https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115639
--- Comment #8 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:5869a881442aa4214d5deed7cfe0d352bcca1fd4 commit r15-9414-g5869a881442aa4214d5deed7cfe0d352bcca1fd4 Author: Patrick Palka <ppa...@redhat.com> Date: Sun Apr 13 11:04:46 2025 -0400 c++: improve constexpr call caching [PR115639] For the testcase from this PR, checking static_assert(0 == big_calc()); takes twice as much time as constexpr int ret = big_calc(); static_assert(0 == ret); ultimately because in the former, we first constant evaluate big_calc() with mce_unknown (as part of warning-dependent folding from cp_build_binary_op). We then constant evaluate it a second time, with mce_true, during finish_static_assert. The result of the first evaluation isn't reused because of the different mce_value, which in general can give a different result. But big_calc() here doesn't depend on mce_value at all (i.e. there's no if consteval or __builtin_is_constant_evaluated calls, nested or otherwise) so we should be able to reuse the result in such cases. Specifically if a constexpr call with mce_unknown succeeds, we can safely reuse the result during a subsequent mce_true or mce_false evaluation. This patch implements this by also caching a successful mce_unknown call result into the corresponding mce_true and mce_false slots, so that such a subsequent evaluation effectively reuses the mce_unknown result. To make it more convenient to access the cache slot for the same call with different mce_value, this patch gives each constexpr_call entry three result slots, one per mce_value, instead of having a distinct constexpr_call entry for each mce_value. And we can no longer use NULL_TREE to denote the call is in progress; instead use unknown_type_node. After this patch compile time for the above two fragments is the same. PR c++/115639 gcc/cp/ChangeLog: * constexpr.cc (struct constexpr_call): Add NSDMIs to each field. Replace 'result' data member with 3-element 'results' array and a 'result' accessor function. Remove 'manifestly_const_eval' data member. (constexpr_call_hasher::equal): Adjust after constexpr_call layout change. (cxx_eval_call_expression): Likewise. Define some local variables closer to their first use. Use unknown_type_node instead of NULL_TREE as the "in progress" result. After successully evaluating a call with mce_unknown, also cache the result in the corresponding mce_true and mce_false slots. Reviewed-by: Jason Merrill <ja...@redhat.com>
