forwprop: Ignore scalar mode vectors in simplify_vector_constructor [PR95528]

As mentioned in the PR, the problem is that at least the x86 backend asumes
that the vec_unpack* and vec_pack* optabs with integral modes are for the
AVX512-ish vector masks rather than for very small vectors done in GPRs.
The only other target that seems to have a scalar mode vec_{,un}pack* optab
is aarch64 as discussed in the PR, so there is also a condition for that.
All other targets have just vector mode optabs.

2020-06-08  Jakub Jelinek  <jakub@redhat.com>

	PR target/95528
	* tree-ssa-forwprop.c (simplify_vector_constructor): Don't use
	VEC_UNPACK*_EXPR or VEC_PACK_TRUNC_EXPR with scalar modes unless the
	type is vector boolean.

	* g++.dg/opt/pr95528.C: New test.

(cherry picked from commit 8be374e02761c9d63d2753d71e4bd4874a1577b1)

gcc/testsuite/g++.dg/opt/pr95528.C

+// PR target/95528
+// { dg-do compile { target c++11 } }
+// { dg-options "-O3" }
+// { dg-additional-options "-march=skylake-avx512" { target i?86-*-*- x86_64-*-* } }
+
+template <typename a> struct b {
+  typedef a c __attribute__((vector_size(sizeof(a) * 4)));
+  union {
+    c d;
+    struct {
+      a e, f, g, h;
+    };
+  };
+  b();
+  b(const b &i) : d(i.d) {}
+  static b j(c);
+  template <typename k> operator b<k>() {
+    b<k>::j(typename b<k>::c{k(e), k(f), k(g), k(h)});
+    return b<k>();
+  }
+};
+template <typename a> using l = b<a>;
+using m = l<char>;
+using n = l<short>;
+m o(n i) { return i; }
+b<short> q;
+void p() { o(q); }

gcc/tree-ssa-forwprop.c

@@ -2401,6 +2401,10 @@ simplify_vector_constructor (gimple_stmt_iterator *gsi)
 	      && (dblvectype
 		  = build_vector_type (TREE_TYPE (TREE_TYPE (orig[0])),
 				       nelts * 2))
+	      /* Only use it for vector modes or for vector booleans represented
+		 as scalar bitmasks.  See PR95528.  */
+	      && (VECTOR_MODE_P (TYPE_MODE (dblvectype))
+		  || VECTOR_BOOLEAN_TYPE_P (dblvectype))
 	      && (optab = optab_for_tree_code (FLOAT_TYPE_P (TREE_TYPE (type))
 					       ? VEC_UNPACK_FLOAT_LO_EXPR
 					       : VEC_UNPACK_LO_EXPR,
@@ -2442,6 +2446,13 @@ simplify_vector_constructor (gimple_stmt_iterator *gsi)
 		   && (halfvectype
 		         = build_vector_type (TREE_TYPE (TREE_TYPE (orig[0])),
 					      nelts / 2))
+		   /* Only use it for vector modes or for vector booleans
+		      represented as scalar bitmasks, or allow halfvectype
+		      be the element mode.  See PR95528.  */
+		   && (VECTOR_MODE_P (TYPE_MODE (halfvectype))
+		       || VECTOR_BOOLEAN_TYPE_P (halfvectype)
+		       || (TYPE_MODE (halfvectype)
+			   == TYPE_MODE (TREE_TYPE (halfvectype))))
 		   && (optab = optab_for_tree_code (VEC_PACK_TRUNC_EXPR,
 						    halfvectype,
 						    optab_default))