phiopt, math-opts: Adjust spaceship_replacement and optimize_spaceship for recent libstdc++ changes [PR121698]

libstdc++ changed its ABI in <compare> for C++20 recently (under the
C++20 is still experimental rule).  In addition to the -1, 0, 1 values
for less, equal, greater it now uses -128 for unordered instead of
former 2 and changes some of the operators, instead of checks like
(_M_value & ~1) == _M_value in some cases it now uses _M_reverse()
which is negation in unsigned char type + conversion back to the original
type. _M_reverse() thus turns the -1, 0, 1, -128 values into
1, 0, -1, -128.  Note libc++ uses value -127 instead of 2/-128.

Now, the middle-end has some optimizations which rely on the particular
implementation and don't optimize if not.  One is optimize_spaceship
which on some targets (currently x86, aarch64 and s390) attempts to use
better comparison instructions (ideally just one floating point comparison
to get all 4 possible outcomes plus some flag tests or magic instead of
2 or 3 floating point comparisons).  This one can actually handle
arbitrary int non-[-1,1] values for unordered but still has a default
of 2.  The patch changes that default to -128 so that even if something
is expanded as branches if it is later during RTL optimizations determined
to convert that into partial_ordering we get better code.

The other optimization (phiopt one) is about optimizing (x <=> y) < 0
etc. into just x < y.  This one actually relies on the exact unordered
value (2) and has code to deal with that (_M_value & ~1) == _M_value
kind of tests and whatever match.pd lowers it.  So, this patch partially
rewrites it to look for -128 instead of 2, drop those
(_M_value & ~1) == _M_value pattern recognitions and instead introduces
pattern recognition of _M_reverse(), i.e. cast to unsigned char, negation
in that type and cast back to the original signed type.

With all these changes we get back the desired optimizations for all
the cases we could optimize previously (note, for HONOR_NANS case
we don't try to optimize say (x <=> y) == 0 because the original
will raise exception if either x or y is a NaN, while turning it into
x == y will not, but (x <=> y) <= 0 is fine (x <= y), because it
does raise those exceptions.

2025-08-30  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/121698
	* tree-ssa-phiopt.cc (spaceship_replacement): Adjust
	to handle spaceship unordered value -128 rather than 2 and
	stmts from the new std::partial_order::_M_reverse() instead
	of (_M_value & ~1) == _M_value etc.
	* doc/md.texi (spaceship@var{m}4): Use -128 instead of 2.
	* tree-ssa-math-opts.cc (optimize_spaceship): Adjust comments
	that libstdc++ unordered value is -128 rather than 2 and use
	that as the default unordered value.
	* config/i386/i386-expand.cc (ix86_expand_fp_spaceship): Use
	GEN_INT (-128) instead of const2_rtx and adjust comment accordingly.
	* config/aarch64/aarch64.cc (aarch64_expand_fp_spaceship): Likewise.
	* config/s390/s390.cc (s390_expand_fp_spaceship): Likewise.

	* gcc.dg/pr94589-2.c: Adjust for expected unordered value -128
	rather than 2 and negations in unsigned char instead of and with
	~1 and comparison against original value.
	* gcc.dg/pr94589-4.c: Likewise.
	* gcc.dg/pr94589-5.c: Likewise.
	* gcc.dg/pr94589-6.c: Likewise.

0 files changed, 0 insertions, 0 deletions