https://gcc.gnu.org/g:96bc77e45c202303b87647a44206c6475f996c5c
commit r16-6766-g96bc77e45c202303b87647a44206c6475f996c5c Author: Richard Biener <[email protected]> Date: Wed Jan 14 10:53:05 2026 +0100 tree-optimization/123190 - allow VF == 1 epilog vectorization The following adjusts the condition where we reject vectorization because the scalar loop runs only for a single iteration (or two, in case we need to peel for gaps). Because this is over-eager when considering the case of VF == 1 where instead the cost model should decide wheter it is worthwhile or not. I'm playing conservative here and exclude the case of two iterations as I do not have benchmark evidence. This helps fixing a regression observed with improved SLP handling, not exactly for the options used in the PR though, but for a more common -O3 -march=x86-64-v3 this speeds up 433.milc by 6%. PR tree-optimization/123190 * tree-vect-loop.cc (vect_analyze_loop_costing): Allow vectorizing loops with a single scalar iteration iff the vectorization factor is 1. * gcc.dg/vect/costmodel/x86_64/costmodel-pr123190-1.c: New testcase. * gcc.dg/vect/slp-28.c: Avoid epilogue vectorization for simplicity. Diff: --- .../vect/costmodel/x86_64/costmodel-pr123190-1.c | 38 ++++++++++++++++++++++ gcc/testsuite/gcc.dg/vect/slp-28.c | 1 + gcc/tree-vect-loop.cc | 8 +++-- 3 files changed, 45 insertions(+), 2 deletions(-) diff --git a/gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr123190-1.c b/gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr123190-1.c new file mode 100644 index 000000000000..4265ac80a43d --- /dev/null +++ b/gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr123190-1.c @@ -0,0 +1,38 @@ +/* { dg-do compile } */ +/* { dg-additional-options "-O3 -mavx2 -mno-avx512f -mtune=generic" } */ + +typedef struct { + double real; + double imag; +} complex; + +typedef struct { complex e[3][3]; } su3_matrix; + +void mult_su3_na( su3_matrix *a, su3_matrix *b, su3_matrix *c ){ +int i,j; +register double t,ar,ai,br,bi,cr,ci; + for(i=0;i<3;i++) + for(j=0;j<3;j++){ + + ar=a->e[i][0].real; ai=a->e[i][0].imag; + br=b->e[j][0].real; bi=b->e[j][0].imag; + cr=ar*br; t=ai*bi; cr += t; + ci=ai*br; t=ar*bi; ci -= t; + + ar=a->e[i][1].real; ai=a->e[i][1].imag; + br=b->e[j][1].real; bi=b->e[j][1].imag; + t=ar*br; cr += t; t=ai*bi; cr += t; + t=ar*bi; ci -= t; t=ai*br; ci += t; + + ar=a->e[i][2].real; ai=a->e[i][2].imag; + br=b->e[j][2].real; bi=b->e[j][2].imag; + t=ar*br; cr += t; t=ai*bi; cr += t; + t=ar*bi; ci -= t; t=ai*br; ci += t; + + c->e[i][j].real=cr; + c->e[i][j].imag=ci; + } +} + +/* { dg-final { scan-tree-dump "optimized: loop vectorized using 32" "vect" } } */ +/* { dg-final { scan-tree-dump "optimized: epilogue loop vectorized using 16 byte vectors and unroll factor 1" "vect" } } */ diff --git a/gcc/testsuite/gcc.dg/vect/slp-28.c b/gcc/testsuite/gcc.dg/vect/slp-28.c index 1f987874f0df..bf6271eed253 100644 --- a/gcc/testsuite/gcc.dg/vect/slp-28.c +++ b/gcc/testsuite/gcc.dg/vect/slp-28.c @@ -1,4 +1,5 @@ /* { dg-require-effective-target vect_int } */ +/* { dg-additional-options "--param vect-epilogues-nomask=0" } */ #include <stdarg.h> #include "tree-vect.h" diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc index 74eecb832e6f..fdf544fa47b0 100644 --- a/gcc/tree-vect-loop.cc +++ b/gcc/tree-vect-loop.cc @@ -1792,9 +1792,13 @@ vect_analyze_loop_costing (loop_vec_info loop_vinfo, } } /* Reject vectorizing for a single scalar iteration, even if - we could in principle implement that using partial vectors. */ + we could in principle implement that using partial vectors. + But allow such vectorization if VF == 1 in case we do not + need to peel for gaps (if we need, avoid vectorization for + reasons of code footprint). */ unsigned peeling_gap = LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo); - if (scalar_niters <= peeling_gap + 1) + if (scalar_niters <= peeling_gap + 1 + && (assumed_vf > 1 || peeling_gap != 0)) { if (dump_enabled_p ()) dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
