https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114346
Bug ID: 114346
Summary: vectorizer generates the same IV twice
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: ---
The following example:
---
double f(int n, double *data, double b) {
double res = b;
for (int i=0;i<n;i++) {
res += data[i] * i;
}
return res;
}
---
generates at -Ofast -march=armv9-a this code:
cntd x5
mov z28.s, w5
index z30.d, #0, #1
.L4:
incw x2
add z1.s, z30.s, z28.s
ld1d z25.d, p7/z, [x3, #1, mul vl]
mov z26.d, z30.d
ld1d z2.d, p7/z, [x3]
sxtw z1.d, p7/m, z1.d
sxtw z26.d, p7/m, z26.d
scvtf z1.d, p7/m, z1.d
scvtf z26.d, p7/m, z26.d
incb x3, all, mul #2
fmla z29.d, p7/m, z25.d, z1.d
incw z30.s
fmla z31.d, p7/m, z2.d, z26.d
cmp w4, w2
bcs .L4
note that the incw is calculating the vectorized IV of i, initialized and z28
is filled with the VL.
so the incw z30.s and the add z1.s, z30.s, z28.s are calculating the same
thing.
there are other issues with this codegen but this ticket is about the double
IVs.
The vectorizer genertes:
# vect_vec_iv_.7_45 = PHI <_49(6), { 0, 1, 2, ... }(15)>
_48 = vect_vec_iv_.7_45 + { POLY_INT_CST [2, 2], ... };
_71 = VIEW_CONVERT_EXPR<vector([2,2]) unsigned int>(vect_vec_iv_.7_45);
_72 = VIEW_CONVERT_EXPR<vector([2,2]) unsigned int>({ POLY_INT_CST [4, 4],
... });
_73 = _71 + _72;
_49 = VIEW_CONVERT_EXPR<vector([2,2]) int>(_73);
so it looks like _48 and _49 are the same value, except that _48 is done as
32-bit IV and _49 is calculated as a 64-bit one and truncated to 32?
