[LLVMdev] MinGW/MSVC++ uses different ABI for sret

Óscar Fuentes ofv at wanadoo.es
Fri Sep 25 14:41:27 PDT 2009

Let's go directly to the example

struct S {
  double dummy1;
  double dummy2;

S bar();

S foo() {
  return bar();

This is the result of g++ -c -S -O2  (focus on the final `ret'):

	pushl	%ebp
	movl	%esp, %ebp
	pushl	%ebx
	subl	$20, %esp
	movl	8(%ebp), %ebx
	movl	%ebx, (%esp)
	call	__Z3barv
	pushl	%eax
	movl	%ebx, %eax
	movl	-4(%ebp), %ebx
	ret	$4

This is the result of cl -O2 -c -Fa (again, focus on the final `ret')

PUBLIC	?foo@@YA?AUS@@XZ				; foo
EXTRN	?bar@@YA?AUS@@XZ:PROC				; bar
; Function compile flags: /Ogtpy
$T2548 = -16						; size = 16
$T2546 = 8						; size = 4
?foo@@YA?AUS@@XZ PROC					; foo, COMDAT
; File c:\dev\exp\bar.cpp
; Line 8
	sub	esp, 16					; 00000010H
; Line 9
	lea	eax, DWORD PTR $T2548[esp+16]
	push	eax
	call	?bar@@YA?AUS@@XZ			; bar
	mov	ecx, DWORD PTR $T2546[esp+16]
	mov	edx, DWORD PTR [eax]
	mov	DWORD PTR [ecx], edx
	mov	edx, DWORD PTR [eax+4]
	mov	DWORD PTR [ecx+4], edx
	mov	edx, DWORD PTR [eax+8]
	mov	eax, DWORD PTR [eax+12]
	mov	DWORD PTR [ecx+8], edx
	mov	DWORD PTR [ecx+12], eax
	mov	eax, ecx
; Line 10
	add	esp, 20					; 00000014H
	ret	0
?foo@@YA?AUS@@XZ ENDP					; foo

Please note how g++ pops 4 bytes from the stack on return, while cl
doesn't. This is reflected on the call to `bar' too, where the callee
takes that into account.

LLVM generates code that follows the gcc behaviour. The result is that
after LLVM code calls a VC++ function that returns a struct, the stack
is corrupted. The "solution" is to not mark external VC++ functions as
sret in any case, but this breaks if the external function was compiled
by gcc, or if you pass a LLVM callback that returns a struct to a VC++
function, etc.

I filed a bug yesterday ( http://llvm.org/bugs/show_bug.cgi?id=5046 )
and Anton kindly explained that LLVM is doing the right thing as per the
ABI (the GCC ABI, I'll add).

 1. Is there a LLVM way of dealing with this without using separate code
 for VC++ and GCC?

 2. Is there a document that thoroughly explains the ABI used by VC++?
 The documentation on MSDN is quite vague
 ( http://msdn.microsoft.com/en-us/library/984x0h58.aspx )

 3. Is a bug that LLVM does not distinguish among GCC and VC++ sret

 4. Why the heck GCC and VC++ follow different ABIs on the same

The last question is rhetoric.


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