https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86504
Bug ID: 86504
Summary: vectorization failure for a nest loop
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jiangning.liu at amperecomputing dot com
Target Milestone: ---
Created attachment 44386
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44386&action=edit
bad vectorizatoin result for boundary size 16
For the case below, the code generated by “gcc -O3” is very ugly, and the inner
loop can be correctly vectorized. Please refer to attached file
test_loop_inner_16.s.
char g_d[1024], g_s1[1024], g_s2[1024];
void test_loop(void)
{
char *d = g_d, *s1 = g_s1, *s2 = g_s2;
for ( int y = 0; y < 128; y++ )
{
for ( int x = 0; x < 16; x++ )
d[x] = s1[x] + s2[x];
d += 16;
}
}
If we change inner loop “for ( int x = 0; x < 16; x++ )” to be like “for ( int
x = 0; x < 32; x++ )”, i.e. the loop boundary size changes from 16 to 32, very
beautiful vectorization code would be generated. For example, the code below is
the aarch64 result for loop boundary size 32, and it the same case for x86.
test_loop:
.LFB0:
.cfi_startproc
adrp x2, g_s1
adrp x3, g_s2
add x2, x2, :lo12:g_s1
add x3, x3, :lo12:g_s2
adrp x0, g_d
adrp x1, g_d+2048
add x0, x0, :lo12:g_d
add x1, x1, :lo12:g_d+2048
ldp q1, q2, [x2]
ldp q3, q0, [x3]
add v1.16b, v1.16b, v3.16b
add v0.16b, v0.16b, v2.16b
.p2align 3,,7
.L2:
str q1, [x0]
str q0, [x0, 16]!
cmp x0, x1
bne .L2
ret
The code generated for loop boundary size 8 is also very bad.
Any idea?
