[SR-13129] [IRGen, Windows] Wrong argument order when calling C++ member functions with indirect return types

[swift-ci]

Previous ID	SR-13129
Radar	rdar://problem/83423212
Original Reporter	MForster (JIRA User)
Type	Bug

Attachment: Download

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, CxxInterop, IRGen, Windows
Assignee	None
Priority	Medium

md5: c3f23723106b510fdc433108dfead71b

Issue Description:

On Windows IRGen passes arguments to non-static C++ member functions in the wrong order if the result value is passed indirectly.

test.h

struct Result {};

struct MyStruct {
  Result foo() { return Result{}; }
};

test.swift

import Test
 
public func bar(arg: inout MyStruct) -> Result { arg.foo() }

The C++ function expects the this pointer first and the result pointer second. Swift's C++ Interop, however, passes the result first and the this pointer second (after bitcasting them).

Look at the three lines starting from %3.

test.ll

define dllexport swiftcc void @"$s4test3bar3argSo6ResultVSo8MyStructVz_tF"(%TSo8MyStructV* nocapture dereferenceable(1) %0) #​0 {
entry:
 %call.aggresult = alloca %TSo6ResultV, align 1
 %1 = bitcast %TSo8MyStructV* %0 to %struct.MyStruct*
 %2 = bitcast %TSo6ResultV* %call.aggresult to i8*
 call void @llvm.lifetime.start.p0i8(i64 1, i8* %2)
 %3 = bitcast %TSo6ResultV* %call.aggresult to %struct.MyStruct*
 %4 = bitcast %struct.MyStruct* %1 to %struct.Result*
 call void @"?foo@MyStruct@@QEAA?AUResult@@XZ"(%struct.MyStruct* noalias nocapture sret %3, %struct.Result* %4)
 %5 = bitcast %TSo6ResultV* %call.aggresult to i8*
 call void @llvm.lifetime.end.p0i8(i64 1, i8* %5)
 ret void
}
; Function Attrs: noinline nounwind optnone uwtable
define linkonce_odr dso_local void @"?foo@MyStruct@@QEAA?AUResult@@XZ"(%struct.MyStruct* %this, %struct.Result* noalias sret align 1 %agg.result) #​1 comdat align 2 {

[swift-ci]

Comment by Michael Forster (JIRA)

I suspect that we need to add a new function calling convention for C++ member functions.

[zoecarver]

@swift-ci create