https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71599
Bug ID: 71599
Summary: An extra unrelated move when compiling
compare_exchange_strong
Product: gcc
Version: 6.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jianjin2016 at fb dot com
Target Milestone: ---
At high optimizations, for the code below:
bool cas(std::atomic<uint64_t>* target, uint64_t expected, uint64_t desired) {
return target->compare_exchange_strong(expected, desired);
}
Gcc gives (link: https://godbolt.org/g/wP8Ah8 ):
movq %rsi, %rax
movq %rsi, -8(%rsp) -----------------------(Note this spurious move)
lock cmpxchgq %rdx, (%rdi)
sete %al
ret
However,
Clang gives (link: https://godbolt.org/g/UmUO7M ):
movq %rsi, %rax
lock cmpxchgq %rdx, (%rdi)
sete %al
retq
This extra and unrelated move will make a difference in our measurable
benchmark result.
