https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83518
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Known to work| |7.2.1
Version|tree-ssa |8.0
Keywords| |missed-optimization
Last reconfirmed| |2018-01-05
CC| |rguenth at gcc dot gnu.org
Ever confirmed|0 |1
Summary|Missing optimization: |[8 Regression] Missing
|useless instructions should |optimization: useless
|be dropped |instructions should be
| |dropped
Target Milestone|--- |8.0
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Works fine with GCC 7. I suppose unroller limits hit and/or we're just lucky
that GCC 7 doesn't vectorize the reduction loop ...
Trunk has
t.C:16:20: note: loop vectorized
t.C:21:10: note: basic block vectorized
resulting in
<bb 2> [local count: 178992762]:
MEM[(int *)&arr] = { 5, 4, 3, 2 };
t_2 = arr[0];
_65 = arr[1];
_46 = MEM[(int *)&arr + 8B];
MEM[(int *)&arr] = _46;
arr[2] = 1;
arr[3] = t_2;
vect__2.5_38 = MEM[(int *)&arr];
vect_sum_21.8_30 = VEC_PERM_EXPR <vect__2.5_38, { 0, 0, 0, 0 }, { 2, 3, 4, 5
}>;
vect_sum_21.8_15 = vect_sum_21.8_30 + vect__2.5_38;
vect_sum_21.8_59 = VEC_PERM_EXPR <vect_sum_21.8_15, { 0, 0, 0, 0 }, { 1, 2,
3, 4 }>;
vect_sum_21.8_60 = vect_sum_21.8_15 + vect_sum_21.8_59;
stmp_sum_21.7_61 = BIT_FIELD_REF <vect_sum_21.8_60, 32, 0>;
sum_27 = stmp_sum_21.7_61 + _65;
_23 = (unsigned int) sum_27;
arr ={v} {CLOBBER};
return _23;
while GCC 7 simply unrolls the loop. DOM is not able to simplify the
vector load from MEM[(int *)&arr] but the scalar loads from the unrolled
variant.
