[Bug target/104882] New: [12 Regression] MVE: Wrong code at -O2 since r12-1434-g046a3beb1673bf4a61c131373b6a5e84158e92bf

["acoplan at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104882

            Bug ID: 104882
           Summary: [12 Regression] MVE: Wrong code at -O2 since
                    r12-1434-g046a3beb1673bf4a61c131373b6a5e84158e92bf
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: acoplan at gcc dot gnu.org
  Target Milestone: ---

Created attachment 52608
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52608&action=edit
broken assembly output

The following code:

int i;
char src[1072];
char dst[72];
int main() {
  for (i = 0; i < 128; i++)
    src[i] = i;
  __builtin_memcpy(dst, src, 7);
  for (i = 0; i < 7; i++)
    if (dst[i] != i)
      __builtin_abort();
}

is miscompiled at -O2 since vectorization was enabled at -O2. With -O2
-ftree-vectorize, it is miscompiled earlier, starting with:

commit 046a3beb1673bf4a61c131373b6a5e84158e92bf
Author: Christophe Lyon <christophe.lyon@linaro.org>
Date:   Thu Jun 3 15:35:50 2021

    arm: Auto-vectorization for MVE: add pack/unpack patterns

It looks like we do some dubious packing of vector elements before storing to
src. If I change the last loop to print the elements of dst instead, I see:

0 8 4 12 1 9 5

it should of course print:

0 1 2 3 4 5 6.

The broken code is attached. The testcase above was reduced from
gcc/testsuite/gcc.c-torture/execute/memcpy-1.c.