https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87062
Bug ID: 87062
Summary: mis-optimized code with -O3 and std::pair
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: tromey at gcc dot gnu.org
Target Milestone: ---
I'm filing this on behalf of someone who posted this bug on reddit.
https://www.reddit.com/r/cpp/comments/99e1ri/interesting_gcc_optimizer_bug/
Copying text from there:
Looks like there is an interesting gcc optimizer bug in gcc 7+.
#include <utility>
std::pair<long, long> fret(long i) { return {i, i}; }
With -O2 gcc generates the expected:
mov rdx, rdi
mov rax, rdi
But with -O3 it generates:
mov QWORD PTR [rsp-24], rdi
movq xmm0, QWORD PTR [rsp-24]
punpcklqdq xmm0, xmm0
movaps XMMWORD PTR [rsp-24], xmm0
mov rax, QWORD PTR [rsp-24]
mov rdx, QWORD PTR [rsp-16]
https://godbolt.org/z/lXoaA4
