bitint: Handle VCE from large/huge _BitInt SSA_NAME from load [PR114156]

When adding checks in which case not to merge a VIEW_CONVERT_EXPR from
large/huge _BitInt to vector/complex etc., I missed the case of loads.
Those are handled differently later.
Anyway, I think the load case is something we can handle just fine,
so the following patch does that instead of preventing the merging
gimple_lower_bitint; we'd then copy from memory to memory and and do the
vce only on the second one, it is just better to vce the first one.

2024-03-01  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/114156
	* gimple-lower-bitint.cc (bitint_large_huge::lower_stmt): Allow
	rhs1 of a VCE to have no underlying variable if it is a load and
	handle that case.

	* gcc.dg/bitint-96.c: New test.

Author: jakubjelinek

gcc/gimple-lower-bitint.cc

@@ -5153,6 +5153,22 @@ bitint_large_huge::lower_stmt (gimple *stmt)
 		  gimple_assign_set_rhs1 (stmt, rhs1);
 		  gimple_assign_set_rhs_code (stmt, SSA_NAME);
 		}
+	      else if (m_names == NULL
+		       || !bitmap_bit_p (m_names, SSA_NAME_VERSION (rhs1)))
+		{
+		  gimple *g = SSA_NAME_DEF_STMT (rhs1);
+		  gcc_assert (gimple_assign_load_p (g));
+		  tree mem = gimple_assign_rhs1 (g);
+		  tree ltype = TREE_TYPE (lhs);
+		  addr_space_t as = TYPE_ADDR_SPACE (TREE_TYPE (mem));
+		  if (as != TYPE_ADDR_SPACE (ltype))
+		    ltype
+		      = build_qualified_type (ltype,
+					      TYPE_QUALS (ltype)
+					      | ENCODE_QUAL_ADDR_SPACE (as));
+		  rhs1 = build1 (VIEW_CONVERT_EXPR, ltype, mem);
+		  gimple_assign_set_rhs1 (stmt, rhs1);
+		}
 	      else
 		{
 		  int part = var_to_partition (m_map, rhs1);

gcc/testsuite/gcc.dg/bitint-96.c

@@ -0,0 +1,17 @@
+/* PR middle-end/114156 */
+/* { dg-do compile { target bitint } } */
+/* { dg-options "-O2" } */
+/* { dg-additional-options "-msse2" { target i?86-*-* x86_64-*-* } } */
+
+#if __BITINT_MAXWIDTH__ >= 128
+_BitInt(128) a, b;
+#else
+int a, b;
+#endif
+
+void
+foo (void)
+{
+  int u = b;
+  __builtin_memmove (&a, &b, sizeof (a));
+}