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tree-optimization/123190 - allow VF == 1 epilog vectorization

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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.
This commit is contained in:
Richard Biener
2026-01-14 10:53:05 +01:00
committed by Richard Biener
parent 9167c9eeea
commit 96bc77e45c
3 changed files with 45 additions and 2 deletions
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38
gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr123190-1.c Normal file
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@@ -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" } } */

1
gcc/testsuite/gcc.dg/vect/slp-28.c
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@@ -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"

8
gcc/tree-vect-loop.cc
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@@ -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,