[Bug tree-optimization/64811] New: fold_inf_compare bogus for NaNs (given -ftrapping-math)

[Bug tree-optimization/64811] New: fold_inf_compare bogus for NaNs (given -ftrapping-math)

[jsm28 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64811

            Bug ID: 64811
           Summary: fold_inf_compare bogus for NaNs (given
                    -ftrapping-math)
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jsm28 at gcc dot gnu.org

Various optimizations in fold_inf_compare can cause "invalid" floating-point
exceptions to be present when they should be absent, or vice versa.

Folding x > +Inf to 0 should not be about HONOR_SNANS - ordered comparisons of
both quiet and signaling NaNs should raise invalid.  So if -ftrapping-math and
HONOR_NANS, this is invalid (given a strict interpretation of -ftrapping-math,
anyway).

x <= +Inf is not the same as x == x, because again that loses an exception
(equality comparisons don't raise exceptions except for signaling NaNs).

x == +Inf is not the same as x > DBL_MAX, and a similar issue applies with the
x != +Inf case - that transformation causes a spurious exception (in
particular, I've seen this in __gcc_qadd for powerpc-linux-gnu soft-float,
causing glibc testsuite failures, which is how I found this bug), not just a
missing exception.  (The existing HONOR_NANS code for NE_EXPR is only about the
result, not the exceptions raised.)