diff --git a/doc/langref.html.in b/doc/langref.html.in
index 9123b1df74..05e804d78a 100644
--- a/doc/langref.html.in
+++ b/doc/langref.html.in
@@ -5645,7 +5645,7 @@ UseDecl = "use" Expression ";"
 
 ExternDecl = "extern" option(String) (FnProto | VariableDeclaration) ";"
 
-FnProto = option("nakedcc" | "stdcallcc" | "extern") "fn" option(Symbol) ParamDeclList option("align" "(" Expression ")") option("section" "(" Expression ")") option("!") TypeExpr
+FnProto = option("nakedcc" | "stdcallcc" | "extern" | "async") "fn" option(Symbol) ParamDeclList option("align" "(" Expression ")") option("section" "(" Expression ")") option("!") TypeExpr
 
 FnDef = option("inline" | "export") FnProto Block
 
diff --git a/src/all_types.hpp b/src/all_types.hpp
index 694a2f64d9..b87899c526 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -406,6 +406,7 @@ enum CallingConvention {
     CallingConventionCold,
     CallingConventionNaked,
     CallingConventionStdcall,
+    CallingConventionAsync,
 };
 
 struct AstNodeFnProto {
@@ -2152,6 +2153,8 @@ struct IrInstructionCall {
     bool is_comptime;
     LLVMValueRef tmp_ptr;
     FnInline fn_inline;
+    bool is_async;
+    IrInstruction *async_allocator;
 };
 
 struct IrInstructionConst {
diff --git a/src/analyze.cpp b/src/analyze.cpp
index 4545db9d78..4d6b218d0e 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -884,6 +884,7 @@ static const char *calling_convention_name(CallingConvention cc) {
         case CallingConventionCold: return "coldcc";
         case CallingConventionNaked: return "nakedcc";
         case CallingConventionStdcall: return "stdcallcc";
+        case CallingConventionAsync: return "async";
     }
     zig_unreachable();
 }
@@ -895,6 +896,7 @@ static const char *calling_convention_fn_type_str(CallingConvention cc) {
         case CallingConventionCold: return "coldcc ";
         case CallingConventionNaked: return "nakedcc ";
         case CallingConventionStdcall: return "stdcallcc ";
+        case CallingConventionAsync: return "async ";
     }
     zig_unreachable();
 }
diff --git a/src/codegen.cpp b/src/codegen.cpp
index 4d9b0279d0..7c80c33754 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -381,6 +381,8 @@ static LLVMCallConv get_llvm_cc(CodeGen *g, CallingConvention cc) {
             } else {
                 return LLVMCCallConv;
             }
+        case CallingConventionAsync:
+            return LLVMFastCallConv;
     }
     zig_unreachable();
 }
diff --git a/src/ir.cpp b/src/ir.cpp
index d220efde79..7e95d98427 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -986,7 +986,7 @@ static IrInstruction *ir_build_union_field_ptr_from(IrBuilder *irb, IrInstructio
 
 static IrInstruction *ir_build_call(IrBuilder *irb, Scope *scope, AstNode *source_node,
         FnTableEntry *fn_entry, IrInstruction *fn_ref, size_t arg_count, IrInstruction **args,
-        bool is_comptime, FnInline fn_inline)
+        bool is_comptime, FnInline fn_inline, bool is_async, IrInstruction *async_allocator)
 {
     IrInstructionCall *call_instruction = ir_build_instruction<IrInstructionCall>(irb, scope, source_node);
     call_instruction->fn_entry = fn_entry;
@@ -995,21 +995,25 @@ static IrInstruction *ir_build_call(IrBuilder *irb, Scope *scope, AstNode *sourc
     call_instruction->fn_inline = fn_inline;
     call_instruction->args = args;
     call_instruction->arg_count = arg_count;
+    call_instruction->is_async = is_async;
+    call_instruction->async_allocator = async_allocator;
 
     if (fn_ref)
         ir_ref_instruction(fn_ref, irb->current_basic_block);
     for (size_t i = 0; i < arg_count; i += 1)
         ir_ref_instruction(args[i], irb->current_basic_block);
+    if (async_allocator)
+        ir_ref_instruction(async_allocator, irb->current_basic_block);
 
