> -----Original Message-----
> From: Richard Biener <[email protected]>
> Sent: 10 February 2026 10:33
> To: Victor Do Nascimento <[email protected]>
> Cc: [email protected]; Tamar Christina <[email protected]>
> Subject: Re: [PATCH] vect: disable vectorization of non-gather elementwise
> loads [PR124037]
>
> On Tue, 10 Feb 2026, Victor Do Nascimento wrote:
>
> > For the vectorization of non-contiguous memory accesses such as the
> > vectorization of loads from a particular struct member, specifically
> > when vectorizing with unknown bounds (thus using a pointer and not an
> > array) it is observed that inadequate alignment checking allows for
> > the crossing of a page boundary within a single vectorized loop
> > iteration. This leads to potential segmentation faults in the
> > resulting binaries.
> >
> > For example, for the given datatype:
> >
> > typedef struct {
> > uint64_t a;
> > uint64_t b;
> > uint32_t flag;
> > uint32_t pad;
> > } Data;
> >
> > and a loop such as:
> >
> > int
> > foo (Data *ptr) {
> > if (ptr == NULL)
> > return -1;
> >
> > int cnt;
> > for (cnt = 0; cnt < MAX; cnt++) {
> > if (ptr->flag == 0)
> > break;
> > ptr++;
> > }
> > return cnt;
> > }
> >
> > the vectorizer yields the following loop logic on armhf:
> >
> > <bb 1>:
> > _41 = ptr_4(D) + 16;
> > <bb 2>:
> > _44 = MEM[(unsigned int *)ivtmp_42];
> > ivtmp_45 = ivtmp_42 + 24;
> > _46 = MEM[(unsigned int *)ivtmp_45];
> > ivtmp_47 = ivtmp_45 + 24;
> > _48 = MEM[(unsigned int *)ivtmp_47];
> > ivtmp_49 = ivtmp_47 + 24;
> > _50 = MEM[(unsigned int *)ivtmp_49];
> > vect_cst__51 = {_44, _46, _48, _50};
> > mask_patt_6.17_52 = vect_cst__51 == { 0, 0, 0, 0 };
> > if (mask_patt_6.17_52 != { 0, 0, 0, 0 })
> > goto <bb 4>;
> > else
> > ivtmp_43 = ivtmp_42 + 96;
> > goto <bb 2>;
> > <bb4>
> > ...
> >
> > without any proper address alignment checks on the starting address
> > or on whether alignment is preserved across iterations.
> >
> > Reject the vectorization of such cases.
>
> Why is peeling for alignment not applied? This is not a
> STMT_VINFO_STRIDED_P access and VMAT_ELEMENTWISE is only a fallback
> for the access, it should not disable the peeling for alignment
> requirement here?
My understanding from a quick look at this before was that the requested
alignment ends up as not a power of two, which gets the vectorizer to correctly
say that the alignment is not reachable. And when you over-align it the check
disappeared.
My guess is because of this target alignment is not set correctly, and the block
/* If this DR needs alignment for correctness, we must ensure the target
alignment is a constant power-of-two multiple of the amount read per
vector iteration or force masking. */
if (dr_safe_speculative_read_required (stmt_info)
&& (*alignment_support_scheme == dr_aligned
&& !mat_gather_scatter_p (*memory_access_type)))
{
Does not end up rejecting it. If this would be true then I agree with you in
that
the likely problem is that DR_TARGET_ALIGNMENT on the DR is incorrect and
is why we don't reject the access.
So I wonder if we override it somewhere?
Thanks,
Tamar
>
> Richard.
>
> >
> > gcc/ChangeLog:
> >
> > PR tree-optimization/124037
> > * tree-vect-stmts.cc (get_load_store_type): Add check for use
> > context of VMAT_ELEMENTWISE loads.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * gcc.dg/vect/pr123588.c: New.
> > ---
> > gcc/testsuite/gcc.dg/vect/pr124037.c | 57
> ++++++++++++++++++++++++++++
> > gcc/tree-vect-stmts.cc | 13 +++++++
> > 2 files changed, 70 insertions(+)
> > create mode 100644 gcc/testsuite/gcc.dg/vect/pr124037.c
> >
> > diff --git a/gcc/testsuite/gcc.dg/vect/pr124037.c
> b/gcc/testsuite/gcc.dg/vect/pr124037.c
> > new file mode 100644
> > index 00000000000..3121380454b
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.dg/vect/pr124037.c
> > @@ -0,0 +1,57 @@
> > +/* PR tree-optimization/124037 */
> > +/* { dg-options "-O3" } */
> > +/* { dg-require-effective-target mmap } */
> > +/* { dg-require-effective-target vect_early_break } */
> > +#include <stdint.h>
> > +#include <stdio.h>
> > +#include <string.h>
> > +#include <sys/mman.h>
> > +#include <unistd.h>
> > +
> > +#define MAX 65536
> > +
> > +typedef struct {
> > + uint64_t a;
> > + uint64_t b;
> > + uint32_t flag;
> > + uint32_t pad;
> > +} Data;
> > +
> > +int __attribute__ ((noinline))
> > +foo (Data *ptr) {
> > + if (ptr == NULL)
> > + return -1;
> > +
> > + int cnt;
> > + for (cnt = 0; cnt < MAX; cnt++) {
> > + if (ptr->flag == 0)
> > + break;
> > + ptr++;
> > + }
> > + return cnt;
> > +}
> > +
> > +int main() {
> > + long pgsz = sysconf (_SC_PAGESIZE);
> > + if (pgsz == -1) {
> > + fprintf (stderr, "sysconf failed\n");
> > + return 0;
> > + }
> > +
> > + /* Allocate 2 consecutive pages. */
> > + void *mem = mmap (NULL, pgsz * 2, PROT_READ | PROT_WRITE,
> > + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> > + if (mem == MAP_FAILED) {
> > + fprintf (stderr, "mmap failed\n");
> > + return 0;
> > + }
> > +
> > + memset (mem, 1, pgsz);
> > + uint8_t *ptr = (uint8_t*) mem + pgsz;
> > + memset (ptr - 16, 0, 16);
> > +
> > + mprotect (ptr, pgsz, PROT_NONE);
> > + Data *data = (Data *) (ptr - 80);
> > + __builtin_printf("%d\n", foo(data));
> > +}
> > +
> > diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
> > index ee98e72d1e5..7434d934463 100644
> > --- a/gcc/tree-vect-stmts.cc
> > +++ b/gcc/tree-vect-stmts.cc
> > @@ -2581,6 +2581,19 @@ get_load_store_type (vec_info *vinfo,
> stmt_vec_info stmt_info,
> > return false;
> > }
> >
> > + if (loop_vinfo
> > + && !inbounds
> > + && dr_safe_speculative_read_required (stmt_info)
> > + && *memory_access_type == VMAT_ELEMENTWISE
> > + && !STMT_VINFO_GATHER_SCATTER_P (stmt_info))
> > + {
> > + if (dump_enabled_p ())
> > + dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> > + "unsafe use of elementwise access for buffer with"
> > + "unknown bounds\n");
> > + return false;
> > + }
> > +
> > /* If this DR needs alignment for correctness, we must ensure the target
> > alignment is a constant power-of-two multiple of the amount read per
> > vector iteration or force masking. */
> >
>
> --
> Richard Biener <[email protected]>
> SUSE Software Solutions Germany GmbH,
> Frankenstrasse 146, 90461 Nuernberg, Germany;
> GF: Jochen Jaser, Andrew McDonald, Werner Knoblich; (HRB 36809, AG
> Nuernberg)
