llvm dev - Jul 2010 - [LLVMdev] X86 gcc and clang have incompatible calling conventions for returning some small structs?

Hello,

I think I have come across an inconsistency between gcc and clang/llvm with
respect to returning small structs. Given the
following code:

> struct s {
>     int a;
>     int b;
> };
>
> struct s3 {
>     int a;
>     int b;
>     int c;
> };
>
> struct s new_s(int v){
>     struct s res;
>     res.a = v;
>     res.b = -v;
>     return res;
> }
>
> struct s3 new_s3(int v){
>     struct s3 res;
>     res.a = v;
>     res.b = -v;
>     res.c = v<<2;
>     return res;
> }
For the function new_s() both the current Ubuntu gcc and the current Ubuntu
clang generate code to return struct s in registers.
However for the function new_s3() the gcc compiler uses the stack, whereas clang
uses registers, so the generated code is not
compatible.

$ gcc --version
gcc (Ubuntu 4.4.3-4ubuntu5) 4.4.3
$ clang --version
clang version 1.1 (branches/release_27)
Target: x86_64-pc-linux-gnu
Thread model: posix


It is possible that this is specific to the Ubuntu clang or gcc, but it seems
unlikely that the Ubuntu people would tinker with this.


More generally, I'm looking for the exact specification of the calling
conventions that llvm supports, in particular for returning
small structs. I'm also looking for a way to let llvm always use the stack
for such small structures, since that's the calling
convention on the platform I'm porting to (Minix 3).

Thanks for any help with this.

-- 
Dr. Ir. Kees van Reeuwijk, Vrije Universiteit Amsterdam

On Thu, Jul 8, 2010 at 5:58 AM, Kees van Reeuwijk <reeuwijk at few.vu.nl>
wrote:
> For the function new_s() both the current Ubuntu gcc and the current Ubuntu
clang generate code to return struct s in registers.
> However for the function new_s3() the gcc compiler uses the stack, whereas
clang uses registers, so the generated code is not
> compatible.
clang code:
new_s3:
	movl	%edi, %edx
	movl	%edx, %eax
	negl	%eax
	shlq	$32, %rax
	addq	%rdx, %rax
	shll	$2, %edx
	ret

gcc code:
new_s3:
	movl	%edi, %eax
	leal	0(,%rdi,4), %edx
	movl	%edi, -24(%rsp)
	negl	%eax
	movl	%eax, -20(%rsp)
	movq	-24(%rsp), %rax
	ret

Both compilers are in fact returning the struct in RAX and RDX.  gcc
is just preferring to compute the value of RAX using the stack;
-24(%rsp) is scratch space.

The relevant specification is http://www.x86-64.org/documentation/abi.pdf .

-Eli