https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117717
Bug ID: 117717
Summary: Auto-vectorization resulting in poor performance
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: iansseijelly at berkeley dot edu
Target Milestone: ---
See: https://github.com/iansseijelly/what-is-wrong/tree/main for scripts and
code to reproduce the issue.
When activating autovectorization (-O3 for gcc11 and -O2 & -O3 for gcc12+), it
would produce the following optimization message:
```
sort.c:31:26: optimized: basic block part vectorized using 8 byte vectors
```
Leading to the creation of the following assembly:
```
void bubble_sort (uint32_t *a, uint32_t n) {
11d0: 48 89 f8 mov %rdi,%rax
s = 0;
11d3: 45 31 c0 xor %r8d,%r8d
11d6: 66 66 2e 0f 1f 84 00 data16 cs nopw 0x0(%rax,%rax,1)
11dd: 00 00 00 00
11e1: 66 66 2e 0f 1f 84 00 data16 cs nopw 0x0(%rax,%rax,1)
11e8: 00 00 00 00
11ec: 66 66 2e 0f 1f 84 00 data16 cs nopw 0x0(%rax,%rax,1)
11f3: 00 00 00 00
11f7: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
11fe: 00 00
if (a[i] < a[i - 1]) {
1200: f3 0f 7e 00 movq (%rax),%xmm0
1204: 66 0f 70 c8 e5 pshufd $0xe5,%xmm0,%xmm1
1209: 66 0f 7e c2 movd %xmm0,%edx
120d: 66 0f 7e c9 movd %xmm1,%ecx
1211: 39 d1 cmp %edx,%ecx
1213: 73 0f jae 1224 <bubble_sort+0x64>
a[i - 1] = t;
1215: 66 0f 70 c0 e1 pshufd $0xe1,%xmm0,%xmm0
s = 1;
121a: 41 b8 01 00 00 00 mov $0x1,%r8d
a[i - 1] = t;
1220: 66 0f d6 00 movq %xmm0,(%rax)
for (i = 1; i < n; i++) {
1224: 48 83 c0 04 add $0x4,%rax
1228: 48 39 f0 cmp %rsi,%rax
122b: 75 d3 jne 1200 <bubble_sort+0x40>
while (s) {
122d: 45 85 c0 test %r8d,%r8d
1230: 75 9e jne 11d0 <bubble_sort+0x10>
}
1232: c3 ret
1233: 66 66 2e 0f 1f 84 00 data16 cs nopw 0x0(%rax,%rax,1)
123a: 00 00 00 00
123e: 66 90 xchg %ax,%ax
```
For the bubble sort code:
```
void bubble_sort (uint32_t *a, uint32_t n) {
uint32_t i, t, s = 1;
while (s) {
s = 0;
for (i = 1; i < n; i++) {
if (a[i] < a[i - 1]) {
t = a[i];
a[i] = a[i - 1];
a[i - 1] = t;
s = 1;
}
}
}
}
```
This inefficient use of fixed-width simd registers made the code ~2x worse than
O0 and ~5.5x worse than O3 with vectorization explicitly disabled.
However, this also means the part for populating arrays are not vectorized,
leading to performance drops.
This phenomenum of pathologically using avx for sorting is observable on other
x86_64 machines like AMD and Xeon, and is not observable on clang+x86,
clang+arm, gcc+arm, or gcc+riscv.
Additionally, when use fdo or autofdo in gcc11, it seems that gcc also tries to
vectorize the sorting basic block, despite under O2 (where %xmm should not be
used at all).
