https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102780
--- Comment #4 from CVS 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:9927ecbb42d5be48fa933adc26f8601fab5007ca commit r12-4769-g9927ecbb42d5be48fa933adc26f8601fab5007ca Author: Patrick Palka <ppa...@redhat.com> Date: Thu Oct 28 10:05:14 2021 -0400 c++: quadratic constexpr behavior for left-assoc logical exprs [PR102780] In the testcase below the two left fold expressions each expand into a constant logical expression with 1024 terms, for which potential_const_expr takes more than a minute to return true. This happens because p_c_e_1 performs trial evaluation of the first operand of a &&/|| in order to determine whether to consider the potentiality of the second operand. And because the expanded expression is left-associated, this trial evaluation causes p_c_e_1 to be quadratic in the number of terms of the expression. This patch fixes this quadratic behavior by making p_c_e_1 preemptively compute potentiality of the second operand of a &&/||, and perform trial evaluation of the first operand only if the second operand isn't potentially constant. We must be careful to avoid emitting bogus diagnostics during the preemptive computation; to that end, we perform this shortcut only when tf_error is cleared, and when tf_error is set we now first check potentiality of the whole expression quietly and replay the check noisily for diagnostics. Apart from fixing the quadraticness for left-associated logical exprs, this change also reduces compile time for the libstdc++ testcase 20_util/variant/87619.cc by about 15% even though our <variant> uses right folds instead of left folds. Likewise for the testcase in the PR, for which compile time is reduced by 30%. The reason for these speedups is that p_c_e_1 no longer performs expensive trial evaluation of each term of large constant logical expressions when determining their potentiality. PR c++/102780 gcc/cp/ChangeLog: * constexpr.c (potential_constant_expression_1) <case TRUTH_*_EXPR>: When tf_error isn't set, preemptively check potentiality of the second operand before performing trial evaluation of the first operand. (potential_constant_expression_1): When tf_error is set, first check potentiality quietly and return true if successful, otherwise proceed noisily to give errors. gcc/testsuite/ChangeLog: * g++.dg/cpp1z/fold13.C: New test.
