https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96862
--- Comment #12 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-9 branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:25542b27aa6a75ad0f7316c3767c957cce912fb7 commit r9-8912-g25542b27aa6a75ad0f7316c3767c957cce912fb7 Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Sep 3 20:11:43 2020 +0200 c++: Disable -frounding-math during manifestly constant evaluation [PR96862] As discussed in the PR, fold-const.c punts on floating point constant evaluation if the result is inexact and -frounding-math is turned on. /* Don't constant fold this floating point operation if the result may dependent upon the run-time rounding mode and flag_rounding_math is set, or if GCC's software emulation is unable to accurately represent the result. */ if ((flag_rounding_math || (MODE_COMPOSITE_P (mode) && !flag_unsafe_math_optimizations)) && (inexact || !real_identical (&result, &value))) return NULL_TREE; Jonathan said that we should be evaluating them anyway, e.g. conceptually as if they are done with the default rounding mode before user had a chance to change that, and e.g. in C in initializers it is also ignored. In fact, fold-const.c for C initializers turns off various other options: /* Perform constant folding and related simplification of initializer expression EXPR. These behave identically to "fold_buildN" but ignore potential run-time traps and exceptions that fold must preserve. */ int saved_signaling_nans = flag_signaling_nans;\ int saved_trapping_math = flag_trapping_math;\ int saved_rounding_math = flag_rounding_math;\ int saved_trapv = flag_trapv;\ int saved_folding_initializer = folding_initializer;\ flag_signaling_nans = 0;\ flag_trapping_math = 0;\ flag_rounding_math = 0;\ flag_trapv = 0;\ folding_initializer = 1; flag_signaling_nans = saved_signaling_nans;\ flag_trapping_math = saved_trapping_math;\ flag_rounding_math = saved_rounding_math;\ flag_trapv = saved_trapv;\ folding_initializer = saved_folding_initializer; So, shall cxx_eval_outermost_constant_expr instead turn off all those options (then warning_sentinel wouldn't be the right thing to use, but given the 8 or how many return stmts in cxx_eval_outermost_constant_expr, we'd need a RAII class for this. Not sure about the folding_initializer, that one is affecting complex multiplication and division constant evaluation somehow. 2020-09-03 Jakub Jelinek <ja...@redhat.com> PR c++/96862 * constexpr.c (cxx_eval_outermost_constant_expr): Temporarily disable flag_rounding_math during manifestly constant evaluation. * g++.dg/cpp1z/constexpr-96862.C: New test. (cherry picked from commit 6641d6d3fe79113f8d9f3ced355aea79bffda822)
