[gcc r16-6578] vect: Generalise vect_add_slp_permutation [PR122793]
Richard Biener
rguenth@gcc.gnu.org
Thu Jan 8 14:28:57 GMT 2026
https://gcc.gnu.org/g:ece83c6a5aa7ac1cc68d3941797a25601293f310
commit r16-6578-gece83c6a5aa7ac1cc68d3941797a25601293f310
Author: Richard Sandiford <rdsandiford@googlemail.com>
Date: Fri Dec 12 17:40:54 2025 +0000
vect: Generalise vect_add_slp_permutation [PR122793]
The problem seems to be with a packing permutation:
op0[4] op0[5] op0[6] op0[7]
and with the identity_offset parameter to vect_add_slp_permutation.
Both the repeating_p and !repeating_p paths correctly realise that this
permutation reduces to an identity. But the !repeating_p path ends up with
first_node and second_node both set to the second VEC_PERM_EXPR operand
(since that path works elementwise, and since no elements are taken from
the first input). Therefore, the call:
vect_add_slp_permutation (vinfo, gsi, node, first_def,
second_def, mask_vec, mask[0]);
works regardless of whether vect_add_slp_permutation picks first_def or
second_def. In that sense, the parameters to vect_add_slp_permutation are
already “canonical”.
The repeating_p path instead passes vector 2N as first_def and vector 2N+1
as second_def, with mask[0] indicating the position of the identity within
the concatenation of first_def and second_def. However,
vect_add_slp_permutation doesn't expect this and instead ignores the
identity_offset parameter.
PR tree-optimization/122793
* tree-vect-slp.cc (vect_add_slp_permutation): Document the existing
identity_offset parameter. Handle identities that take from the
second input rather than the first.
* gcc.dg/vect/vect-pr122793.c: New testcase.
Co-authored-by: Richard Biener <rguenther@suse.de>
Diff:
---
gcc/testsuite/gcc.dg/vect/vect-pr122793.c | 28 +++++++++++++
gcc/tree-vect-slp.cc | 67 +++++++++++++++++++------------
2 files changed, 69 insertions(+), 26 deletions(-)
diff --git a/gcc/testsuite/gcc.dg/vect/vect-pr122793.c b/gcc/testsuite/gcc.dg/vect/vect-pr122793.c
new file mode 100644
index 000000000000..18af3235b845
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/vect-pr122793.c
@@ -0,0 +1,28 @@
+/* { dg-additional-options "-msse4" { target sse4_runtime } } */
+
+#include "tree-vect.h"
+
+static void
+foo (unsigned char *d, unsigned char *s, int e, int f)
+{
+ for (int i = 0; i < 4; i++)
+ {
+ d[0] = s[-2];
+ d[5] = (s[5] + s[6]) * 2 - (s[4] + s[7]);
+ d[6] = (s[6] + s[7]) * 2 - (s[5] + s[8]);
+ d[7] = (s[7] + s[8]) * 2 - (s[6] + s[9]);
+ d += e;
+ s += f;
+ }
+}
+
+unsigned char s[128] = { 2 }, d[128];
+
+int
+main ()
+{
+ check_vect ();
+ foo (d, s + 2, 16, 16);
+ if (d[5] != 0)
+ __builtin_abort ();
+}
diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
index 4e01adf4cd30..e4e0320c6781 100644
--- a/gcc/tree-vect-slp.cc
+++ b/gcc/tree-vect-slp.cc
@@ -11365,7 +11365,11 @@ vect_transform_slp_perm_load (vec_info *vinfo,
otherwise add:
- <new SSA name> = FIRST_DEF. */
+ <new SSA name> = VEC_PERM_EXPR <FIRST_DEF, SECOND_DEF,
+ { N, N+1, N+2, ... }>
+
+ where N == IDENTITY_OFFSET which is either zero or equal to the
+ number of elements of the result. */
static void
vect_add_slp_permutation (vec_info *vinfo, gimple_stmt_iterator *gsi,
@@ -11405,36 +11409,47 @@ vect_add_slp_permutation (vec_info *vinfo, gimple_stmt_iterator *gsi,
first_def, second_def,
mask_vec);
}
- else if (!types_compatible_p (TREE_TYPE (first_def), vectype))
+ else
{
- /* For identity permutes we still need to handle the case
- of offsetted extracts or concats. */
- unsigned HOST_WIDE_INT c;
- auto first_def_nunits
- = TYPE_VECTOR_SUBPARTS (TREE_TYPE (first_def));
- if (known_le (TYPE_VECTOR_SUBPARTS (vectype), first_def_nunits))
+ auto def_nunits = TYPE_VECTOR_SUBPARTS (TREE_TYPE (first_def));
+ unsigned HOST_WIDE_INT vecno;
+ poly_uint64 eltno;
+ if (!can_div_trunc_p (poly_uint64 (identity_offset), def_nunits,
+ &vecno, &eltno))
+ gcc_unreachable ();
+ tree def = vecno & 1 ? second_def : first_def;
+ if (!types_compatible_p (TREE_TYPE (def), vectype))
{
- unsigned HOST_WIDE_INT elsz
- = tree_to_uhwi (TYPE_SIZE (TREE_TYPE (TREE_TYPE (first_def))));
- tree lowpart = build3 (BIT_FIELD_REF, vectype, first_def,
- TYPE_SIZE (vectype),
- bitsize_int (identity_offset * elsz));
- perm_stmt = gimple_build_assign (perm_dest, lowpart);
+ /* For identity permutes we still need to handle the case
+ of offsetted extracts or concats. */
+ unsigned HOST_WIDE_INT c;
+ if (known_le (TYPE_VECTOR_SUBPARTS (vectype), def_nunits))
+ {
+ unsigned HOST_WIDE_INT elsz
+ = tree_to_uhwi (TYPE_SIZE (TREE_TYPE (TREE_TYPE (def))));
+ tree lowpart = build3 (BIT_FIELD_REF, vectype, def,
+ TYPE_SIZE (vectype),
+ bitsize_int (eltno * elsz));
+ perm_stmt = gimple_build_assign (perm_dest, lowpart);
+ }
+ else if (constant_multiple_p (TYPE_VECTOR_SUBPARTS (vectype),
+ def_nunits, &c) && c == 2)
+ {
+ gcc_assert (known_eq (identity_offset, 0U));
+ tree ctor = build_constructor_va (vectype, 2,
+ NULL_TREE, first_def,
+ NULL_TREE, second_def);
+ perm_stmt = gimple_build_assign (perm_dest, ctor);
+ }
+ else
+ gcc_unreachable ();
}
- else if (constant_multiple_p (TYPE_VECTOR_SUBPARTS (vectype),
- first_def_nunits, &c) && c == 2)
+ else
{
- tree ctor = build_constructor_va (vectype, 2, NULL_TREE, first_def,
- NULL_TREE, second_def);
- perm_stmt = gimple_build_assign (perm_dest, ctor);
+ /* We need a copy here in case the def was external. */
+ gcc_assert (known_eq (eltno, 0U));
+ perm_stmt = gimple_build_assign (perm_dest, def);
}
- else
- gcc_unreachable ();
- }
- else
- {
- /* We need a copy here in case the def was external. */
- perm_stmt = gimple_build_assign (perm_dest, first_def);
}
vect_finish_stmt_generation (vinfo, NULL, perm_stmt, gsi);
/* Store the vector statement in NODE. */
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