https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71088
Bug ID: 71088
Summary: [i386, AVX-512, Perf] vpermi2ps instead of vpermps
emitted
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: iverbin at gcc dot gnu.org
CC: ienkovich at gcc dot gnu.org, izamyatin at gmail dot com,
kyukhin at gcc dot gnu.org, ubizjak at gmail dot com
Target Milestone: ---
Testcase:
float foo (float *arr1, float *arr2, float *max_x, int M, float s)
{
float *res = new float[M];
for (int i = M - 1; i >= 0; i--)
for (int j = 0; j <= i; j++)
{
float x = arr1[j] * arr2[i - j] + s;
res[j] = x > max_x[j] ? x : max_x[j];
}
return res[0];
}
To reproduce:
$ g++ -S test.cpp -Ofast -funroll-loops -march=knl
GCC emits vpermi2ps instruction to rearrange elements of arr2 backwards,
however
this instruction writes the result into the index register, therefore there are
additional movs before each vpermi2ps to restore indexes [1].
Also there are some weird movs after each vpermi2ps [2]. It's not clear why
the
result from vpermi2ps isn't passed directly to vfmadd132ps.
.L1:
vmovups (%r11), %zmm9
vmovdqa64 %zmm2, %zmm1 # [1]
vpermi2ps %zmm9, %zmm9, %zmm1
vmovdqa64 %zmm2, %zmm16 # [1]
vmovaps %zmm1, %zmm10 # [2]
vmovdqa64 %zmm2, %zmm1 # [1]
vmovups -64(%r11), %zmm12
vfmadd132ps (%rax,%r9), %zmm3, %zmm10
vpermi2ps %zmm12, %zmm12, %zmm1
vmaxps (%rcx,%r9), %zmm10, %zmm11
vmovaps %zmm1, %zmm13 # [2]
vmovdqa64 %zmm2, %zmm1 # [1]
vmovups -128(%r11), %zmm15
vfmadd132ps 64(%rax,%r9), %zmm3, %zmm13
vmovups -192(%r11), %zmm6
vpermi2ps %zmm15, %zmm15, %zmm1
vpermi2ps %zmm6, %zmm6, %zmm16
vmovaps %zmm1, %zmm4 # [2]
vmovaps %zmm16, %zmm7 # [2]
vmaxps 64(%rcx,%r9), %zmm13, %zmm14
vfmadd132ps 128(%rax,%r9), %zmm3, %zmm4
vfmadd132ps 192(%rax,%r9), %zmm3, %zmm7
vmaxps 128(%rcx,%r9), %zmm4, %zmm5
leal 4(%r15), %r15d
vmaxps 192(%rcx,%r9), %zmm7, %zmm8
cmpl %esi, %r15d
vmovups %zmm11, (%r8,%r9)
leaq -256(%r11), %r11
vmovups %zmm14, 64(%r8,%r9)
vmovups %zmm5, 128(%r8,%r9)
vmovups %zmm8, 192(%r8,%r9)
leaq 256(%r9), %r9
jb .L1
Instead of this, vpermps can be used. It doesn't overwrite the index register,
what allows to get rid of 8 movs in this loop:
.L2:
lea (,%r12,4), %r10
negq %r10
addq %rbx, %r10
vpermps -64(%r10), %zmm3, %zmm4
vpermps -128(%r10), %zmm3, %zmm6
vpermps -192(%r10), %zmm3, %zmm8
vpermps -256(%r10), %zmm3, %zmm10
vfmadd132ps (%r11,%r12,4), %zmm2, %zmm4
vfmadd132ps 64(%r11,%r12,4), %zmm2, %zmm6
vfmadd132ps 128(%r11,%r12,4), %zmm2, %zmm8
vfmadd132ps 192(%r11,%r12,4), %zmm2, %zmm10
vmaxps (%r13,%r12,4), %zmm4, %zmm5
vmovups %zmm5, (%rdi,%r12,4)
vmaxps 64(%r13,%r12,4), %zmm6, %zmm7
vmovups %zmm7, 64(%rdi,%r12,4)
vmaxps 128(%r13,%r12,4), %zmm8, %zmm9
vmovups %zmm9, 128(%rdi,%r12,4)
vmaxps 192(%r13,%r12,4), %zmm10, %zmm11
vmovups %zmm11, 192(%rdi,%r12,4)
addq $64, %r12
cmpq %rax, %r12
jb .L2
