https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121290
--- Comment #5 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
In gimple that's
<bb 10> [local count: 108459]:
x_22 = a[0];
_69 = {x_22, x_22, x_22, x_22};
<bb 4> [local count: 10737416]:
# ivtmp_83 = PHI <ivtmp_84(11), 0(10)>
<bb 5> [local count: 1063004408]:
# i_43 = PHI <i_24(12), 0(4)>
# ivtmp_34 = PHI <ivtmp_33(12), 32000(4)>
# vect_x_36.8_70 = PHI <vect_x_9.10_72(12), _69(4)>
# vect_vec_iv_.12_76 = PHI <_77(12), { 0, 0, 0, 0 }(4)>
# vect_index_39.13_78 = PHI <vect_index_12.14_79(12), { 0, 0, 0, 0 }(4)>
_4 = a[i_43];
vect_cst__68 = {_4, _4, _4, _4};
mask__16.9_71 = vect_cst__68 > vect_x_36.8_70;
vect_index_12.14_79 = VEC_COND_EXPR <mask__16.9_71, vect_vec_iv_.12_76,
vect_index_39.13_78>;
vect_x_9.10_72 = VEC_COND_EXPR <mask__16.9_71, vect_cst__68, vect_x_36.8_70>;
i_24 = i_43 + 1;
ivtmp_33 = ivtmp_34 - 1;
_77 = vect_vec_iv_.12_76 + { 1, 1, 1, 1 };
if (ivtmp_33 != 0)
goto <bb 12>; [98.99%]
else
goto <bb 8>; [1.01%]
The SLP tree seems to mostly be working on lanes of externals:
note: Vectorizing SLP tree:
note: node 0x42900120 (max_nunits=4, refcnt=1) vector(4) float
note: op template: x_41 = PHI <x_9(5)>
note: [l] stmt 0 x_41 = PHI <x_9(5)>
note: children 0x429001c8
note: node 0x429001c8 (max_nunits=4, refcnt=2) vector(4) float
note: op template: x_9 = _16 ? _4 : x_36;
note: stmt 0 x_9 = _16 ? _4 : x_36;
note: children 0x42900270 0x42900318 0x429003c0
note: node 0x42900270 (max_nunits=4, refcnt=2) vector(4) <signed-boolean:32>
note: op template: _16 = _4 > x_36;
note: stmt 0 _16 = _4 > x_36;
note: children 0x42900318 0x429003c0
note: node 0x42900318 (max_nunits=4, refcnt=2) vector(4) float
note: op template: _4 = a[i_43];
note: stmt 0 _4 = a[i_43];
note: node 0x429003c0 (max_nunits=4, refcnt=2) vector(4) float
note: op template: x_36 = PHI <x_9(12), x_22(4)>
note: stmt 0 x_36 = PHI <x_9(12), x_22(4)>
note: children 0x429001c8 0x42900468
note: node (external) 0x42900468 (max_nunits=1, refcnt=1) vector(4) float
note: { x_22 }
it also looks like we missed simplifying a > b ? a : b into just a max.
Before we failed during analysis in the block that was removed:
missed: Unsupported loop-closed phi in outer-loop.
missed: bad operation or unsupported loop bound
and now it's a costing issue, as it's an inner loop,
You can reduce it down to
#define iterations 100000
#define LEN_1D 32000
float a[LEN_1D];
int main()
{
float x;
for (int nl = 0; nl < iterations; nl++) {
x = a[0];
for (int i = 0; i < LEN_1D; ++i) {
if (a[i] > x) {
x = a[i];
}
}
}
return x > 1;
}
It looks like the access of a[0] in the outer loop is making it treat the inner
loop as only being able to access one element at a time.
