[Bug c/20328] New: assembly constraints fail unless optimizing code

[Bug c/20328] New: assembly constraints fail unless optimizing code

[mrd at alkemio dot org]

The following code (taken from L4Ka::Pistachio) compiles correctly on gcc
(3.3.4, 3.3.5, and 3.4.4) when given any optimization level, but fails when you
don't use any:

$ cat foo.c
unsigned char
inb (unsigned long port)
{
  unsigned char tmp;

  if (port < 0x100)
    __asm__ __volatile__ ("inb %w1, %0":"=al" (tmp):"dN" (port));
  else
    __asm__ __volatile__ ("inb %%dx, %0": "=al" (tmp):"d" (port));

  return tmp;
}
$ gcc -c foo.c
foo.c: In function `inb':
foo.c:7: error: impossible constraint in `asm'
foo.c:9: error: impossible constraint in `asm'
$ gcc -c foo.c -O
$

-- 
           Summary: assembly constraints fail unless optimizing code
           Product: gcc
           Version: 3.4.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mrd at alkemio dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20328

[Bug c/20328] assembly constraints fail unless optimizing code

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2005-03-05 01:26 -------
Which target triplet?

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20328

[Bug c/20328] assembly constraints fail unless optimizing code

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-05 01:31 -------
You don't want the "al" contstraint at all.

Read the docs to figure out which constaint you really want.  the constraint "al" means pick either the 
"a" constraint or the "l" constraint

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20328

[Bug c/20328] assembly constraints fail unless optimizing code

[mrd at alkemio dot org]

------- Additional Comments From mrd at alkemio dot org  2005-03-05 06:44 -------
Subject: Re:  assembly constraints fail unless optimizing code

On Sat, 2005-03-05 at 01:26 +0000, giovannibajo at libero dot it wrote:
> Which target triplet?

i386-pc-linux-gnu



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20328

[Bug c/20328] assembly constraints fail unless optimizing code

[mrd at alkemio dot org]

------- Additional Comments From mrd at alkemio dot org  2005-03-05 06:52 -------
Subject: Re:  assembly constraints fail unless optimizing code

On Sat, 2005-03-05 at 01:31 +0000, pinskia at gcc dot gnu dot org wrote:
> You don't want the "al" contstraint at all.
> 
> Read the docs to figure out which constaint you really want.  the constraint "al" means pick either the 
> "a" constraint or the "l" constraint

The original code's not mine at all, so I don't know what constraint is
actually wanted.  However, after looking through section 5.36 of the gcc
3.4.3 manual, I can't figure out what the "l" constraint means for the
i386 architecture anyways.

So maybe the original code *shouldn't* be using the "=al" constraint,
but either the manual's missing a description of the "l" constraint for
i386 so I can't decide whether it's correct or not, or gcc's not
complaining about an illegal constraint when optimizations are enabled
(or maybe that's an optimization feature?).



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20328