Re: [PATCH] Fix immediate asm constraint for gcc 3 x86_64

From: Mathieu Desnoyers
Date: Wed May 21 2008 - 09:31:50 EST

* H. Peter Anvin (hpa@xxxxxxxxx) wrote:
> Mathieu Desnoyers wrote:
>> It seems that include/asm-x86/immediate.h in sched-devel.git causes
>> this. gcc-3.4 does not seem to like the "i" (&name##__imv) constraint. I
>> have seen no such problem with gcc-4.1. This is weird. It seems that
>> relaxing the constraint helps fixing this, but it's not clear whether
>> fixing the code or gcc-3.4 is the correct solution... here is the fix :
>> Fix immediate asm constraint for gcc 3 x86_64
> It might make it compile, but it's completely *wrong* for the purpose
> intended.
> It permits gcc to present the address in a register, for example, so there
> is no guarantee that you end up with an immediate.
> -hpa

Hrm, you are right. Let's see a simple toy case which generates the
problem :

struct test_struct {
int a;
int b;
int c;

static struct test_struct testa;

int main()
asm (
".quad %c0\n\t"
: "i" (&testa.c));
return 0;

gcc 4.1 generates : .quad testa+8

and doesn't complain. However, gcc-3.4 stops with :
compudj@amd64:~/test$ gcc-3.4 -S -o oldgcc.S oldgcc.c
oldgcc.c: In function `main':
oldgcc.c:11: warning: asm operand 0 probably doesn't match constraints
oldgcc.c:11: error: impossible constraint in `asm'

It's understandable given that changing the "i" for a "g" constraint
changes the assembly result for :

movl $testa, %eax
addq $8, %rax
.quad %rax


Is there any way we could get a symbol assigned to &testa.c to make gcc
3 happy ?


Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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