[Bug target/56944] New: Better isfinite in some cases?

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56944



             Bug #: 56944

           Summary: Better isfinite in some cases?

    Classification: Unclassified

           Product: gcc

           Version: 4.9.0

            Status: UNCONFIRMED

          Keywords: missed-optimization

          Severity: enhancement

          Priority: P3

         Component: target

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: gli...@gcc.gnu.org

            Target: x86_64-linux-gnu





Hello,



for isfinite, gcc typically generates this sequence:



    movsd    .LC0(%rip), %xmm1

    andpd    %xmm1, %xmm0

    movsd    .LC1(%rip), %xmm1

    ucomisd    %xmm0, %xmm1

    setae    %al



With -fno-trapping-math, I tried this shorter sequence instead, which should be

valid:



    subsd    %xmm0, %xmm0

    ucomisd    %xmm0, %xmm0

    setnp    %al



Depending on the tests, it seemed to be either the same speed or 15% faster,

whether the argument is normal, infinite or nan. For a denormal argument, it is

15% slower (but then both codes take 100 times as long as the normal case). The

results might also be different on a more recent processor.



I don't know if we want to try and generate this code when -fno-trapping-math

is present.



(related to PR 30652)