[PATCH] Improve DSE to handle redundant zero initializations.

[Matthew Beliveau]

This patch improves DSE to handle missing optimizations for zero
initializations.

Thank you,
Matthew Beliveau

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

2019-08-12  Matthew Beliveau  <mbeli...@redhat.com>

	PR c++/DSE.patch
	* tree-ssa-dse.c (dse_optimize_redundant_stores): Improved check to
	catch more redundant zero initalization cases.
	(dse_dom_walker::dse_optimize_stmt): Improved check to catch more
	redundant zero initialization cases.

	* gcc.dg/tree-ssa/redundant-assign-zero-1.c: New test.
	* gcc.dg/tree-ssa/redundant-assign-zero-2.c: New test.

diff --git gcc/testsuite/gcc.dg/tree-ssa/redundant-assign-zero-1.c gcc/testsuite/gcc.dg/tree-ssa/redundant-assign-zero-1.c
new file mode 100644
index 00000000000..b8d01d1644b
--- /dev/null
+++ gcc/testsuite/gcc.dg/tree-ssa/redundant-assign-zero-1.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-dse-details" } */
+
+void blah (char *);
+
+void bar ()
+{
+  char a[256] = "";
+  a[3] = 0; 
+  blah (a);
+}
+
+/* { dg-final { scan-tree-dump-times "Deleted redundant store" 1 "dse1"} } */
diff --git gcc/testsuite/gcc.dg/tree-ssa/redundant-assign-zero-2.c gcc/testsuite/gcc.dg/tree-ssa/redundant-assign-zero-2.c
new file mode 100644
index 00000000000..8cefa6f0cb7
--- /dev/null
+++ gcc/testsuite/gcc.dg/tree-ssa/redundant-assign-zero-2.c
@@ -0,0 +1,18 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-dse-details" } */
+
+#include <string.h>
+
+void blahd (double *);
+
+void fubar ()
+{
+  double d;
+  double *x = &d;
+
+  memset (&d, 0 , sizeof d);
+  *x = 0.0;
+  blahd (x);
+}
+
+/* { dg-final { scan-tree-dump-times "Deleted redundant store" 1 "dse1"} } */
diff --git gcc/tree-ssa-dse.c gcc/tree-ssa-dse.c
index 5b7c4fc6d1a..dcaeb8edbfe 100644
--- gcc/tree-ssa-dse.c
+++ gcc/tree-ssa-dse.c
@@ -628,11 +628,8 @@ dse_optimize_redundant_stores (gimple *stmt)
       tree fndecl;
       if ((is_gimple_assign (use_stmt)
 	   && gimple_vdef (use_stmt)
-	   && ((gimple_assign_rhs_code (use_stmt) == CONSTRUCTOR
-	        && CONSTRUCTOR_NELTS (gimple_assign_rhs1 (use_stmt)) == 0
-	        && !gimple_clobber_p (stmt))
-	       || (gimple_assign_rhs_code (use_stmt) == INTEGER_CST
-		   && integer_zerop (gimple_assign_rhs1 (use_stmt)))))
+	   && initializer_zerop (gimple_op (use_stmt, 1), NULL)
+	   && !gimple_clobber_p (stmt))
 	  || (gimple_call_builtin_p (use_stmt, BUILT_IN_NORMAL)
 	      && (fndecl = gimple_call_fndecl (use_stmt)) != NULL
 	      && (DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET
@@ -1027,15 +1024,13 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
     {
       bool by_clobber_p = false;
 
-      /* First see if this store is a CONSTRUCTOR and if there
-	 are subsequent CONSTRUCTOR stores which are totally
-	 subsumed by this statement.  If so remove the subsequent
-	 CONSTRUCTOR store.
+      /* Check if this store initalizes zero, or some aggregate of zeros,
+	 and check if there are subsequent stores which are subsumed by this
+	 statement.  If so, remove the subsequent store.
 
 	 This will tend to make fewer calls into memset with longer
 	 arguments.  */
-      if (gimple_assign_rhs_code (stmt) == CONSTRUCTOR
-	  && CONSTRUCTOR_NELTS (gimple_assign_rhs1 (stmt)) == 0
+      if (initializer_zerop (gimple_op (stmt, 1), NULL)
 	  && !gimple_clobber_p (stmt))
 	dse_optimize_redundant_stores (stmt);