llvm dev - Jan 2012 - [LLVMdev] Odd weak symbol thing on i386

Hi,

I'm compiling lldiv.c from the NetBSD standard library. It works on ARM,
Mips,
Microblaze,ppc, ppc64, and x86_64. On i386 a very strange thing happens.
Here's the source:

#include <stdlib.h>
#define       __weak_alias(sym) __attribute__ ((weak, alias (#sym)))
    
lldiv_t lldiv(long long int num, long long int denom) __weak_alias(_lldiv);

lldiv_t _lldiv(long long num, long long denom)
{
        lldiv_t r;
        r.quot = num / denom;
        r.rem = num % denom;
        if (num >= 0 && r.rem < 0) {
                r.quot++;
                r.rem -= denom;
        }
        return (r);
}

I get the following code emitted for the return if the alias line is present:

LBB0_3:                                # %if.end
	movl	64(%esp), %eax
	movsd	24(%esp), %xmm0
	movsd	32(%esp), %xmm1
	movsd	%xmm1, 8(%eax)
	movsd	%xmm0, (%eax)
	addl	$56, %esp
	popl	%esi
	ret
.Ltmp0:
	.size	_lldiv, .Ltmp0-_lldiv


	.weak	lldiv
lldiv = _lldiv

And this if it isn't:
LBB0_3:                                # %if.end
	movl	64(%esp), %eax
	movsd	24(%esp), %xmm0
	movsd	32(%esp), %xmm1
	movsd	%xmm1, 8(%eax)
	movsd	%xmm0, (%eax)
	addl	$56, %esp
	popl	%esi
	ret	$4
.Ltmp0:
	.size	_lldiv, .Ltmp0-_lldiv

Other than the $4 on the return address, the code is otherwise identical. The 
alias version crashes, of course. Is there supposed to be an implicit pop of 
the return structure pointer? What could cause an alias to mess this up?

-Rich

On Fri, Jan 13, 2012 at 2:53 PM, Richard Pennington <rich at pennware.com>
wrote:
> Hi,
>
> I'm compiling lldiv.c from the NetBSD standard library. It works on
ARM, Mips,
> Microblaze,ppc, ppc64, and x86_64. On i386 a very strange thing happens.
> Here's the source:
>
> #include <stdlib.h>
> #define       __weak_alias(sym) __attribute__ ((weak, alias (#sym)))
>
> lldiv_t lldiv(long long int num, long long int denom) __weak_alias(_lldiv);
>
> lldiv_t _lldiv(long long num, long long denom)
> {
>        lldiv_t r;
>        r.quot = num / denom;
>        r.rem = num % denom;
>        if (num >= 0 && r.rem < 0) {
>                r.quot++;
>                r.rem -= denom;
>        }
>        return (r);
> }
>
> I get the following code emitted for the return if the alias line is
present:
>
> LBB0_3:                                # %if.end
>        movl    64(%esp), %eax
>        movsd   24(%esp), %xmm0
>        movsd   32(%esp), %xmm1
>        movsd   %xmm1, 8(%eax)
>        movsd   %xmm0, (%eax)
>        addl    $56, %esp
>        popl    %esi
>        ret
> .Ltmp0:
>        .size   _lldiv, .Ltmp0-_lldiv
>
>
>        .weak   lldiv
> lldiv = _lldiv
>
> And this if it isn't:
> LBB0_3:                                # %if.end
>        movl    64(%esp), %eax
>        movsd   24(%esp), %xmm0
>        movsd   32(%esp), %xmm1
>        movsd   %xmm1, 8(%eax)
>        movsd   %xmm0, (%eax)
>        addl    $56, %esp
>        popl    %esi
>        ret     $4
> .Ltmp0:
>        .size   _lldiv, .Ltmp0-_lldiv
>
> Other than the $4 on the return address, the code is otherwise identical.
The
> alias version crashes, of course. Is there supposed to be an implicit pop
of
> the return structure pointer? What could cause an alias to mess this up?
Bad version:
define void @_lldiv(%struct.lldiv_t* noalias %agg.result, i64 %num,
i64 %denom) nounwind {

Good version:
define void @_lldiv(%struct.lldiv_t* noalias sret %agg.result, i64
%num, i64 %denom) nounwind {

Looks like a bug in clang's IR generation; please file a bug.

-Eli

> Looks like a bug in clang's IR generation; please file a bug.
I reported pr11770. Thanks Richard!
> -Eli
> 
Cheers,
Rafael