[Bug middle-end/106805] [13 Regression] Undue optimisation of floating-point comparisons

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106805

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:109148dd16e4bcd50faee19c49082de69d0ba26e

commit r13-4493-g109148dd16e4bcd50faee19c49082de69d0ba26e
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Mon Dec 5 11:54:45 2022 +0100

    match.pd: Don't fold nan < x etc. for -ftrapping-math [PR106805]

    As reported in the PR, the following pr106805.c testcase is miscompiled
    with the default -ftrapping-math, because we fold all the comparisons into
    constants and don't raise any exceptions.

    The match.pd pattern handles just simple comparisons, from those
    EQ/NE are quiet and don't raise exceptions on anything but sNaN, while
    GT/GE/LT/LE are signaling and do raise exceptions even on qNaN.

    fold_relational_const handles this IMHO correctly:
          /* Handle the cases where either operand is a NaN.  */
          if (real_isnan (c0) || real_isnan (c1))
            {
              switch (code)
                {
                case EQ_EXPR:
                case ORDERED_EXPR:
                  result = 0;
                  break;

                case NE_EXPR:
                case UNORDERED_EXPR:
                case UNLT_EXPR:
                case UNLE_EXPR:
                case UNGT_EXPR:
                case UNGE_EXPR:
                case UNEQ_EXPR:
                  result = 1;
                  break;

                case LT_EXPR:
                case LE_EXPR:
                case GT_EXPR:
                case GE_EXPR:
                case LTGT_EXPR:
                  if (flag_trapping_math)
                    return NULL_TREE;
                  result = 0;
                  break;

                default:
                  gcc_unreachable ();
                }

              return constant_boolean_node (result, type);
            }
    by folding the signaling comparisons only if -fno-trapping-math.
    The following patch does the same in match.pd.

    Unfortunately the pr106805.c testcase still fails, but no longer because of
    match.pd, but on the trunk because of the still unresolved ranger problems
    (same issue as for fold-overflow-1.c etc.) and on 12 branch (and presumably
    trunk too) somewhere during expansion the comparisons are also expanded
    into constants (which is ok for -fno-trapping-math, but not ok with that).

    Though, I think the patch is a small step in the direction, so I'd like
    to commit this patch without the gcc.dg/pr106805.c testcase for now.

    2022-12-05  Jakub Jelinek  <jakub@redhat.com>

            PR middle-end/106805
            * match.pd (cmp @0 REAL_CST@1): Don't optimize x cmp NaN
            or NaN cmp x to false/true for cmp >/>=/</<= if -ftrapping-math.

            * c-c++-common/pr57371-4.c: Revert 2021-09-19 changes.
            * c-c++-common/pr57371-5.c: New test.
            * gcc.c-torture/execute/ieee/fp-cmp-6.x: Add -fno-trapping-math.
            * gcc.c-torture/execute/ieee/fp-cmp-9.c: New test.
            * gcc.c-torture/execute/ieee/fp-cmp-9.x: New file.