https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110449
Bug ID: 110449
Summary: Vect: use a small step to calculate the loop induction
if the loop is unrolled during loop vectorization
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: hliu at amperecomputing dot com
Target Milestone: ---
This is inspired by clang. Compile the follwing case with "-mcpu=neoverse-n2
-O3":
void foo(int *arr, int val, int step) {
for (int i = 0; i < 1024; i++) {
arr[i] = val;
val += step;
}
}
It will be unrolled by 2 during vectorization. GCC generates code:
fmov s29, w2 # step
shl v27.2s, v29.2s, 3 # 8*step
shl v28.2s, v29.2s, 2 # 4*step
...
.L2:
mov v30.16b, v31.16b
add v31.4s, v31.4s, v27.4s # += 8*step
add v29.4s, v30.4s, v28.4s # += 4*step
stp q30, q29, [x0]
add x0, x0, 32
cmp x1, x0
bne .L2
The v27 (i.e. "8*step") is actually not necessary. We can use v29 + v28 (i.e.
"+ 4*step") and generate simpler code:
fmov s29, w2 # step
shl v28.2s, v29.2s, 2 # 4*step
...
.L2:
add v29.4s, v30.4s, v28.4s # += 4*step
stp q30, q29, [x0]
add x0, x0, 32
add v30.4s, v29.4s, v28.4s # += 4*step
cmp x1, x0
bne .L2
This has two benefits:
(1) Save 1 vector register and one "mov" instructon
(2) For floating point, the result value of small step should be closer to the
original scalar result value than large step. I.e. "A + 4*step + ... + 4*step"
should be closer to "A + step + ... + step" than "A + 8*step + ... 8*step".
Do you think if this is reasonable?
I have a simple patch to enhance the tree-vect-loop.cc
"vectorizable_induction()" to achieve this. Will send out the patch for code
review later.
