tree-dse: Fix mem* head trimming if call has lhs [PR94130]

As the testcase shows, if DSE decides to head trim {mem{set,cpy,move},strncpy}
and the call has lhs, it is incorrect to leave the lhs as is, because it
will then point to the adjusted address (base + head_trim) instead of the
original base.
The following patch fixes that by dropping the lhs of the call and assigning
lhs the original base in a following statement.

2020-03-12  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/94130
	* tree-ssa-dse.c: Include gimplify.h.
	(increment_start_addr): If stmt has lhs, drop the lhs from call and
	set it after the call to the original value of the first argument.
	Formatting fixes.
	(decrement_count): Formatting fix.

	* gcc.c-torture/execute/pr94130.c: New test.

gcc/ChangeLog

@@ -1,6 +1,15 @@
 2020-03-17  Jakub Jelinek  <jakub@redhat.com>
 
 	Backported from mainline
+	2020-03-12  Jakub Jelinek  <jakub@redhat.com>
+
+	PR tree-optimization/94130
+	* tree-ssa-dse.c: Include gimplify.h.
+	(increment_start_addr): If stmt has lhs, drop the lhs from call and
+	set it after the call to the original value of the first argument.
+	Formatting fixes.
+	(decrement_count): Formatting fix.
+
 	2020-03-11  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/94134

gcc/testsuite/ChangeLog

@@ -1,6 +1,11 @@
 2020-03-17  Jakub Jelinek  <jakub@redhat.com>
 
 	Backported from mainline
+	2020-03-12  Jakub Jelinek  <jakub@redhat.com>
+
+	PR tree-optimization/94130
+	* gcc.c-torture/execute/pr94130.c: New test.
+
 	2020-03-11  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/94134

gcc/testsuite/gcc.c-torture/execute/pr94130.c

@@ -0,0 +1,16 @@
+/* PR tree-optimization/94130 */
+
+int
+main ()
+{
+  int a[8];
+  char *b = __builtin_memset (a, 0, sizeof (a));
+  a[0] = 1;
+  a[1] = 2;
+  a[2] = 3;
+  if (b != (char *) a)
+    __builtin_abort ();
+  else
+    asm volatile ("" : : "g" (a) : "memory");
+  return 0;
+}

gcc/tree-ssa-dse.c

@@ -36,6 +36,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "params.h"
 #include "alias.h"
 #include "tree-ssa-loop.h"
+#include "gimplify.h"
 
 /* This file implements dead store elimination.
 
@@ -394,29 +395,38 @@ decrement_count (gimple *stmt, int decrement)
   gcc_assert (TREE_CODE (*countp) == INTEGER_CST);
   *countp = wide_int_to_tree (TREE_TYPE (*countp), (TREE_INT_CST_LOW (*countp)
 						    - decrement));
-
 }
 
 static void
 increment_start_addr (gimple *stmt, tree *where, int increment)
 {
+  if (tree lhs = gimple_call_lhs (stmt))
+    if (where == gimple_call_arg_ptr (stmt, 0))
+      {
+	gassign *newop = gimple_build_assign (lhs, unshare_expr (*where));
+	gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
+	gsi_insert_after (&gsi, newop, GSI_SAME_STMT);
+	gimple_call_set_lhs (stmt, NULL_TREE);
+	update_stmt (stmt);
+      }
+
   if (TREE_CODE (*where) == SSA_NAME)
     {
       tree tem = make_ssa_name (TREE_TYPE (*where));
       gassign *newop
-        = gimple_build_assign (tem, POINTER_PLUS_EXPR, *where,
+	= gimple_build_assign (tem, POINTER_PLUS_EXPR, *where,
 			       build_int_cst (sizetype, increment));
       gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
       gsi_insert_before (&gsi, newop, GSI_SAME_STMT);
       *where = tem;
-      update_stmt (gsi_stmt (gsi));
+      update_stmt (stmt);
       return;
     }
 
   *where = build_fold_addr_expr (fold_build2 (MEM_REF, char_type_node,
-                                             *where,
-                                             build_int_cst (ptr_type_node,
-                                                            increment)));
+					      *where,
+					      build_int_cst (ptr_type_node,
+							     increment)));
 }
 
 /* STMT is builtin call that writes bytes in bitmap ORIG, some bytes are dead