https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80738
David Stone <david at doublewise dot net> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |david at doublewise dot net
--- Comment #6 from David Stone <david at doublewise dot net> ---
All of the standard library types in C++ have a hand-written version of
std::swap to work around the problem. Fundamental types do not need anything
set to 0 in the middle so they don't show the problem. The simplest case to
show this would probably be a struct containing a unique_ptr (although it
applies to any resource-owning type that doesn't have a swap written
specifically for it):
```
#include <memory>
#include <utility>
struct wrapper {
std::unique_ptr<int> ptr;
};
void slow(wrapper & lhs, wrapper & rhs) {
std::swap(lhs, rhs);
}
void fast(wrapper & lhs, wrapper & rhs) {
std::swap(lhs.ptr, rhs.ptr);
}
```
At `-O3`, this generates the assembly
```
slow(wrapper&, wrapper&):
mov rax, QWORD PTR [rdi]
mov QWORD PTR [rdi], 0
mov rdx, QWORD PTR [rsi]
mov QWORD PTR [rsi], 0
mov QWORD PTR [rdi], rdx
mov rdi, QWORD PTR [rsi]
mov QWORD PTR [rsi], rax
test rdi, rdi
je .L1
mov esi, 4
jmp operator delete(void*, unsigned long)
.L1:
ret
fast(wrapper&, wrapper&):
mov rax, QWORD PTR [rdi]
mov rdx, QWORD PTR [rsi]
mov QWORD PTR [rdi], rdx
mov QWORD PTR [rsi], rax
ret
```
Where `fast` is better only because the C++ standard library maintainers
hand-wrote a faster version.
