https://gcc.gnu.org/g:5584b489341924d042988ad27a2dc25fa8fda62a
commit r16-5957-g5584b489341924d042988ad27a2dc25fa8fda62a Author: Richard Biener <[email protected]> Date: Sun Dec 7 11:30:48 2025 +0100 tree-optimization/123038 - FFS/CTZ pattern incompatible with reductions The pattern ends up using the argument more than one time which isn't supported. When FFS directly maps to CTZ + 1 it works though. PR tree-optimization/123038 * tree-vect-patterns.cc (vect_recog_ctz_ffs_pattern): Reject pattern for reductions when the call argument is used multiple times. * gcc.dg/vect/pr123038.c: New testcase. Diff: --- gcc/testsuite/gcc.dg/vect/pr123038.c | 8 ++++++++ gcc/tree-vect-patterns.cc | 14 ++++++++++++++ 2 files changed, 22 insertions(+) diff --git a/gcc/testsuite/gcc.dg/vect/pr123038.c b/gcc/testsuite/gcc.dg/vect/pr123038.c new file mode 100644 index 000000000000..bca831f9c085 --- /dev/null +++ b/gcc/testsuite/gcc.dg/vect/pr123038.c @@ -0,0 +1,8 @@ +/* { dg-do compile } */ + +unsigned char f(int b) +{ + for (int a = 0; a < 10; a += 1) + b = __builtin_ffs(b); + return b; +} diff --git a/gcc/tree-vect-patterns.cc b/gcc/tree-vect-patterns.cc index 555986b4fec5..af64cb84e418 100644 --- a/gcc/tree-vect-patterns.cc +++ b/gcc/tree-vect-patterns.cc @@ -1914,6 +1914,9 @@ vect_recog_ctz_ffs_pattern (vec_info *vinfo, stmt_vec_info stmt_vinfo, && val_new == prec) || (ifnnew == IFN_POPCOUNT && ifn == IFN_CTZ)) { + if (vect_is_reduction (stmt_vinfo)) + return NULL; + /* .CTZ (X) = PREC - .CLZ ((X - 1) & ~X) .CTZ (X) = .POPCOUNT ((X - 1) & ~X). */ if (ifnnew == IFN_CLZ) @@ -1952,6 +1955,9 @@ vect_recog_ctz_ffs_pattern (vec_info *vinfo, stmt_vec_info stmt_vinfo, } else if (ifnnew == IFN_CLZ) { + if (vect_is_reduction (stmt_vinfo)) + return NULL; + /* .CTZ (X) = (PREC - 1) - .CLZ (X & -X) .FFS (X) = PREC - .CLZ (X & -X). */ sub = prec - (ifn == IFN_CTZ); @@ -1971,6 +1977,9 @@ vect_recog_ctz_ffs_pattern (vec_info *vinfo, stmt_vec_info stmt_vinfo, } else if (ifnnew == IFN_POPCOUNT) { + if (vect_is_reduction (stmt_vinfo)) + return NULL; + /* .CTZ (X) = PREC - .POPCOUNT (X | -X) .FFS (X) = (PREC + 1) - .POPCOUNT (X | -X). */ sub = prec + (ifn == IFN_FFS); @@ -1993,6 +2002,11 @@ vect_recog_ctz_ffs_pattern (vec_info *vinfo, stmt_vec_info stmt_vinfo, /* .FFS (X) = .CTZ (X) + 1. */ add = 1; val_cmp++; + + if (vect_is_reduction (stmt_vinfo) + && defined_at_zero + && (!defined_at_zero_new || val != val_cmp)) + return NULL; } /* Create B = .IFNNEW (A). */
