Created attachment 55183 [details]
Open-source stream benchmark

The stream benchmark performance deteriorates. The loop peeling policy in the vect_enhance_data_refs_alignment function is modified to degrade the benchmark performance,which can be demonstrated on example from attachment. 

Alternatively, you can obtain it from https://github.com/jeffhammond/stream/archive/master.zip.

Compiling & Running:
gcc -fopenmp -O -DSTREAM_ARRAY_SIZE=100000000 stream.c  -o stream
./stream

Patches to modify the loop stripping policy of the vect_enhance_data_refs_alignment function are available from:
https://gcc.gnu.org/git/?p=gcc.git&a=commit;h=49ab46214e9288ee1268f87ddcd64dacfd21c31d

After you open OpenMP,

Before modification: (Add subitem)
ldr d0, [x5, x1, lsl #3]
fadd d0, d0, d1
str d0, [x4, x1, lsl #3]
mov w4, w2
sub w7, w7, w2
add x4, x4, x1
ldr x1, [x10, #888]
lsl x4, x4, #3
lsr w8, w7, #1
add x6, x4, x6
add x5, x4, x5
mov w2, #0x0 /
add x4, x4, x1
mov x1, #0x0 /
ldr q0, [x5, x1]
add w2, w2, #0x1
ldr q1, [x6, x1]
cmp w2, w8
fadd v0.2d, v0.2d, v1.2d
str q0, [x4, x1]
add x1, x1, #0x10
b.cc 4012d8 <main._omp_fn.4+0xd8>
and w1, w7, #0xfffffffe
add w0, w0, w1
cmp w7, w1
b.eq 401348 <main._omp_fn.4+0x148>
ldr x5, [x9, #880]
sxtw x1, w0
ldr x4, [x11, #896]
add w0, w0, #0x1
ldr d1, [x5, x1, lsl #3]
cmp w3, w0
ldr x2, [x10, #888]
ldr d0, [x4, x1, lsl #3]
fadd d0, d0, d1
str d0, [x2, x1, lsl #3]
b.le 401348 <main._omp_fn.4+0x148>
sxtw x0, w0
ldr d0, [x5, x0, lsl #3]
ldr d1, [x4, x0, lsl #3]
fadd d0, d0, d1
str d0, [x2, x0, lsl #3]
ldr x19, [sp, #16]
ldp x29, x30, [sp], #32

After the modification:
mov x29, sp
str x19, [sp, #16]
bl 4006e0 <omp_get_num_threads@plt>
mov w19, w0
bl 4006b0 <omp_get_thread_num@plt>
mov w2, #0x8000 /
movk w2, #0x61a, lsl #16
sdiv w1, w2, w19
msub w2, w1, w19, w2
cmp w0, w2
b.ge 401238 <main._omp_fn.4+0x38>
add w1, w1, #0x1
mov w2, #0x0 /
madd w0, w1, w0, w2
add w1, w1, w0
cmp w0, w1
b.ge 4012d8 <main._omp_fn.4+0xd8>
sub w2, w1, w0
adrp x8, 401000 <main._omp_fn.3+0x100>
adrp x9, 401000 <main._omp_fn.3+0x100>
adrp x7, 401000 <main._omp_fn.3+0x100>
cmp w2, #0x1
b.eq 4012b8 <main._omp_fn.4+0xb8>
ldr x1, [x7, #760]
sbfiz x4, x0, #3, #32
ldr x6, [x8, #744]
lsr w10, w2, #1
ldr x5, [x9, #752]
mov w3, #0x0 /
add x6, x4, x6
add x5, x4, x5
add x4, x4, x1
mov x1, #0x0 /
ldr q0, [x6, x1]
add w3, w3, #0x1
ldr q1, [x5, x1]
cmp w3, w10
fadd v0.2d, v0.2d, v1.2d
str q0, [x4, x1]
add x1, x1, #0x10
b.cc 401288 <main._omp_fn.4+0x88>
and w1, w2, #0xfffffffe
add w0, w0, w1
cmp w2, w1
b.eq 4012d8 <main._omp_fn.4+0xd8>
ldr x3, [x9, #752]
sxtw x0, w0
ldr x2, [x8, #744]
ldr x1, [x7, #760]
ldr d0, [x3, x0, lsl #3]
ldr d1, [x2, x0, lsl #3]
fadd d0, d0, d1
str d0, [x1, x0, lsl #3]
ldr x19, [sp, #16]
ldp x29, x30, [sp], #32
ret

After modifying the peeling policy, the vectorization of the for loop in the Add subitem does not attempt to peel the loop, but the performance eventually degrades.