https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109362
--- Comment #1 from Barry Revzin <barry.revzin at gmail dot com> ---
Sorry, in this reduced example, it doesn't actually consume an extra register -
only rdi is used.
In this slightly less reduced example:
#include <atomic>
struct S {
std::atomic<long> size;
std::atomic<char*> read_ptr;
auto peek() const -> const char* {
auto const s = size.load(std::memory_order_acquire);
return read_ptr.load(std::memory_order_acquire);
}
};
auto with_atomic(S const& v) {
while (true) {
if (v.peek()) {
return true;
}
}
}
the codegen is:
with_atomic(WithAtomic const&):
lea rdx, [rdi+8]
.L2:
mov rax, QWORD PTR [rdi]
mov rax, QWORD PTR [rdx]
test rax, rax
je .L2
mov eax, 1
ret
which now does consume rdx unnecessarily.