     return &call_instruction->base;
 }
 
 static IrInstruction *ir_build_call_from(IrBuilder *irb, IrInstruction *old_instruction,
         FnTableEntry *fn_entry, IrInstruction *fn_ref, size_t arg_count, IrInstruction **args,
-        bool is_comptime, FnInline fn_inline)
+        bool is_comptime, FnInline fn_inline, bool is_async, IrInstruction *async_allocator)
 {
     IrInstruction *new_instruction = ir_build_call(irb, old_instruction->scope,
-            old_instruction->source_node, fn_entry, fn_ref, arg_count, args, is_comptime, fn_inline);
+            old_instruction->source_node, fn_entry, fn_ref, arg_count, args, is_comptime, fn_inline, is_async, async_allocator);
     ir_link_new_instruction(new_instruction, old_instruction);
     return new_instruction;
 }
@@ -3754,7 +3758,7 @@ static IrInstruction *ir_gen_builtin_fn_call(IrBuilder *irb, Scope *scope, AstNo
                 }
                 FnInline fn_inline = (builtin_fn->id == BuiltinFnIdInlineCall) ? FnInlineAlways : FnInlineNever;
 
-                return ir_build_call(irb, scope, node, nullptr, fn_ref, arg_count, args, false, fn_inline);
+                return ir_build_call(irb, scope, node, nullptr, fn_ref, arg_count, args, false, fn_inline, false, nullptr);
             }
         case BuiltinFnIdTypeId:
             {
@@ -3888,11 +3892,17 @@ static IrInstruction *ir_gen_fn_call(IrBuilder *irb, Scope *scope, AstNode *node
             return args[i];
     }
 
-    if (node->data.fn_call_expr.is_async) {
-        zig_panic("TODO ir_gen_fn_call for async fn calls");
+    bool is_async = node->data.fn_call_expr.is_async;
+    IrInstruction *async_allocator = nullptr;
+    if (is_async) {
+        if (node->data.fn_call_expr.async_allocator) {
+            async_allocator = ir_gen_node(irb, node->data.fn_call_expr.async_allocator, scope);
+            if (async_allocator == irb->codegen->invalid_instruction)
+                return async_allocator;
+        }
     }
 
-    return ir_build_call(irb, scope, node, nullptr, fn_ref, arg_count, args, false, FnInlineAuto);
+    return ir_build_call(irb, scope, node, nullptr, fn_ref, arg_count, args, false, FnInlineAuto, is_async, async_allocator);
 }
 
 static IrInstruction *ir_gen_if_bool_expr(IrBuilder *irb, Scope *scope, AstNode *node) {
@@ -10584,6 +10594,11 @@ static TypeTableEntry *ir_analyze_instruction_export(IrAnalyze *ira, IrInstructi
                         buf_sprintf("exported function must specify calling convention"));
                     add_error_note(ira->codegen, msg, fn_entry->proto_node, buf_sprintf("declared here"));
                 } break;
+                case CallingConventionAsync: {
+                    ErrorMsg *msg = ir_add_error(ira, target,
+                        buf_sprintf("exported function cannot be async"));
+                    add_error_note(ira->codegen, msg, fn_entry->proto_node, buf_sprintf("declared here"));
+                } break;
                 case CallingConventionC:
                 case CallingConventionNaked:
                 case CallingConventionCold:
@@ -10963,6 +10978,13 @@ static TypeTableEntry *ir_analyze_fn_call(IrAnalyze *ira, IrInstructionCall *cal
         }
         return ira->codegen->builtin_types.entry_invalid;
     }
+    if (fn_type_id->cc == CallingConventionAsync && !call_instruction->is_async) {
+        ErrorMsg *msg = ir_add_error(ira, fn_ref, buf_sprintf("must use async keyword to call async function"));
+        if (fn_proto_node) {
+            add_error_note(ira->codegen, msg, fn_proto_node, buf_sprintf("declared here"));
+        }
+        return ira->codegen->builtin_types.entry_invalid;
+    }
 
 
     if (fn_type_id->is_var_args) {
@@ -11258,7 +11280,7 @@ static TypeTableEntry *ir_analyze_fn_call(IrAnalyze *ira, IrInstructionCall *cal
 
         size_t impl_param_count = impl_fn->type_entry->data.fn.fn_type_id.param_count;
         IrInstruction *new_call_instruction = ir_build_call_from(&ira->new_irb, &call_instruction->base,
-                impl_fn, nullptr, impl_param_count, casted_args, false, fn_inline);
+                impl_fn, nullptr, impl_param_count, casted_args, false, fn_inline, false, nullptr);
 
