https://bugs.llvm.org/show_bug.cgi?id=38405
Bug ID: 38405
Summary: Suboptimal machine code for atomic bool load + test
Product: new-bugs
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: gonzalob...@gmail.com
CC: chandl...@gmail.com, efrie...@codeaurora.org,
hfin...@anl.gov, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
The following C++ code compiled with clang -O3 -std=c++17
(https://godbolt.org/g/CAMK9k):
#include <atomic>
std::atomic<bool> flag_atomic{false};
bool flag_nonatomic{false};
extern void f1();
extern void f2();
void branchAtomic() {
if (flag_atomic.load(std::memory_order_relaxed)) {
f1();
} else {
f2();
}
}
void branchNonatomic() {
if (flag_nonatomic) {
f1();
} else {
f2();
}
}
produces different code for the atomic and non-atomic functions, but in this
particular case, it should probably be the same and emit a cmpb instruction on
both (right? I am not 100% sure):
branchAtomic(): # @branchAtomic()
movb flag_atomic(%rip), %al
testb $1, %al
jne .LBB0_1
jmp _Z2f2v # TAILCALL
.LBB0_1:
jmp _Z2f1v # TAILCALL
branchNonatomic(): # @branchNonatomic()
cmpb $0, flag_nonatomic(%rip)
je .LBB1_2
jmp _Z2f1v # TAILCALL
.LBB1_2:
jmp _Z2f2v # TAILCALL
flag_atomic:
.zero 1
flag_nonatomic:
.byte 0 # 0x0
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