https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121698
--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <[email protected]>: https://gcc.gnu.org/g:4ee9e8319abfeec1a9ba452661cb3d55aef684fa commit r16-3474-g4ee9e8319abfeec1a9ba452661cb3d55aef684fa Author: Jakub Jelinek <[email protected]> Date: Sat Aug 30 18:29:04 2025 +0200 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 <[email protected]> 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.
