https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111770
Bug ID: 111770
Summary: predicated loads inactive lane values not modelled
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tnfchris at gcc dot gnu.org
Target Milestone: ---
For this example:
int foo(int n, char *a, char *b) {
int sum = 0;
for (int i = 0; i < n; ++i) {
sum += a[i] * b[i];
}
return sum;
}
we generate with -O3 -march=armv8-a+sve
.L3:
ld1b z29.b, p7/z, [x1, x3]
ld1b z31.b, p7/z, [x2, x3]
add x3, x3, x4
sel z31.b, p7, z31.b, z28.b
whilelo p7.b, w3, w0
udot z30.s, z29.b, z31.b
b.any .L3
uaddv d30, p6, z30.s
fmov w0, s30
ret
Which is pretty good, but we completely ruin it with the SEL.
In gimple this is:
vect__7.12_81 = .MASK_LOAD (_21, 8B, loop_mask_77);
masked_op1_82 = .VCOND_MASK (loop_mask_77, vect__7.12_81, { 0, ... });
vect_patt_33.13_83 = DOT_PROD_EXPR <vect__3.9_78, masked_op1_82,
vect_sum_19.6_74>;
The missed optimization here is that we don't model what happens with
predicated operations that zero inactive lanes.
i.e. in this case .MASK_LOAD will zero the unactive lanes, so the .VCOND_MASK
is completely superfluous.
I'm not entirely sure how we should go about fixing this generally.
