen:
             ir_print_alloca_gen(irp, (IrInstructionAllocaGen *)instruction);
             break;
+        case IrInstructionIdEndExpr:
+            ir_print_end_expr(irp, (IrInstructionEndExpr *)instruction);
+            break;
     }
     fprintf(irp->f, "\n");
 }