public inbox for gcc-cvs@sourceware.org help / color / mirror / Atom feed
From: Jason Merrill <jason@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc r13-3162] gimplify: prevent some C++ temporary elision Date: Fri, 7 Oct 2022 13:22:37 +0000 (GMT) [thread overview] Message-ID: <20221007132237.78C513858D3C@sourceware.org> (raw) https://gcc.gnu.org/g:d3e5465757c599ea64f611290b7793d3141a6b7c commit r13-3162-gd3e5465757c599ea64f611290b7793d3141a6b7c Author: Jason Merrill <jason@redhat.com> Date: Wed Oct 5 11:50:59 2022 -0400 gimplify: prevent some C++ temporary elision In this testcase, we were optimizing away the temporary for f(), but C++17 and above are clear that there is a temporary, and because its destructor has visible side-effects we can't optimize it away under the as-if rule. So disable this optimization for TREE_ADDRESSABLE type. I moved the declaration of volatile_p after the call to gimple_fold_indirect_ref_rhs to minimize indentation changes; I don't see any way the value of that flag could be affected by the call. gcc/ChangeLog: * gimplify.cc (gimplify_modify_expr_rhs): Don't optimize x = *(A*)&<expr> to x = <expr> for a TREE_ADDRESSABLE type. gcc/testsuite/ChangeLog: * g++.dg/init/elide9.C: New test. Diff: --- gcc/gimplify.cc | 15 ++++++++------- gcc/testsuite/g++.dg/init/elide9.C | 25 +++++++++++++++++++++++++ 2 files changed, 33 insertions(+), 7 deletions(-) diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc index 95e16f600b5..d4209ea74a3 100644 --- a/gcc/gimplify.cc +++ b/gcc/gimplify.cc @@ -5620,7 +5620,7 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p, } break; case INDIRECT_REF: - { + if (!TREE_ADDRESSABLE (TREE_TYPE (*from_p))) /* If we have code like *(const A*)(A*)&x @@ -5629,11 +5629,13 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p, of "A"), treat the entire expression as identical to "x". This kind of code arises in C++ when an object is bound to a const reference, and if "x" is a TARGET_EXPR we want - to take advantage of the optimization below. */ - bool volatile_p = TREE_THIS_VOLATILE (*from_p); - tree t = gimple_fold_indirect_ref_rhs (TREE_OPERAND (*from_p, 0)); - if (t) + to take advantage of the optimization below. But not if + the type is TREE_ADDRESSABLE; then C++17 says that the + TARGET_EXPR needs to be a temporary. */ + if (tree t + = gimple_fold_indirect_ref_rhs (TREE_OPERAND (*from_p, 0))) { + bool volatile_p = TREE_THIS_VOLATILE (*from_p); if (TREE_THIS_VOLATILE (t) != volatile_p) { if (DECL_P (t)) @@ -5646,8 +5648,7 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p, ret = GS_OK; changed = true; } - break; - } + break; case TARGET_EXPR: { diff --git a/gcc/testsuite/g++.dg/init/elide9.C b/gcc/testsuite/g++.dg/init/elide9.C new file mode 100644 index 00000000000..810d60a94a4 --- /dev/null +++ b/gcc/testsuite/g++.dg/init/elide9.C @@ -0,0 +1,25 @@ +// The static_cast should prevent temporary elision. +// { dg-do run { target c++11 } } + +int d; +struct A +{ + int i; + A() { } + ~A() { ++d; } +}; + +A f() { return A(); } + +struct B +{ + A a; + B(): a(static_cast<A&&>(f())) {} +}; + +int main() +{ + { B b; } + if (d != 2) + return -1; +}
reply other threads:[~2022-10-07 13:22 UTC|newest] Thread overview: [no followups] expand[flat|nested] mbox.gz Atom feed
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20221007132237.78C513858D3C@sourceware.org \ --to=jason@gcc.gnu.org \ --cc=gcc-cvs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).