This patch addresses PR middle-end/111701 where optimization of signbit(x*x)
using tree_nonnegative_p incorrectly eliminates a floating point
multiplication when the operands may potentially be signaling NaNs.

The above bug fix also provides a solution or work-around to the tricky
issue in PR middle-end/111701, that the results of IEEE operations on NaNs
are specified to return a NaN result, but fail to (precisely) specify
the exact NaN representation of this result.  Hence for the operation
"-NaN*-NaN" different hardware implementations (targets) return different
results.  Ultimately knowing what the resulting NaN "payload" of an
operation is can only be known by executing that operation at run-time,
and I'd suggest that GCC's -fsignaling-nans provides a mechanism for
handling code that uses NaN representations for communication/signaling
(which is a different but related concept to IEEE's sNaN).

One nice thing about this patch, which may or may not be a P2 regression
fix, is that it only affects (improves) code compiled with -fsignaling-nans
so should be extremely safe even for this point in stage 3.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures.  Ok for mainline?


2024-04-26  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR middle-end/111701
        * fold-const.cc (tree_binary_nonnegative_warnv_p) <case MULT_EXPR>:
        Split handling of floating point and integer types.  For equal
        floating point operands, avoid optimization if the operand may be
        a signaling NaN.

gcc/testsuite/ChangeLog
        PR middle-end/111701
        * gcc.dg/pr111701-1.c: New test case.
        * gcc.dg/pr111701-2.c: Likewise.


Thanks in advance,
Roger
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