[Bug tree-optimization/108068] [10/11/12/13 Regression] decimal floating point signed zero is not honored

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108068

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jsm28 at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
           Priority|P3                          |P2

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Optimizing statement if (_5 != 0)

Visiting conditional with predicate: if (_5 != 0)

With known ranges
        _5: [unsupported_range] UNDEFINED

Predicate evaluates to: DON'T KNOW
LKUP STMT _5 ne_expr 0
0>>> COPY _5 = 0
<<<< COPY _5 = 0

the issue is we do

              bool can_infer_simple_equiv
                = !(HONOR_SIGNED_ZEROS (op1)
                    && (TREE_CODE (op1) == SSA_NAME || real_zerop (op1)));

but real_zerop is false for Decimal zero.  That's because "Trailing zeroes
matter for decimal float constants, so don't return 1 for them.".  We'd
need a real_maybe_zerop () for this usage.  We have other !real_zerop
checks in match.pd and elsewhere, those are susceptible as well.

Joseph, do you think adding DECIMAL_FLOAT_MODE_P checks in users is what
we want to do or do you think a real_nonzerop would be more appropriate
here?  I guess DOM want's to ask whether op1 may compare equal to zero.