On Wed, Jan 18, 2017 at 10:10 AM, Martin Liška <mli...@suse.cz> wrote: > Hello. > > After basic understanding of loop predictive commoning, the problematic > combined chain is: > > Loads-only chain 0x38b6730 (combined) > max distance 0 > references: > MEM[(real(kind=8) *)vectp_a.29_81] (id 1) > offset 20 > distance 0 > MEM[(real(kind=8) *)vectp_a.38_141] (id 3) > offset 20 > distance 0 > > Loads-only chain 0x38b68b0 (combined) > max distance 0 > references: > MEM[(real(kind=8) *)vectp_a.23_102] (id 0) > offset 0 > distance 0 > MEM[(real(kind=8) *)vectp_a.33_33] (id 2) > offset 0 > distance 0 > > Combination chain 0x38b65b0 > max distance 0, may reuse first > equal to 0x38b6730 + 0x38b68b0 in type vector(2) real(kind=8) > references: > combination ref > in statement predreastmp.48_10 = vect__32.31_78 + vect__28.25_100; > > distance 0 > combination ref > in statement predreastmp.50_17 = vect__42.41_138 + vect__38.36_29; > > distance 0 > > It's important to note that distance is equal to zero (happening within a > same loop iteration). > Aforementioned chains correspond to: > > ... > r2: vect__28.25_100 = MEM[(real(kind=8) *)vectp_a.23_102]; > vectp_a.23_99 = vectp_a.23_102 + 16; > vect__28.26_98 = MEM[(real(kind=8) *)vectp_a.23_99]; > vect__82.27_97 = vect__22.22_108; > vect__82.27_96 = vect__22.22_107; > vect__79.28_95 = vect__82.27_97 + vect__84.17_120; > vect__79.28_94 = vect__82.27_96 + vect__84.17_119; > r1: vect__32.31_78 = MEM[(real(kind=8) *)vectp_a.29_81]; > vectp_a.29_77 = vectp_a.29_81 + 16; > vect__32.32_76 = MEM[(real(kind=8) *)vectp_a.29_77]; > vect__38.35_39 = MEM[(real(kind=8) *)vectp_a.33_57]; > r2': vectp_a.33_33 = vectp_a.33_57 + 16; > vect__38.36_29 = MEM[(real(kind=8) *)vectp_a.33_33]; > vect__56.37_23 = vect__38.35_39; > vect__56.37_15 = vect__32.32_76; > vect__42.40_161 = MEM[(real(kind=8) *)vectp_a.38_163]; > vectp_a.38_141 = vectp_a.38_163 + 16; > r1': vect__42.41_138 = MEM[(real(kind=8) *)vectp_a.38_141]; > vect__54.42_135 = vect__42.40_161 + vect__56.37_23; > r1'+r2': predreastmp.50_17 = vect__42.41_138 + vect__38.36_29; > predreastmp.51_18 = vect__56.37_15; > vect__54.42_134 = predreastmp.50_17; > r1+r2: predreastmp.48_10 = vect__32.31_78 + vect__28.25_100; > ... > > Problematic construct is that while having load-only chains r1->r1' and > r2->r2', the combination > is actually r1'+r2'->r1+r2, which cause the troubles. I believe the proper > fix is to reject such > combinations where combined root stmt does not dominate usages. It's probably > corner case as it does > not reuse any values among loop iterations (which is main motivation of the > pass), it's doing PRE > if I'm right. I think this is OK, though I can't approve it. Later DOM pass should be able to pick up the redundant combination operation between predreastmp.50_17 and predreastmp.48_10, could you double check this?
Thanks, bin > > Patch can bootstrap on ppc64le-redhat-linux and survives regression tests. > > Ready to be installed? > Martin >
