https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110916
Bug ID: 110916
Summary: [12/13/14 Regression] Architecture-dependent missed
optimizations for double swapping
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: janschultke at googlemail dot com
Target Milestone: ---
GCC's ability to eliminate redundant stores and loads is oddly dependent on the
architecture. Even on the same overall arch, compiling for Skylake in
particular performs always performs best.
On x86_64 -march=x86-64-v2, GCC 11 provides the optimal output. GCC 12/13/14
provide suboptimal output compared to -march=skylake.
On ARM64, a strange load and store from/to the same register is emitted. This
is the case for all version of GCC available on Compiler Explorer.
## Code to Reproduce (https://godbolt.org/z/d7Kcdn8fo)
static void swap(int* restrict a, int* restrict b) {
const int tmp = *a;
*a = *b;
*b = tmp;
}
void double_swap_alias(int* a, int* b) {
swap(a, b);
swap(a, b);
}
## Expected Output (x86_64 GCC 14 -O3 -march=skylake)
ret
## Actual Output (x86_64 GCC 14 -O3 -march=x86-64-v2)
mov edx, DWORD PTR [rsi]
mov eax, DWORD PTR [rdi]
mov DWORD PTR [rdi], edx
mov DWORD PTR [rsi], eax
mov edx, DWORD PTR [rdi]
mov DWORD PTR [rdi], eax
mov DWORD PTR [rsi], edx
ret
## Actual Output (ARM64 GCC 14 -O3)
ldr w0, [x1]
str w0, [x1]
ret
