[LLVMdev] X86 gcc and clang have incompatible calling conventions for returning some small structs?

Eli Friedman eli.friedman at gmail.com
Thu Jul 8 10:32:58 PDT 2010

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:
	movl	%edi, %edx
	movl	%edx, %eax
	negl	%eax
	shlq	$32, %rax
	addq	%rdx, %rax
	shll	$2, %edx

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

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 .


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