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From: "lennox.ho at intel dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/110381] New: Incorrect loop unrolling for structs of floating point types Date: Fri, 23 Jun 2023 18:06:33 +0000 [thread overview] Message-ID: <bug-110381-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110381 Bug ID: 110381 Summary: Incorrect loop unrolling for structs of floating point types Product: gcc Version: 12.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: lennox.ho at intel dot com Target Milestone: --- We believe gcc is incorrectly unrolling loops while performing summation of structs with floating point members: Here's a minimal example: ``` #include <iostream> using value_type = double; struct FOO { value_type a = 0; value_type b = 0; value_type c = 0; }; value_type sum_8_foos(const FOO* foos) { value_type sum = 0; for (int i = 0; i < 8; ++i) { auto foo = foos[i]; sum += foo.c; sum += foo.b; sum += foo.a; } return sum; } int main() { FOO foos[8]; foos[0].b = 5; std::cout << sum_8_foos(foos) << '\n'; return 0; } ``` With -O1, we get 5. With -O2, we get 10. godbolt link: https://godbolt.org/z/7cxeb3Gsv Slightly reorganising the assembly output for the loop, ``` .L2 add rdi, 48 addsd sum, QWORD PTR [rdi-48] // c addsd sum, QWORD PTR [rdi-40] // b addsd sum, QWORD PTR [rdi-32] // a addsd sum, QWORD PTR [rdi-24] // c addsd sum, QWORD PTR [rdi-16] // b addsd sum, QWORD PTR [rdi-8] // a add rax, 24 addsd sum, QWORD PTR [rax-16] // b addsd sum, QWORD PTR [rax-24] // c cmp rdi, end jne .L2 ``` There appears to be duplicate additions for the members b and c. This behaviour appears on gcc 12.1 and is still present in gcc 13.1.
next reply other threads:[~2023-06-23 18:06 UTC|newest] Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-06-23 18:06 lennox.ho at intel dot com [this message] 2023-06-23 18:08 ` [Bug c++/110381] " lennox.ho at intel dot com 2023-06-23 19:48 ` [Bug tree-optimization/110381] [11/12/13/14 Regression] double counting for sum of " pinskia at gcc dot gnu.org 2023-06-23 20:01 ` lennox.ho at intel dot com 2023-06-23 20:07 ` pinskia at gcc dot gnu.org 2023-06-23 22:34 ` mpolacek at gcc dot gnu.org 2023-06-26 8:58 ` rguenth at gcc dot gnu.org 2023-06-26 9:33 ` rguenth at gcc dot gnu.org 2023-06-26 9:41 ` rguenth at gcc dot gnu.org 2023-06-26 10:50 ` rguenth at gcc dot gnu.org 2023-06-26 12:18 ` cvs-commit at gcc dot gnu.org 2023-06-26 12:18 ` [Bug tree-optimization/110381] [11/12/13 " rguenth at gcc dot gnu.org 2023-06-29 16:01 ` clyon at gcc dot gnu.org 2023-06-30 6:35 ` cvs-commit at gcc dot gnu.org 2023-06-30 6:40 ` rguenth at gcc dot gnu.org 2023-06-30 14:05 ` clyon at gcc dot gnu.org 2023-06-30 20:50 ` pinskia at gcc dot gnu.org 2023-07-03 7:54 ` clyon at gcc dot gnu.org 2023-07-03 7:57 ` pinskia at gcc dot gnu.org 2023-07-03 8:03 ` cvs-commit at gcc dot gnu.org 2023-07-03 8:05 ` clyon at gcc dot gnu.org 2023-07-07 12:06 ` [Bug tree-optimization/110381] [11/12 " cvs-commit at gcc dot gnu.org
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