[Bug inline-asm/31386] New: wrong registers used in cmov instruction

[Bug inline-asm/31386] New: wrong registers used in cmov instruction

[ramiro at lisha dot ufsc dot br]

This is reproduceable with gcc 4.0.0 and gcc SVN, but not with gcc 3.4.6.

I still don't really know what's the problem, but I think it may be that not
enough registers are available for output, and gcc is using the same register
for 2 different outputs.

gccs were built with:
--prefix=/usr --enable-languages=c --program-suffix=-{svn,4.0.0}

file was compiled with:
gcc-{svn,4.0.0} -v -save-temps -O3 -c regs.c

The corresponding inline asm that triggers the error is the following, when
used many times in a function inside many ifs:
asm volatile (
    "cmpl %0, %3\n\t"
    "cmovl %3, %0\n\t"
    "cmovl %4, %1\n\t"
    "cmovl %5, %2\n\t"
    : "+r" (dmin), "+r" (dx), "+r" (dy)
    : "r" (d), "r" (val1), "r" (val2)
    );

The problem can be seen at the end of the assembly file:
# 47 "regs.c" 1
    cmpl %edx, %esi
    cmovl %esi, %edx
    cmovl %edi, %eax
    cmovl %ecx, %eax

# 0 "" 2
#NO_APP
    movl    %eax, %ebx
    movl    %eax, %edi
    movl    %eax, -36(%ebp)
    movl    %eax, -24(%ebp)
    movl    %edx, dmin

eax is used twice, both for dx and dy.

Test code minimized from motion_est.c from FFmpeg. Could be minimized further.


-- 
           Summary: wrong registers used in cmov instruction
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: inline-asm
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ramiro at lisha dot ufsc dot br
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31386

[Bug inline-asm/31386] wrong registers used in cmov instruction

[ramiro at lisha dot ufsc dot br]

------- Comment #1 from ramiro at lisha dot ufsc dot br  2007-03-28 22:39 -------
Created an attachment (id=13296)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=13296&action=view)
.c, .i and .s files


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31386

[Bug inline-asm/31386] wrong registers used in cmov instruction

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2007-03-28 22:49 -------
Actually I think the inline-asm should be:
#define CMOV_COMBO( val1, val2 ) \
                    asm volatile ( \
                            "cmpl %0, %3\n\t" \
                            "cmovl %3, %0\n\t" \
                            "cmovl %4, %1\n\t" \
                            "cmovl %5, %2\n\t" \
    : "+&r" (dmin), "+&r" (dx), "+&r" (dy) \
    : "r" (d), "r" (val1), "r" (val2) \
                                 )


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31386

[Bug inline-asm/31386] wrong registers used in cmov instruction

[ramiro at lisha dot ufsc dot br]

------- Comment #3 from ramiro at lisha dot ufsc dot br  2007-04-02 21:24 -------
Hello,

That and many other combinations fix the issue. Including changing the code
around the inline asm, or using different optimizations.

What seems wrong is that gcc uses the same register for 2 outputs. Maybe on
that specific register pressure it allocates the same register for 2 outputs.

Is gcc ever supposed to allow 2 output operands to use the same register?

Ramiro Polla


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31386