         TypeTableEntry *return_type = impl_fn->type_entry->data.fn.fn_type_id.return_type;
         ir_add_alloca(ira, new_call_instruction, return_type);
@@ -11324,12 +11346,16 @@ static TypeTableEntry *ir_analyze_fn_call(IrAnalyze *ira, IrInstructionCall *cal
 
     assert(next_arg_index == call_param_count);
 
+    if (call_instruction->is_async) {
+        zig_panic("TODO handle async fn call");
+    }
+
     TypeTableEntry *return_type = fn_type_id->return_type;
     if (type_is_invalid(return_type))
         return ira->codegen->builtin_types.entry_invalid;
 
     IrInstruction *new_call_instruction = ir_build_call_from(&ira->new_irb, &call_instruction->base,
-            fn_entry, fn_ref, call_param_count, casted_args, false, fn_inline);
+            fn_entry, fn_ref, call_param_count, casted_args, false, fn_inline, false, nullptr);
 
     ir_add_alloca(ira, new_call_instruction, return_type);
     return ir_finish_anal(ira, return_type);
@@ -16491,7 +16517,10 @@ static TypeTableEntry *ir_analyze_instruction_tag_type(IrAnalyze *ira, IrInstruc
 }
 
 static TypeTableEntry *ir_analyze_instruction_cancel(IrAnalyze *ira, IrInstructionCancel *instruction) {
-    IrInstruction *casted_target = ir_implicit_cast(ira, instruction->target->other, ira->codegen->builtin_types.entry_promise);
+    IrInstruction *target_inst = instruction->target->other;
+    if (type_is_invalid(target_inst->value.type))
+        return ira->codegen->builtin_types.entry_invalid;
+    IrInstruction *casted_target = ir_implicit_cast(ira, target_inst, ira->codegen->builtin_types.entry_promise);
     if (type_is_invalid(casted_target->value.type))
         return ira->codegen->builtin_types.entry_invalid;
 
diff --git a/src/parser.cpp b/src/parser.cpp
index fc682ee62a..ee7141a910 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -2333,7 +2333,7 @@ static AstNode *ast_parse_block(ParseContext *pc, size_t *token_index, bool mand
 }
 
 /*
-FnProto = option("nakedcc" | "stdcallcc" | "extern") "fn" option(Symbol) ParamDeclList option("align" "(" Expression ")") option("section" "(" Expression ")") option("!") TypeExpr
+FnProto = option("nakedcc" | "stdcallcc" | "extern" | "async") "fn" option(Symbol) ParamDeclList option("align" "(" Expression ")") option("section" "(" Expression ")") option("!") TypeExpr
 */
 static AstNode *ast_parse_fn_proto(ParseContext *pc, size_t *token_index, bool mandatory, VisibMod visib_mod) {
     Token *first_token = &pc->tokens->at(*token_index);
@@ -2345,6 +2345,10 @@ static AstNode *ast_parse_fn_proto(ParseContext *pc, size_t *token_index, bool m
         *token_index += 1;
         fn_token = ast_eat_token(pc, token_index, TokenIdKeywordFn);
         cc = CallingConventionNaked;
+    } else if (first_token->id == TokenIdKeywordAsync) {
+        *token_index += 1;
+        fn_token = ast_eat_token(pc, token_index, TokenIdKeywordFn);
+        cc = CallingConventionAsync;
     } else if (first_token->id == TokenIdKeywordStdcallCC) {
         *token_index += 1;
         fn_token = ast_eat_token(pc, token_index, TokenIdKeywordFn);
diff --git a/std/mem.zig b/std/mem.zig
index 1dfef86a8f..2adb647ef6 100644
--- a/std/mem.zig
+++ b/std/mem.zig
@@ -124,7 +124,7 @@ pub const Allocator = struct {
             return self.allocator.allocFn(self.allocator, byte_count, alignment);
         }
 
-        fn free(self: &const AsyncAllocator, old_mem: []u8) {
+        fn free(self: &const AsyncAllocator, old_mem: []u8) void {
             return self.allocator.freeFn(self.allocator, old_mem);
         }
     };