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From: "lh_mouse at 126 dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug pending/103875] New: Dead writes are not optimized out Date: Fri, 31 Dec 2021 13:08:59 +0000 [thread overview] Message-ID: <bug-103875-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103875 Bug ID: 103875 Summary: Dead writes are not optimized out Product: gcc Version: 11.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: pending Assignee: unassigned at gcc dot gnu.org Reporter: lh_mouse at 126 dot com Target Milestone: --- I was reading <https://quuxplusone.github.io/blog/2018/04/17/downsides-of-omitting-trivial-destructor-calls/>, which I think is a great article. But the issue that was mentioned in that article seems to be specific to `std::vector`. Specifically, for this hand-written 'static vector', neither GCC or Clang is able to optimize out any dead writes (godbolt [1]): ``` struct foo { using value_type = int; unsigned long size; value_type data[10]; value_type& back() { return this->data[this->size-1]; } void pop() { this->size --; this->data[this->size].~value_type(); // the last element is destroyed here } }; void use_foo(foo& f) { f.back() *= 42; // this write should be dead f.pop(); } ``` ``` use_foo(foo&): mov rdx, QWORD PTR [rdi] lea rax, [rdx-1] imul edx, DWORD PTR [rdi+4+rdx*4], 42 mov DWORD PTR [rdi+8+rax*4], edx # dead write is here mov QWORD PTR [rdi], rax ret ``` Making `value_type` non-trivial as suggested does not help (godbolt [2]): ``` struct foo { struct value_type { int val = 0; int& operator*=(int x) { return this->val *= x; } ~value_type() { } // non-trivial }; unsigned long size; value_type data[10]; value_type& back() { return this->data[this->size-1]; } void pop() { this->size --; this->data[this->size].~value_type(); } }; void use_foo(foo& f) { f.back() *= 42; f.pop(); } ``` ``` use_foo(foo&): mov rdx, QWORD PTR [rdi] lea rax, [rdx-1] imul edx, DWORD PTR [rdi+4+rdx*4], 42 mov DWORD PTR [rdi+8+rax*4], edx # dead write is still here mov QWORD PTR [rdi], rax ret ``` [1] https://gcc.godbolt.org/z/f9xx3c6Y7 [2] https://gcc.godbolt.org/z/Ysss5GMjj
next reply other threads:[~2021-12-31 13:08 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-12-31 13:08 lh_mouse at 126 dot com [this message] 2021-12-31 14:14 ` [Bug ipa/103875] " pinskia at gcc dot gnu.org 2022-01-04 14:09 ` rguenth at gcc dot gnu.org
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