https://bugs.llvm.org/show_bug.cgi?id=43915
Bug ID: 43915
Summary: Missed optimizations for std::optional<int>::emplace()
Product: clang
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: LLVM Codegen
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected]
https://godbolt.org/z/6-0fT3
#include <optional>
std::optional<int> x;
void test() {
x.emplace(7);
}
clang -O3 codegen:
test(): # @test()
cmp byte ptr [rip + x+4], 0
je .LBB0_2
mov byte ptr [rip + x+4], 0
.LBB0_2:
mov dword ptr [rip + x], 7
mov byte ptr [rip + x+4], 1
ret
Note the double store to [rip + x+4]. If the initial store was eliminated, that
should also kill the unnecessary comparison+branch because the end result is
the same regardless of the initial state of the object.
gcc -O3 codegen:
test():
mov DWORD PTR x[rip], 7
mov BYTE PTR x[rip+4], 1
ret
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