This backport contains 3 patchs from gcc main stream tree.
The commit id of these patchs list as following in the order of time.

0001-tree-ssa-dse.c-initialize_ao_ref_for_dse-Handle-_chk.patch
4a61cf9c62212fd04f21704efc2decffe9544651

0002-tree-ssa-dse.c-initialize_ao_ref_for_dse-Fix-formatt.patch
caffb6e56c2914e64e65f3c336b770c178f265a3

0003-tree-ssa-dse.c-initialize_ao_ref_for_dse-Handle-strn.patch
192ece9e15d25fd9b6534b2a8bd271684bf76d38

diff --git a/gcc/testsuite/gcc.c-torture/execute/builtins/builtins.exp b/gcc/testsuite/gcc.c-torture/execute/builtins/builtins.exp
index 3560a1ff2..e9d3c9aab 100644
--- a/gcc/testsuite/gcc.c-torture/execute/builtins/builtins.exp
+++ b/gcc/testsuite/gcc.c-torture/execute/builtins/builtins.exp
@@ -37,7 +37,7 @@ load_lib c-torture.exp
 torture-init
 set-torture-options $C_TORTURE_OPTIONS {{}} $LTO_TORTURE_OPTIONS
 
-set additional_flags "-fno-tree-loop-distribute-patterns -fno-tracer -fno-ipa-ra -fno-inline-functions"
+set additional_flags "-fno-tree-dse -fno-tree-loop-distribute-patterns -fno-tracer -fno-ipa-ra -fno-inline-functions"
 if [istarget "powerpc-*-darwin*"] {
    lappend additional_flags "-Wl,-multiply_defined,suppress"
 }
diff --git a/gcc/testsuite/gcc.dg/builtin-stringop-chk-1.c b/gcc/testsuite/gcc.dg/builtin-stringop-chk-1.c
index afd07ddd0..40cfa0472 100644
--- a/gcc/testsuite/gcc.dg/builtin-stringop-chk-1.c
+++ b/gcc/testsuite/gcc.dg/builtin-stringop-chk-1.c
@@ -1,7 +1,7 @@
 /* Test whether buffer overflow warnings for __*_chk builtins
    are emitted properly.  */
 /* { dg-do compile } */
-/* { dg-options "-O2 -Wno-format -std=gnu99 -ftrack-macro-expansion=0" } */
+/* { dg-options "-O2 -Wno-format -std=gnu99 -ftrack-macro-expansion=0 -fno-tree-dse" } */
 // { dg-skip-if "packed attribute missing for t" { "epiphany-*-*" } }
 
 extern void abort (void);
diff --git a/gcc/testsuite/gcc.dg/memcpy-2.c b/gcc/testsuite/gcc.dg/memcpy-2.c
index 7f839d27a..6ad887416 100644
--- a/gcc/testsuite/gcc.dg/memcpy-2.c
+++ b/gcc/testsuite/gcc.dg/memcpy-2.c
@@ -1,6 +1,6 @@
 /* PR middle-end/38454 */
 /* { dg-do compile } */
-/* { dg-options "-O2" } */
+/* { dg-options "-O2 -fno-tree-dse" } */
 
 typedef __SIZE_TYPE__ size_t;
 
diff --git a/gcc/testsuite/gcc.dg/pr40340-1.c b/gcc/testsuite/gcc.dg/pr40340-1.c
index 8fbb206a2..6307e064c 100644
--- a/gcc/testsuite/gcc.dg/pr40340-1.c
+++ b/gcc/testsuite/gcc.dg/pr40340-1.c
@@ -1,6 +1,6 @@
 /* PR middle-end/40340 */
 /* { dg-do compile } */
-/* { dg-options "-O2 -Wall -Wno-system-headers" } */
+/* { dg-options "-O2 -Wall -Wno-system-headers -fno-tree-dse" } */
 
