[PATCH] veclower: Fix up vec_shl matching of VEC_PERM_EXPR [PR100239]

Jakub Jelinek jakub@redhat.com
Tue Apr 27 13:05:02 GMT 2021

```Hi!

The following testcase ICEs at -O0, because lower_vec_perm sees the
_1 = { 0, 0, 0, 0, 0, 0, 0, 0 };
_2 = VEC_COND_EXPR <_1, { -1, -1, -1, -1, -1, -1, -1, -1 }, { 0, 0, 0, 0, 0, 0, 0, 0 }>;
_3 = { 6, 0, 0, 0, 0, 0, 0, 0 };
_4 = VEC_PERM_EXPR <{ 0, 0, 0, 0, 0, 0, 0, 0 }, _2, _3>;
and as the ISA is SSE2, there is no support for the particular permutation
nor for variable mask permutation.  But, the code to match vec_shl matches
it, because the permutation has the first operand a zero vector and the
mask picks all elements randomly from that vector.
So, in the end that isn't a vec_shl, but the permutation could be in theory
optimized into the first argument.  As we keep it as is, it will fail
during expansion though, because that for vec_shl correctly requires that
it actually is a shift:
unsigned firstidx = 0;
for (unsigned int i = 0; i < nelt; i++)
{
if (known_eq (sel[i], nelt))
{
if (i == 0 || firstidx)
return NULL_RTX;
firstidx = i;
}
else if (firstidx
? maybe_ne (sel[i], nelt + i - firstidx)
: maybe_ge (sel[i], nelt))
return NULL_RTX;
}

if (firstidx == 0)
return NULL_RTX;
first = firstidx;
The if (firstidx == 0) return NULL; is what is missing a counterpart
on the lower_vec_perm side.
As with optimize != 0 we fold it in other spots, I think it is not needed
to optimize this cornercase in lower_vec_perm (which would mean we'd need
to recurse on the newly created _4 = { 0, 0, 0, 0, 0, 0, 0, 0 };
whether it is supported or not).

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2021-04-27  Jakub Jelinek  <jakub@redhat.com>

PR tree-optimization/100239
* tree-vect-generic.c (lower_vec_perm): Don't accept constant
permutations with all indices from the first zero element as vec_shl.

* gcc.dg/pr100239.c: New test.

--- gcc/tree-vect-generic.c.jj	2021-01-27 19:30:20.763625450 +0100
+++ gcc/tree-vect-generic.c	2021-04-26 14:41:37.909432994 +0200
@@ -1515,7 +1515,7 @@ lower_vec_perm (gimple_stmt_iterator *gs
elements + i - first)
: maybe_ge (poly_uint64 (indices[i]), elements))
break;
-	  if (i == elements)
+	  if (first && i == elements)
{
update_stmt (stmt);
--- gcc/testsuite/gcc.dg/pr100239.c.jj	2021-04-26 14:45:28.517819255 +0200
+++ gcc/testsuite/gcc.dg/pr100239.c	2021-04-26 14:45:09.985029312 +0200
@@ -0,0 +1,12 @@
+/* PR tree-optimization/100239 */
+/* { dg-do compile } */
+/* { dg-options "-O0" } */
+
+typedef short __attribute__((__vector_size__ (8 * sizeof (short)))) V;
+V v, w;
+
+void
+foo (void)
+{
+  w = __builtin_shuffle (v != v, 0 < (V) {}, (V) {192} >> 5);
+}

Jakub

```