https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107715
Bug ID: 107715
Summary: TSVC s161 for double runs at zen4 30 times slower when
vectorization is enabled
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: hubicka at gcc dot gnu.org
Target Milestone: ---
jh@alberti:~/tsvc/bin> more test.c
typedef double real_t;
#define iterations 100000
#define LEN_1D 32000
#define LEN_2D 256
real_t a[LEN_1D],b[LEN_1D],c[LEN_1D],d[LEN_1D],e[LEN_1D];
int
main()
{
for (int nl = 0; nl < iterations/2; nl++) {
for (int i = 0; i < LEN_1D-1; ++i) {
if (b[i] < (real_t)0.) {
goto L20;
}
a[i] = c[i] + d[i] * e[i];
goto L10;
L20:
c[i+1] = a[i] + d[i] * d[i];
L10:
;
}
}
return 0;
}
jh@alberti:~/tsvc/bin> ~/trunk-install/bin/gcc -Ofast -march=native test.c
-fno-tree-vectorize
jh@alberti:~/tsvc/bin> time ./a.out
real 0m1.170s
user 0m1.170s
sys 0m0.000s
jh@alberti:~/tsvc/bin> ~/trunk-install/bin/gcc -Ofast -march=native test.c
jh@alberti:~/tsvc/bin> time ./a.out
real 0m37.269s
user 0m37.258s
sys 0m0.004s
It is not quite clear to me why this happens. It seems that all the time is
spent by movapd:
│ b0:┌─→vmovapd 0x6bc880(%rax),%zmm2
│ │ vmovapd 0x63f880(%rax),%zmm0
0.00 │ │ vcmpltpd %zmm1,%zmm2,%k1
│ │ vmovapd 0x6fb080(%rax),%zmm2
│ │ vfmadd132pd %zmm0,%zmm2,%zmm0
│ │ vmovapd 0x6bc880(%rax),%zmm2
│ │ vmovupd %zmm0,0x67e088(%rax){%k1}
99.94 │ │ vmovapd 0x63f880(%rax),%zmm0
│ │ add $0x40,%rax
│ │ vcmpgepd %zmm1,%zmm2,%k1
│ │ vmovapd 0x67e040(%rax),%zmm2
0.02 │ │ vfmadd132pd 0x601040(%rax),%zmm2,%zmm0
0.04 │ │ vmovapd 0x6fb040(%rax),%zmm2
0.00 │ │ vblendmpd %zmm0,%zmm2,%zmm0{%k1}
│ │ vmovapd %zmm0,0x6fb040(%rax)
│ │ cmp $0x3e7c0,%rax
│ └──jne b0
Since I do not initialize the array in reduced testcase we always execute the
jump to L20.
Exctending the testcase by array initialization:
typedef double real_t;
#define iterations 100000
#define LEN_1D 32000
#define LEN_2D 256
real_t a[LEN_1D],b[LEN_1D],c[LEN_1D],d[LEN_1D],e[LEN_1D];
enum {SET1D_RECIP_IDX = -1, SET1D_RECIP_IDX_SQ = -2};
void set_1d_array(real_t * arr, int length, real_t value, int stride)
{
if (stride == SET1D_RECIP_IDX) {
for (int i = 0; i < length; i++) {
arr[i] = 1. / (real_t) (i+1);
}
} else if (stride == SET1D_RECIP_IDX_SQ) {
for (int i = 0; i < length; i++) {
arr[i] = 1. / (real_t) ((i+1) * (i+1));
}
} else {
for (int i = 0; i < length; i += stride) {
arr[i] = value;
}
}
}
int
main()
{
set_1d_array(a, LEN_1D, 1.,1);
set_1d_array(b, LEN_1D, 1.,1);
set_1d_array(c, LEN_1D, 1.,1);
set_1d_array(d, LEN_1D, 1.,1);
set_1d_array(e, LEN_1D, 1.,1);
for (int nl = 0; nl < iterations/2; nl++) {
for (int i = 0; i < LEN_1D-1; ++i) {
if (b[i] < (real_t)0.) {
goto L20;
}
a[i] = c[i] + d[i] * e[i];
goto L10;
L20:
c[i+1] = a[i] + d[i] * d[i];
L10:
;
}
}
return 0;
}
jh@alberti:~/tsvc/bin> ~/trunk-install/bin/gcc -Ofast -march=native test.c
-fno-tree-vectorize
jh@alberti:~/tsvc/bin> time ./a.out
real 0m0.910s
user 0m0.910s
sys 0m0.000s
jh@alberti:~/tsvc/bin> ~/trunk-install/bin/gcc -Ofast -march=native test.c
jh@alberti:~/tsvc/bin> time ./a.out
real 0m1.866s
user 0m1.866s
sys 0m0.000s
jh@alberti:~/tsvc/bin>
still gets about 2x regression for vectorization.
