middle-end/118795 - fix can_vec_perm_const_p query in match.pd

When expanding to RTL we always use vec_perm_indices with two operands
which can make a difference with respect to supported vs. unsupported.
So the following adjusts a query in match.pd for target support which
got this "wrong" and using 1 for a equal operand permute.

	PR middle-end/118795
	* match.pd (vec_perm <vec_perm <a, b>> -> vec_perm <a, b>):
	Use the appropriate check to see whether the original
	outer permute was supported.

	* g++.dg/torture/pr118795.C: New testcase.

2 files changed, 16 insertions, 1 deletions

diff --git a/gcc/match.pd b/gcc/match.pd
index 98f6373..ba036e5 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -11121,7 +11121,7 @@ and,
     (with
      {
        vec_perm_indices sel0 (builder0, 2, nelts);
-       vec_perm_indices sel1 (builder1, 1, nelts);
+       vec_perm_indices sel1 (builder1, 2, nelts);
 
        for (int i = 0; i < nelts; i++)
 	 builder2.quick_push (sel0[sel1[i].to_constant ()]);
diff --git a/gcc/testsuite/g++.dg/torture/pr118795.C b/gcc/testsuite/g++.dg/torture/pr118795.C
new file mode 100644
index 0000000..fc40f81
--- /dev/null
+++ b/gcc/testsuite/g++.dg/torture/pr118795.C
@@ -0,0 +1,15 @@
+// { dg-do compile }
+
+unsigned char *a();
+struct b {
+  void c() const;
+};
+void b::c() const {
+  unsigned char *d = a(), *e = a();
+  for (long f; f; ++f) {
+    e[0] = e[1] = e[2] = d[0];
+    e[3] = d[0];
+    d += 4;
+    e += 4;
+  }
+}