[Bug c/55299] New: missed optimization: ASR idiom

[olly at survex dot com]

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: unassig...@gcc.gnu.org

        ReportedBy: o...@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.