which registers must be preserved for the caller

[Andrew Makhorin]

On Mon, 2017-08-21 at 15:39 +0200, Mason wrote:
> On 19/08/2017 09:10, Andrew Makhorin wrote:
> 
> > The document "System V ABI: Intel386 Architecture Processor Supplement"
> > says that contents of %ebx, %esi, and %edi must be preserved for the
> > caller. Does GCC for 32-bit GNU/Linux and for 32-bit Cygwin conform to
> > this ABI? If not, how to determine which registers are expected to be
> > saved by calling C functions?

Thank you for your answer.

> 
> (NB: GCC supports dozens of platforms)
> 
> You seem to be interested in the x86 "cdecl" ABI.
> 
> https://en.wikipedia.org/wiki/X86_calling_conventions#cdecl

The C Standard says nothing about calling conventions, so I meant that
one that is used by default (which is probably cdecl).

> 
> I think the SysV and Microsoft ABI are close, but may differ
> over details, e.g. returning structs.

I asked my question, because GCC doesn't seem to follow System V ABI.
In the following example %ebx, %esi, and %edi are not preserved being
clobbered by the generated code.

mao@corvax:~/Desktop/foo$ cat foo.c
int foo(int x[], int y[], int z[], int i, int j, int k)
{
      int t;
      t = (x[i+j] + y[j+k] + 2) * (z[i+j+k] + 1) * (z[k+1] + 2);
      return t;
}
mao@corvax:~/Desktop/foo$ gcc -c -O2 -save-temps foo.c
mao@corvax:~/Desktop/foo$ cat foo.s
	.file	"foo.c"
	.text
	.p2align 4,,15
	.globl	foo
	.type	foo, @function
foo:
.LFB0:
	.cfi_startproc
	subl	$12, %esp
	.cfi_def_cfa_offset 16
	movl	32(%esp), %eax
	movl	36(%esp), %edx
	movl	%ebx, (%esp)
	.cfi_offset 3, -16
	movl	28(%esp), %ebx
	movl	%esi, 4(%esp)
	movl	24(%esp), %ecx
	movl	%edi, 8(%esp)
	.cfi_offset 6, -12
	.cfi_offset 7, -8
	movl	20(%esp), %edi
	leal	(%eax,%edx), %esi
	addl	%eax, %ebx
	movl	16(%esp), %eax
	movl	(%eax,%ebx,4), %eax
	addl	%edx, %ebx
	movl	(%ecx,%ebx,4), %ebx
	addl	(%edi,%esi,4), %eax
	movl	4(%ecx,%edx,4), %edx
	movl	4(%esp), %esi
	addl	$1, %ebx
	addl	$2, %eax
	movl	8(%esp), %edi
	imull	%ebx, %eax
	movl	(%esp), %ebx
	addl	$12, %esp
	.cfi_restore 7
	.cfi_restore 6
	.cfi_restore 3
	.cfi_def_cfa_offset 4
	addl	$2, %edx
	imull	%edx, %eax
	ret
	.cfi_endproc
.LFE0:
	.size	foo, .-foo
	.ident	"GCC: (Debian 4.7.2-5) 4.7.2"
	.section	.note.GNU-stack,"",@progbits
mao@corvax:~/Desktop/foo$ gcc --version
gcc (Debian 4.7.2-5) 4.7.2