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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 cfg 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. We therefore
fix the handling of such cases.
To correct this, we modify the logic in `get_load_store_type',
particularly the logic responsible for ensuring we don't read more
than the scalar code would in the context of early breaks, extending
it from handling not only gather-scatter and strided SLP accesses but
also allowing it to properly handle element-wise accesses, wherein we
specify that these need correct block alignment, thus promoting their
`alignment_support_scheme' from `dr_unaligned_supported' to
`dr_aligned'.
gcc/ChangeLog:
PR tree-optimization/124037
* tree-vect-stmts.cc (get_load_store_type): Fix
alignment_support_scheme categorization for early
break VMAT_ELEMENTWISE accesses.
gcc/testsuite/ChangeLog:
* gcc.dg/vect/vect-pr124037.c: New.
* g++.dg/vect/vect-pr124037.cc: New.
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