 #include "pr40340.h"
 
diff --git a/gcc/testsuite/gcc.dg/pr40340-2.c b/gcc/testsuite/gcc.dg/pr40340-2.c
index 10083acd1..ea76e1008 100644
--- a/gcc/testsuite/gcc.dg/pr40340-2.c
+++ b/gcc/testsuite/gcc.dg/pr40340-2.c
@@ -1,6 +1,6 @@
 /* PR middle-end/40340 */
 /* { dg-do compile } */
-/* { dg-options "-O2 -Wall -Wno-system-headers" } */
+/* { dg-options "-O2 -Wall -Wno-system-headers -fno-tree-dse" } */
 
 #include "pr40340.h"
 
diff --git a/gcc/testsuite/gcc.dg/pr40340-5.c b/gcc/testsuite/gcc.dg/pr40340-5.c
index 0e48a2ca9..99e58f2ab 100644
--- a/gcc/testsuite/gcc.dg/pr40340-5.c
+++ b/gcc/testsuite/gcc.dg/pr40340-5.c
@@ -1,6 +1,6 @@
 /* PR middle-end/40340 */
 /* { dg-do compile } */
-/* { dg-options "-O2 -Wall -Wsystem-headers -g" } */
+/* { dg-options "-O2 -Wall -Wsystem-headers -g -fno-tree-dse" } */
 
 #define TEST3
 #include "pr40340.h"
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-37.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-37.c
new file mode 100644
index 000000000..56251fc34
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-37.c
@@ -0,0 +1,60 @@
+/* { dg-options "-O2 -fdump-tree-dse-details -fno-tree-fre" } */
+
+
+#ifndef SCOPE
+#define SCOPE
+#endif
+
+extern void frob (char *);
+
+void g (char *s)
+{
+  SCOPE char a[8];
+  __builtin_strncpy (a, s, sizeof a);
+  __builtin_memset (a, 0, sizeof a); 
+  frob (a);
+}
+
+void h (char *s)
+{
+  SCOPE char a[8];
+  __builtin_memset (a, 0, sizeof a); 
+  __builtin_strncpy (a, s, sizeof a);
+  frob (a);
+}
+
+void i (char *s)
+{
+  SCOPE char a[8];
+  __builtin_strncpy (a, s, sizeof a);
+  __builtin_memset (a, 0, sizeof a - 5); 
+  frob (a);
+}
+
+void j (char *s)
+{
+  SCOPE char a[8];
+  __builtin_memset (a, 0, sizeof a); 
+  __builtin_strncpy (a, s, sizeof a - 5);
+  frob (a);
+}
+
+void l (char *s)
+{
+  SCOPE char a[8];
+  __builtin_strncpy (a, s, sizeof a);
+  __builtin_memset (a + 2, 0, sizeof a - 2); 
+  frob (a);
+}
+
+void m (char *s)
+{
+  SCOPE char a[8];
+  __builtin_memset (a, 0, sizeof a); 
+  __builtin_strncpy (a + 2, s, sizeof a - 2);
+  frob (a);
+}
+
+/* { dg-final { scan-tree-dump-times "Deleted dead call" 2 "dse1" } } */
+/* { dg-final { scan-tree-dump-times "Trimming statement " 4 "dse1" } } */
+
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-38.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-38.c
new file mode 100644
index 000000000..7ae33bfd1
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-38.c
@@ -0,0 +1,12 @@
+/* { dg-options "-O2 -fdump-tree-dse-details -fno-tree-fre" } */
+
+
+/* This changes the scope of the destination object and exposes
+   missed optimizations in DSE.  */
+#define SCOPE extern
+#include "ssa-dse-37.c"
+
+/* { dg-final { scan-tree-dump-times "Deleted dead call" 2 "dse1" { xfail *-*-* } } } */
+/* { dg-final { scan-tree-dump-times "Trimming statement " 4 "dse1" { xfail *-*-* } } } */
+
+
diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
index 2fb471b69..b593b0d81 100644
--- a/gcc/tree-ssa-dse.c
+++ b/gcc/tree-ssa-dse.c
@@ -100,39 +100,42 @@ initialize_ao_ref_for_dse (gimple *stmt, ao_ref *write)
     {
       switch (DECL_FUNCTION_CODE (gimple_call_fndecl (stmt)))
 	{
-	  case BUILT_IN_MEMCPY:
-	  case BUILT_IN_MEMMOVE:
-	  case BUILT_IN_MEMSET:
-	    {
-	      tree size = NULL_TREE;
-	      if (gimple_call_num_args (stmt) == 3)
-		size = gimple_call_arg (stmt, 2);
-	      tree ptr = gimple_call_arg (stmt, 0);
-	      ao_ref_init_from_ptr_and_size (write, ptr, size);
-	      return true;
-	    }
+	case BUILT_IN_MEMCPY:
+	case BUILT_IN_MEMMOVE:
+	case BUILT_IN_MEMSET:
+	case BUILT_IN_MEMCPY_CHK:
+	case BUILT_IN_MEMMOVE_CHK:
+	case BUILT_IN_MEMSET_CHK:
+	case BUILT_IN_STRNCPY:
+	case BUILT_IN_STRNCPY_CHK:
+	  {
+	    tree size = gimple_call_arg (stmt, 2);
+	    tree ptr = gimple_call_arg (stmt, 0);
+	    ao_ref_init_from_ptr_and_size (write, ptr, size);
+	    return true;
+	  }
 
