https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81943
Bug ID: 81943
Summary: Wrong ABI in class methods returning structs for the
Windows amd64 target
Product: gcc
Version: 7.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: blaffablaffa at gmail dot com
Target Milestone: ---
test case:
struct retstruct{
int a;
};
struct badABI{
retstruct f(int arg){
retstruct ret;
ret.a = arg;
arg++;
return ret;
}
};
int main(){
badABI test;
auto testret = test.f(234);
};
g++ assembly output (x86_64-w64-mingw32-g++ -S -std=c++11 -o main.S main.cpp)
around the badABI::f call:
leaq -1(%rbp), %rax
movl $234, %edx #2nd arg, int
movq %rax, %rcx #1st arg, this pointer
call _ZN6badABI1fEi
movl %eax, -8(%rbp) #returned struct in eax
cl.exe (version 19, Visual Studio 2017) output:
mov r8d, 234 ; 000000eaH #3rd argument, int
lea rdx, QWORD PTR testret$[rsp] #2nd argument, return buffer
lea rcx, QWORD PTR test$[rsp] #1st argument, this pointer
call badABI::f
Note that this is not fixed by -fpcc-struct-return, because the this pointer
and the hidden return struct buffer pointer are inverted:
leaq -8(%rbp), %rax
leaq -1(%rbp), %rdx #2nd argument, this pointer
movl $234, %r8d #3rd argument, int
movq %rax, %rcx #1st argument, return buffer
call _ZN6badABI1fEi
As far as I know this last convention isn't used anywhere on windows, so it's
completely wrong. On the other hand, the first convention appears to follow the
specifications for small struct returns
(https://msdn.microsoft.com/en-us/library/7572ztz4(v=vs.120).aspx), but I can't
explain why the latest cl.exe does not appear to use it.
There are projects where this discrepancy has been taken into account, the one
I know of is https://github.com/SteamRE/open-steamworks : it is a reverse
engineering of the Steamworks SDK (started when it wasn't public), and they
have a number of instances (see for example
https://github.com/SteamRE/open-steamworks/blob/master/Open%20Steamworks/ISteamUser017.h#L44
macro STEAMWORKS_STRUCT_RETURN_0) where the return of structs is fixed by
actually returning void and passing a reference to the return buffer.
clang appears to emit the right assembly when using -cc1 -triplet
x86_64-pc-windows-msvc18.0.0 (default of clang-cl):
leaq 48(%rsp), %rcx #1st argument, this pointer
leaq 40(%rsp), %rdx #2nd argument, return buffer
movl $234, %r8d #3rd argument, int
callq "?f@badABI@@QEAA?AUretstruct@@H@Z"
excerpt of badABI::f:
movq %rdx, %rax #return buffer pointer in rax (correct as per
microsoft ABI)
movl %r8d, 12(%rsp)
movq %rcx, (%rsp)
movl 12(%rsp), %r8d
movl %r8d, (%rdx)
movl 12(%rsp), %r8d
addl $1, %r8d
movl %r8d, 12(%rsp)
