[Bug middle-end/65474] New: sub-optimal code for __builtin_abs

[Bug middle-end/65474] New: sub-optimal code for __builtin_abs

[wmi at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65474

            Bug ID: 65474
           Summary: sub-optimal code for __builtin_abs
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wmi at google dot com

int foo(int x) {
  return __builtin_abs(x);
}

~/workarea/gcc-r221398/build/install/bin/gcc -O2 -S 1.c -o 1.gcc.s
        .cfi_startproc
        movl    %edi, %edx
        movl    %edi, %eax
        sarl    $31, %edx
        xorl    %edx, %eax
        subl    %edx, %eax
        ret
        .cfi_endproc

~/workarea/llvm-r224097/build/bin/clang -O2 -S 1.c -o 1.llvm.s
        .cfi_startproc
        movl    %edi, %eax
        negl    %eax
        cmovll  %edi, %eax
        retq
        .cfi_endproc

[Bug target/65474] sub-optimal code for __builtin_abs

[wmi at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65474

--- Comment #2 from wmi at google dot com ---
Created attachment 35069
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35069&action=edit
microbench

[Bug target/65474] sub-optimal code for __builtin_abs

[wmi at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65474

--- Comment #3 from wmi at google dot com ---
Thanks. You are right. I wrote a microbenchmark (attached), and tested it on
different intel microarchitectures.

westmere:
1.gcc.out:    19.42
1.llvm.out:   19.32

sandybridge:
1.gcc.out:    18.61
1.llvm.out:   19.16

ivybridge:
1.gcc.out:    15.79
1.llvm.out:   15.87

On sandybridge, llvm's version was slower. On other microarchitectures, they
were close to each other. So gcc's choose makes sense.

[Bug target/65474] sub-optimal code for __builtin_abs

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65474

Richard Biener <rguenth at gcc dot gnu.org> changed:

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

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Thus wontfix.