[Bug inline-asm/48347] New: The wrong assembler code is emitted for my architecture

[Bug inline-asm/48347] New: The wrong assembler code is emitted for my architecture

[normvcr at telus dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48347

           Summary: The wrong assembler code is emitted for my
                    architecture
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: inline-asm
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: normvcr@telus.net


Created attachment 23808
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23808
Complete output of compilation

When using the flags "-g -O2", the right assembler code is emitted.  When
dropping the -O2, and using only "-g", the wrong assembler code is emitted.

[Bug inline-asm/48347] The wrong assembler code is emitted for my architecture

[normvcr at telus dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48347

--- Comment #1 from Norman Goldstein <normvcr at telus dot net> 2011-03-30 00:15:12 UTC ---
Created attachment 23809
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23809
preporcessor output

[Bug inline-asm/48347] The wrong assembler code is emitted for my architecture

[normvcr at telus dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48347

--- Comment #2 from Norman Goldstein <normvcr at telus dot net> 2011-03-30 00:16:54 UTC ---
I installed and built the source RPM mediatomb-0.12.1-5.fc14.src.rpm, using the
default compile flags, -g -O2, and this is fine, except that debugging was
difficult, so I rebuilt using only -g. This is when the assembler complained:

Quote:
../src/zmm/atomic.h: Assembler messages:
../src/zmm/atomic.h:81: Error: bad register name `%sil'
Line 81 in atomic.h is the inline asm statement:

Code:

    static inline bool atomic_dec(mt_atomic_t *at)
    {
        unsigned char c;
        __asm__ __volatile__(
            ASM_LOCK "decl %0; sete %1"
            :"=m" (at->x), "=g" (c)
            :"m" (at->x)
            :"cc"
        );
        return (c!=0);
    }

I edited the compile command, replacing, -c with -S, and .o with .asm, to take
a look at the generated assembler code. Here is the relevant snippet:

Code:

# 81 "../src/zmm/atomic.h" 1
    lock; decl (%eax); sete %sil

Sure enough, the gcc inline module inserted a reference to the "%sil" register,
which is not part of the x86 32-bit architecture. The only difference on the
command line was the absence of the "-O2" flag.

I did some more experimenting, and found that "-O1" produces proper code. Then
I did a compile replacing "-O1" with all the implied optimization flags, but
the .asm file had the improper reference to "%sil" !

The gcc documentation does explain that "-On" is not exactly equivalent to
using the implied flags, and I guess that my experience, here, corroborates
this.

[Bug inline-asm/48347] The wrong assembler code is emitted for my architecture

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48347

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-30 09:19:42 UTC ---
I'm sure that sete doesn't work with any general operand.  Use proper
constraints.

[Bug inline-asm/48347] The wrong assembler code is emitted for my architecture

[normvcr at telus dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48347

--- Comment #4 from Norman Goldstein <normvcr at telus dot net> 2011-03-30 19:35:48 UTC ---
(In reply to comment #3)
> I'm sure that sete doesn't work with any general operand.  Use proper
> constraints.

You are right.  sete must operate on an 8-bit register or an 8-bit memory
location.  GCC knows that the reference is to an 8-bit unsigned char, and GCC
does the right thing under the "-g -O2" flags, so why not under the "-g" flag? 
I was not able to find a way to change the "=g" constraint to force GCC to use
an 8-bit register/memory location.

[Bug inline-asm/48347] The wrong assembler code is emitted for my architecture

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48347

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-03-30 19:41:02 UTC ---
Use "=q" isntead of "=g".

*** This bug has been marked as a duplicate of bug 10153 ***