linux-kernel - Re: [PATCH] Fix immediate asm constraint for gcc 3 x86

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Date:	Wed, 21 May 2008 09:31:22 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Fix immediate asm constraint for gcc 3 x86_64

* H. Peter Anvin (hpa@...or.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@...64:~/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

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