Messages in this thread | | | Date | Wed, 21 May 2008 09:31:22 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] Fix immediate asm constraint for gcc 3 x86_64 |
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* H. Peter Anvin (hpa@zytor.com) 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 #APP .quad %rax
#NO_APP
Is there any way we could get a symbol assigned to &testa.c to make gcc 3 happy ?
Mathieu
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