-	  /* A calloc call can never be dead, but it can make
-	     subsequent stores redundant if they store 0 into
-	     the same memory locations.  */
-	  case BUILT_IN_CALLOC:
-	    {
-	      tree nelem = gimple_call_arg (stmt, 0);
-	      tree selem = gimple_call_arg (stmt, 1);
-	      tree lhs;
-	      if (TREE_CODE (nelem) == INTEGER_CST
-		  && TREE_CODE (selem) == INTEGER_CST
-		  && (lhs = gimple_call_lhs (stmt)) != NULL_TREE)
-		{
-		  tree size = fold_build2 (MULT_EXPR, TREE_TYPE (nelem),
-					   nelem, selem);
-		  ao_ref_init_from_ptr_and_size (write, lhs, size);
-		  return true;
-		}
-	    }
+	/* A calloc call can never be dead, but it can make
+	   subsequent stores redundant if they store 0 into
+	   the same memory locations.  */
+	case BUILT_IN_CALLOC:
+	  {
+	    tree nelem = gimple_call_arg (stmt, 0);
+	    tree selem = gimple_call_arg (stmt, 1);
+	    tree lhs;
+	    if (TREE_CODE (nelem) == INTEGER_CST
+		&& TREE_CODE (selem) == INTEGER_CST
+		&& (lhs = gimple_call_lhs (stmt)) != NULL_TREE)
+	      {
+		tree size = fold_build2 (MULT_EXPR, TREE_TYPE (nelem),
+					 nelem, selem);
+		ao_ref_init_from_ptr_and_size (write, lhs, size);
+		return true;
+	      }
+	  }
 
-	  default:
-	    break;
+	default:
+	  break;
 	}
     }
   else if (is_gimple_assign (stmt))
@@ -459,6 +462,10 @@ maybe_trim_memstar_call (ao_ref *ref, sbitmap live, gimple *stmt)
     {
     case BUILT_IN_MEMCPY:
     case BUILT_IN_MEMMOVE:
+    case BUILT_IN_STRNCPY:
+    case BUILT_IN_MEMCPY_CHK:
+    case BUILT_IN_MEMMOVE_CHK:
+    case BUILT_IN_STRNCPY_CHK:
       {
 	int head_trim, tail_trim;
 	compute_trims (ref, live, &head_trim, &tail_trim, stmt);
@@ -480,6 +487,7 @@ maybe_trim_memstar_call (ao_ref *ref, sbitmap live, gimple *stmt)
       }
 
