https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116416
--- Comment #9 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:12de1942a0a673f9f2f1c2bfce4279a666061ffc commit r15-6052-g12de1942a0a673f9f2f1c2bfce4279a666061ffc Author: Marek Polacek <pola...@redhat.com> Date: Thu Aug 29 12:58:41 2024 -0400 c++: compile time evaluation of prvalues [PR116416] This PR reports a missed optimization. When we have: Str str{"Test"}; callback(str); as in the test, we're able to evaluate the Str::Str() call at compile time. But when we have: callback(Str{"Test"}); we are not. With this patch (in fact, it's Patrick's patch with a little tweak), we turn callback (TARGET_EXPR <D.2890, <<< Unknown tree: aggr_init_expr 5 __ct_comp D.2890 (struct Str *) <<< Unknown tree: void_cst >>> (const char *) "Test" >>>>) into callback (TARGET_EXPR <D.2890, {.str=(const char *) "Test", .length=4}>) I explored the idea of calling maybe_constant_value for the whole TARGET_EXPR in cp_fold. That has three problems: - we can't always elide a TARGET_EXPR, so we'd have to make sure the result is also a TARGET_EXPR; - the resulting TARGET_EXPR must have the same flags, otherwise Bad Things happen; - getting a new slot is also problematic. I've seen a test where we had "TARGET_EXPR<D.2680, ...>, D.2680", and folding the whole TARGET_EXPR would get us "TARGET_EXPR<D.2681, ...>", but since we don't see the outer D.2680, we can't replace it with D.2681, and things break. With this patch, two tree-ssa tests regressed: pr78687.C and pr90883.C. FAIL: g++.dg/tree-ssa/pr90883.C scan-tree-dump dse1 "Deleted redundant store: .*.a = {}" is easy. Previously, we would call C::C, so .gimple has: D.2590 = {}; C::C (&D.2590); D.2597 = D.2590; return D.2597; Then .einline inlines the C::C call: D.2590 = {}; D.2590.a = {}; // #1 D.2590.b = 0; // #2 D.2597 = D.2590; D.2590 ={v} {CLOBBER(eos)}; return D.2597; then #2 is removed in .fre1, and #1 is removed in .dse1. So the test passes. But with the patch, .gimple won't have that C::C call, so the IL is of course going to look different. The .optimized dump looks the same though so there's no problem. pr78687.C is XFAILed because the test passes with r15-5746 but not with r15-5747 as well. I opened <https://gcc.gnu.org/PR117971>. PR c++/116416 gcc/cp/ChangeLog: * cp-gimplify.cc (cp_fold_r) <case TARGET_EXPR>: Try to fold TARGET_EXPR_INITIAL and replace it with the folded result if it's TREE_CONSTANT. gcc/testsuite/ChangeLog: * g++.dg/analyzer/pr97116.C: Adjust dg-message. * g++.dg/tree-ssa/pr78687.C: Add XFAIL. * g++.dg/tree-ssa/pr90883.C: Adjust dg-final. * g++.dg/cpp0x/constexpr-prvalue1.C: New test. * g++.dg/cpp1y/constexpr-prvalue1.C: New test. Co-authored-by: Patrick Palka <ppa...@redhat.com> Reviewed-by: Jason Merrill <ja...@redhat.com>
