https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94038
--- Comment #7 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:f65a3299a521a44522c1e01724f75e36af22f40b commit r11-550-gf65a3299a521a44522c1e01724f75e36af22f40b Author: Patrick Palka <ppa...@redhat.com> Date: Thu May 21 09:03:43 2020 -0400 c++: template instantiation during fold_for_warn [PR94038] Unfortunately, the previous fix to PR94038 is fragile. When the argument to fold_for_warn is a bare CALL_EXPR, then all is well: the result of maybe_constant_value from fold_for_warn (with uid_sensitive=true) is reused via the cv_cache in the subsequent call to maybe_constant_value from cp_fold (with uid_sensitive=false), so we avoid instantiating bar<int>. But when the argument to fold_for_warn is more complex, e.g. an INDIRECT_REF of a CALL_EXPR, as in the testcase below (due to bar<int>() returning const int& which we need to decay to int) then from fold_for_warn we call maybe_constant_value on the INDIRECT_REF, and from cp_fold we call it on the CALL_EXPR, so there is no reuse via the cv_cache and we therefore end up instantiating bar<int>. So for a more robust solution to this general issue of warning flags affecting code generation, it seems that we need a way to globally avoid template instantiation during constexpr evaluation whenever we're performing warning-dependent folding. To that end, this patch replaces the flag constexpr_ctx::uid_sensitive with a global flag uid_sensitive_constexpr_evaluation_p, and enables it during fold_for_warn using an RAII helper. The patch also adds a counter that keeps track of the number of times uid_sensitive_constexpr_evaluation_p is called and returned true, and we use this to determine whether the result of constexpr evaluation was restricted by the flag. This lets us safely update the cv_cache and fold_cache from fold_for_warn in the most common case where the flag did not restrict constexpr evaluation. gcc/cp/ChangeLog: PR c++/94038 * constexpr.c (constexpr_ctx::uid_sensitive): Remove field. (uid_sensitive_constexpr_evaluation_value): Define. (uid_sensitive_constexpr_evaluation_true_counter): Define. (uid_sensitive_constexpr_evaluation_p): Define. (uid_sensitive_constexpr_evaluation_sentinel): Define its constructor. (uid_sensitive_constexpr_evaluation_checker): Define its constructor and its evaluation_restricted_p method. (get_fundef_copy): Remove 'ctx' parameter. Use u_s_c_e_p instead of constexpr_ctx::uid_sensitive. (cxx_eval_call_expression): Use u_s_c_e_p instead, and test it last. Adjust call to get_fundef_copy. (instantiate_cx_fn_r): Test u_s_c_e_p so that we increment the counter if necessary. (cxx_eval_outermost_constant_expr): Remove 'uid_sensitive' parameter. Adjust function body accordingly. (maybe_constant_value): Remove 'uid_sensitive' parameter and adjust function body accordingly. Set up a uid_sensitive_constexpr_evaluation_checker, and use it to conditionally update the cv_cache. * cp-gimplify.c (cp_fold): Set up a uid_sensitive_constexpr_evaluation_checker, and use it to conditionally update the fold_cache. * cp-tree.h (maybe_constant_value): Update declaration. (struct uid_sensitive_constexpr_evaluation_sentinel): Define. (struct sensitive_constexpr_evaluation_checker): Define. * expr.c (fold_for_warn): Set up a uid_sensitive_constexpr_evaluation_sentinel before calling the folding subroutines. Drop all but the first argument to maybe_constant_value. gcc/testsuite/ChangeLog: PR c++/94038 * g++.dg/warn/pr94038-2.C: New test.
