https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105490
--- Comment #3 from rguenther at suse dot de <rguenther at suse dot de> --- On Wed, 30 Aug 2023, pinskia at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105490 > > --- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> --- > Here is an even odder case: > ``` > #define N 256 > typedef short T; > extern T a[N]; > extern T b[N]; > extern T c[N]; > extern _Bool pb[N]; > extern _Bool pb1[N]; > extern _Bool pb2[N]; > > void predicate_by_booland() > { > for (int i = 0; i < N; i++) > c[i] = ((pb1[i] != pb[i]) != pb2[i]) ? a[i] : b[i]; > } > ``` > This vectorizes currently with `-O3` but not with `-O3 -fno-tree-vrp`. > IR with -fno-tree-vrp: > ``` > _1 = pb1[i_15]; > _2 = pb[i_15]; > _3 = _1 != _2; > _4 = pb2[i_15]; > iftmp.0_10 = a[i_15]; > _5 = _3 != _4; > iftmp.0_9 = b[i_15]; > ``` > IR without (VRP turned on): > ``` > _1 = pb1[i_15]; > _2 = pb[i_15]; > _3 = _1 ^ _2; > _4 = pb2[i_15]; > iftmp.0_10 = a[i_15]; > _5 = _3 != _4; > iftmp.0_9 = b[i_15]; > ``` > > So it is even more confusing ... This is usually due to limits/bugs in the vectorizer bool pattern recognition code which is supposed to replace "mask" uses of bool with x ? -1 : 0 (or make sure comparisons produce them) and data uses with x ? 1 : 0 (or make sure "data" stmts produce them). There are defenses in vectorizable_* to detect cases where that did go wrong (which it does sometimes), leading to missed vectorizations (or wrong code in the worst case). It's a mess.
