https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118340
--- Comment #7 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:e71c0157478e49188cd754693dcc2059d63573e9 commit r16-1187-ge71c0157478e49188cd754693dcc2059d63573e9 Author: Patrick Palka <ppa...@redhat.com> Date: Thu Jun 5 11:06:04 2025 -0400 c++: quadratic constexpr folding of arith expr [PR118340] Here the PR's testcase demonstrates that the cp_fully_fold calls in cp_build_binary_op (for diagnosing arithmetic overflow) lead to quadratic behavior when building up a large arithmetic constant expression. The problem is ultimately that maybe_constant_value's caching doesn't reuse intermediate values, unlike cp_fold. (And unfortunately we can't leverage the cp_fold cache in this call site because here we want to evaluate constexpr calls even in -O0, which cp_fold avoids.) This patch fixes this by making maybe_constant_value look up each operand of the given expression to see if we've previously reduced it, and if so, rebuild the expression using the (presumably) reduced operands and evaluate that. After this patch each version of the testcase from the PR compiles in ~0.1s on my machine. PR c++/118340 gcc/cp/ChangeLog: * constexpr.cc (maybe_constant_value): First try looking up each operand in the cv_cache and reusing the result. Reviewed-by: Jason Merrill <ja...@redhat.com>
