https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113718
Bug ID: 113718
Summary: std::bit_cast making the compiler generate unnecessary
code.
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: cassio.neri at gmail dot com
Target Milestone: ---
Consider:
#include <bit>
void f();
auto const p1 = &f;
auto const p2 = std::bit_cast<void(*)()>(&f);
bool a() {
return p1 == p2;
}
The code emitted for `a` should be the same as-if `return true;` but the usage
of a "no-op" `std::bit_cast` muddies the waters and the compiler generates:
a():
cmp QWORD PTR p2[rip], OFFSET FLAT:_Z1fv
sete al
ret
FWIW: The following changes make the compiler to generate more efficient code:
1. Move `p1` and `p2` inside the body of `a`.
2. Replace `std::bit_cast` with `static_cast`.
3. Remove the cast altogether.
Things get terribly worse if `p1` and `p2` are made `static` and moved inside
the body of `a`.
Given that the compiler can get confused by a "no-op" `std::bit_cast`, I wonder
if it would do the same for more interesting code than this toy example.
https://godbolt.org/z/daWe5Yod8
