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Subject: [Bug c/55299] New: missed optimization: ASR idiom
Date: Mon, 12 Nov 2012 23:22:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55299

             Bug #: 55299
           Summary: missed optimization: ASR idiom
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: olly@survex.com


This is similar to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54579 but for a
different (and perhaps clearer) idiom for arithmetic shift right.  I've filed
this as a separate PR rather than adding it as a comment, but if it's actually
the same issue underneath, please merge or mark as a duplicate.

int asr(int a, int b)
{
  return a < 0 ? ~((~a) >> b) : a >> b;
}

olly@gcc12:~$ /opt/cfarm/gcc-latest/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/opt/cfarm/gcc-latest/bin/gcc
COLLECT_LTO_WRAPPER=/home/iulius/autobuild/bin/gcc-4.7.1/libexec/gcc/x86_64-unknown-linux-gnu/4.7.1/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.7.1-src/configure
--prefix=/home/iulius/autobuild/bin/gcc-4.7.1
--with-gmp=/home/iulius/autobuild/bin/gmp-5.0.5
--with-mpfr=/home/iulius/autobuild/bin/mpfr-3.1.0
--with-mpc=/home/iulius/autobuild/bin/mpc-0.9 --disable-nls
--enable-threads=posix --disable-multilib --enable-languages=c,c++
Thread model: posix
gcc version 4.7.1 (GCC) 

Compiling with:

LD_LIBRARY_PATH=/opt/cfarm/mpc-latest/lib:/opt/cfarm/mpfr-latest/lib:/opt/cfarm/gmp-latest/lib
/opt/cfarm/gcc-latest/bin/gcc -O2 -S asr.c -o asr.S

Gives this for the function asr:

asr:
.LFB0:
    movl    %edi, %eax
    movl    %esi, %ecx
    sarl    %cl, %eax
    testl    %edi, %edi
    js    .L5
    rep
    ret
    .p2align 4,,10
    .p2align 3
.L5:
    movl    %edi, %eax
    notl    %eax
    sarl    %cl, %eax
    notl    %eax
    ret

Ideally the conditional would be optimised down to just the sarl instruction.

I've checked the older compiler versions on gcc12 (back to 4.1.1) and they all
leave the branch in.  So if this is a regression, it isn't at all recent.