[Bug rtl-optimization/55950] New: Invalid sqrt constant propagation with -frounding-mode

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55950



             Bug #: 55950

           Summary: Invalid sqrt constant propagation with -frounding-mode

    Classification: Unclassified

           Product: gcc

           Version: 4.8.0

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: rtl-optimization

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

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





simplify_const_unary_operation (in simplify-rtx.c) does constant propagation

for SQRT protected only by:



       if (HONOR_SNANS (mode) && real_isnan (&d))

         return 0;



With -frounding-math, it should check whether sqrt was exact and only propagate

in that case (sqrt(1)=1, sqrt(.25)=.5, but sqrt(2) should wait for execution).



(note that this doesn't happen on a trivial sqrt(2.) example, but I've already

seen it happen)