http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53636
Bug #: 53636
Summary: SLP may create invalid unaligned memory accesses
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: uweig...@gcc.gnu.org
The following test case:
void test (unsigned char *dst)
{
short tmp[11 * 8], *tptr;
int i;
fill (tmp);
tptr = tmp;
for (i = 0; i < 8; i++)
{
dst[0] = (-tptr[0] + 9 * tptr[0 + 1] + 9 * tptr[0 + 2] - tptr[0 + 3]) >>
7;
dst[1] = (-tptr[1] + 9 * tptr[1 + 1] + 9 * tptr[1 + 2] - tptr[1 + 3]) >>
7;
dst[2] = (-tptr[2] + 9 * tptr[2 + 1] + 9 * tptr[2 + 2] - tptr[2 + 3]) >>
7;
dst[3] = (-tptr[3] + 9 * tptr[3 + 1] + 9 * tptr[3 + 2] - tptr[3 + 3]) >>
7;
dst[4] = (-tptr[4] + 9 * tptr[4 + 1] + 9 * tptr[4 + 2] - tptr[4 + 3]) >>
7;
dst[5] = (-tptr[5] + 9 * tptr[5 + 1] + 9 * tptr[5 + 2] - tptr[5 + 3]) >>
7;
dst[6] = (-tptr[6] + 9 * tptr[6 + 1] + 9 * tptr[6 + 2] - tptr[6 + 3]) >>
7;
dst[7] = (-tptr[7] + 9 * tptr[7 + 1] + 9 * tptr[7 + 2] - tptr[7 + 3]) >>
7;
dst += 8;
tptr += 11;
}
}
when built on ARM with -mcpu=cortex-a9 -mfpu=neon -mfloat-abi=softfp -O
-ftree-vectorize creates code that uses a VLDR instruction to access unaligned
memory, which causes a Bus error at runtime.
The problem seems to be that the check in vect_compute_data_ref_alignment is
not enough for SLP. Even though SLP only considers a basic blokc, the data-ref
analysis still looks at innermost loops to compute scalar evolutions. This
results in concluding that the access "tptr[0]" is based on "tmp", which is
aligned to 8 bytes, using a step of 22 bytes.
The alignment check now only verified that the *base* is aligned. This is OK
if we're actually vectorizing the loop. But in the SLP case, we really need to
verify instead that the access is aligned on *every* iteration through the loop
...