     case BUILT_IN_MEMSET:
+    case BUILT_IN_MEMSET_CHK:
       {
 	int head_trim, tail_trim;
 	compute_trims (ref, live, &head_trim, &tail_trim, stmt);
@@ -956,54 +964,60 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
       tree fndecl = gimple_call_fndecl (stmt);
       switch (DECL_FUNCTION_CODE (fndecl))
 	{
-	  case BUILT_IN_MEMCPY:
-	  case BUILT_IN_MEMMOVE:
-	  case BUILT_IN_MEMSET:
-	    {
-	      /* Occasionally calls with an explicit length of zero
-		 show up in the IL.  It's pointless to do analysis
-		 on them, they're trivially dead.  */
-	      tree size = gimple_call_arg (stmt, 2);
-	      if (integer_zerop (size))
-		{
-		  delete_dead_or_redundant_call (gsi, "dead");
-		  return;
-		}
-
-	      /* If this is a memset call that initializes an object
-		 to zero, it may be redundant with an earlier memset
-		 or empty CONSTRUCTOR of a larger object.  */
-	      if ((DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET
-		   || DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET_CHK)
-		  && integer_zerop (gimple_call_arg (stmt, 1)))
-		dse_optimize_redundant_stores (stmt);
-
-	      enum dse_store_status store_status;
-	      m_byte_tracking_enabled
-		= setup_live_bytes_from_ref (&ref, m_live_bytes);
-	      store_status = dse_classify_store (&ref, stmt,
-						 m_byte_tracking_enabled,
-						 m_live_bytes);
-	      if (store_status == DSE_STORE_LIVE)
-		return;
-
-	      if (store_status == DSE_STORE_MAYBE_PARTIAL_DEAD)
-		{
-		  maybe_trim_memstar_call (&ref, m_live_bytes, stmt);
-		  return;
-		}
-
-	      if (store_status == DSE_STORE_DEAD)
+	case BUILT_IN_MEMCPY:
+	case BUILT_IN_MEMMOVE:
+	case BUILT_IN_STRNCPY:
+	case BUILT_IN_MEMSET:
+	case BUILT_IN_MEMCPY_CHK:
+	case BUILT_IN_MEMMOVE_CHK:
+	case BUILT_IN_STRNCPY_CHK:
+	case BUILT_IN_MEMSET_CHK:
+	  {
+	    /* Occasionally calls with an explicit length of zero
+	       show up in the IL.  It's pointless to do analysis
+	       on them, they're trivially dead.  */
+	    tree size = gimple_call_arg (stmt, 2);
+	    if (integer_zerop (size))
+	      {
 		delete_dead_or_redundant_call (gsi, "dead");
+		return;
+	      }
+
+	    /* If this is a memset call that initializes an object
+	       to zero, it may be redundant with an earlier memset
+	       or empty CONSTRUCTOR of a larger object.  */
+	    if ((DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET
+		 || DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET_CHK)
+		&& integer_zerop (gimple_call_arg (stmt, 1)))
+	      dse_optimize_redundant_stores (stmt);
+
+	    enum dse_store_status store_status;
+	    m_byte_tracking_enabled
+	      = setup_live_bytes_from_ref (&ref, m_live_bytes);
+	    store_status = dse_classify_store (&ref, stmt,
+					       m_byte_tracking_enabled,
+					       m_live_bytes);
+	    if (store_status == DSE_STORE_LIVE)
 	      return;
-	    }
 
-	  case BUILT_IN_CALLOC:
-	    /* We already know the arguments are integer constants.  */
-	    dse_optimize_redundant_stores (stmt);
+	    if (store_status == DSE_STORE_MAYBE_PARTIAL_DEAD)
+	      {
+		maybe_trim_memstar_call (&ref, m_live_bytes, stmt);
+		return;
+	      }
 
-	  default:
+	    if (store_status == DSE_STORE_DEAD)
+	      delete_dead_or_redundant_call (gsi, "dead");
 	    return;
+	  }
+
+	case BUILT_IN_CALLOC:
+	  /* We already know the arguments are integer constants.  */
+	  dse_optimize_redundant_stores (stmt);
+	  return;
+
+	default:
+	  return;
 	}
     }