https://gcc.gnu.org/g:1cca2bfbdaa4433002d321b580b173294ee88460
commit r17-1490-g1cca2bfbdaa4433002d321b580b173294ee88460 Author: Zhongyao Chen <[email protected]> Date: Wed Jan 14 11:38:02 2026 +0000 tree-optimization - Require size > 1 for SLP reduction subgroups The SLP reduction subgroup analysis can succeed for size-1 groups, but this leads to poor code generation. Size-1 cases should fall back to single-lane reduction instead. Handle size-1 groups by returning false from the analysis function, and add a loop exit check to avoid unnecessary processing. PR tree-optimization/123343 gcc/ChangeLog: * tree-vect-slp.cc (vect_analyze_slp_reduction_group): Return false for group_size <= 1 at entry. (vect_analyze_slp_reductions): Add loop exit check for group_size <= 1. gcc/testsuite/ChangeLog: * gcc.dg/vect/slp-reduc-var.c: New testcase. Signed-off-by: Zhongyao Chen <[email protected]> Diff: --- gcc/testsuite/gcc.dg/vect/slp-reduc-var.c | 25 +++++++++++++++++++++++++ gcc/tree-vect-slp.cc | 7 ++++++- 2 files changed, 31 insertions(+), 1 deletion(-) diff --git a/gcc/testsuite/gcc.dg/vect/slp-reduc-var.c b/gcc/testsuite/gcc.dg/vect/slp-reduc-var.c new file mode 100644 index 000000000000..bb322b1a428d --- /dev/null +++ b/gcc/testsuite/gcc.dg/vect/slp-reduc-var.c @@ -0,0 +1,25 @@ +/* { dg-do compile } */ +/* { dg-additional-options "-O3 --param vect-epilogues-nomask=0" } */ +#include <stdint-gcc.h> + +uint64_t +x264_pixel_var_8x8 (uint8_t *pix, int i_stride) +{ + uint32_t sum = 0, sqr = 0; + for (int y = 0; y < 8; y++) + { + for (int x = 0; x < 8; x++) + { + sum += pix[x]; + sqr += pix[x] * pix[x]; + } + pix += i_stride; + } + return sum + ((uint64_t)sqr << 32); +} + +/* Verify that size-1 reductions fall back to single-lane reduction chains. */ +/* { dg-final { scan-tree-dump "Starting SLP discovery of reduction chain" "vect" } } */ +/* { dg-final { scan-tree-dump-not "SLP discovery of reduction chain failed" "vect" } } */ +/* { dg-final { scan-tree-dump-not "SLP discovery of size 2 reduction group" "vect" } } */ +/* { dg-final { scan-tree-dump-not "SLP discovery of size 1 reduction group" "vect" } } */ diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc index 90c183c8811b..2250f6f74a1a 100644 --- a/gcc/tree-vect-slp.cc +++ b/gcc/tree-vect-slp.cc @@ -4852,8 +4852,11 @@ vect_analyze_slp_reduction_group (loop_vec_info loop_vinfo, unsigned max_tree_size, unsigned *limit, bool *matches) { - /* Try to form a reduction group. */ + /* Try to form a reduction group. Size-1 groups are not suitable + for SLP reduction and should fall back to single-lane reduction. */ unsigned int group_size = scalar_stmts.length (); + if (group_size <= 1) + return false; if (!matches) matches = XALLOCAVEC (bool, group_size); poly_uint64 max_nunits = 1; @@ -5003,6 +5006,8 @@ vect_analyze_slp_reductions (loop_vec_info loop_vinfo, } scalar_stmts.truncate (j); group_size = scalar_stmts.length (); + if (group_size <= 1) + break; if (vect_analyze_slp_reduction_group (loop_vinfo, scalar_stmts, bst_map, max_tree_size, limit, matches))
