* tree-ssa-dse.c (live_bytes_read): New function.
	(dse_classify_store): Ignore reads of dead bytes.

	* testsuite/gcc.dg/tree-ssa/ssa-dse-26.c: New test.
	* testsuite/gcc.dg/tree-ssa/ssa-dse-26.c: Likewise.



diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c
new file mode 100644
index 0000000..6605dfe
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c
@@ -0,0 +1,33 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-dse1-details" } */
+
+enum constraint_expr_type
+{
+  SCALAR, DEREF, ADDRESSOF
+};
+typedef struct constraint_expr
+{
+  enum constraint_expr_type type;
+  unsigned int var;
+  long offset;
+} constraint_expr ;
+typedef struct constraint
+{
+  struct constraint_expr lhs;
+  struct constraint_expr rhs;
+} constraint;
+static _Bool
+constraint_expr_equal (struct constraint_expr x, struct constraint_expr y)
+{
+  return x.type == y.type && x.var == y.var && x.offset == y.offset;
+}
+
+_Bool
+constraint_equal (struct constraint a, struct constraint b)
+{
+  return constraint_expr_equal (a.lhs, b.lhs)
+    && constraint_expr_equal (a.rhs, b.rhs);
+}
+
+/* { dg-final { scan-tree-dump-times "Deleted dead store" 2 "dse1" } } */
+
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-27.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-27.c
new file mode 100644
index 0000000..48dc92e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-27.c
@@ -0,0 +1,21 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-dse1-details -fno-tree-fre -fno-tree-sra" } */
+
+struct S { struct R { int x; int y; } r; int z; } s;
+
+extern void blah (struct S);
+
+void
+foo ()
+{
+  struct S s = { {1, 2}, 3 };
+  s.r.x = 1;
+   s.r.y = 2;
+   struct R r = s.r;
+  s.z = 3;
+  blah (s);
+}
+
+
+/* { dg-final { scan-tree-dump-times "Deleted dead store" 4 "dse1" } } */
+
diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
index a807d6d..f5b53fc 100644
--- a/gcc/tree-ssa-dse.c
+++ b/gcc/tree-ssa-dse.c
@@ -475,6 +475,41 @@ maybe_trim_partially_dead_store (ao_ref *ref, sbitmap live, gimple *stmt)
     }
 }
 
+/* Return TRUE if USE_REF reads bytes from LIVE where live is
+   derived from REF, a write reference.
+
+   While this routine may modify USE_REF, it's passed by value, not
+   location.  So callers do not see those modifications.  */
+
+static bool
+live_bytes_read (ao_ref use_ref, ao_ref *ref, sbitmap live)
+{
+  /* We have already verified that USE_REF and REF hit the same object.
+     Now verify that there's actually an overlap between USE_REF and REF.  */
+  if ((use_ref.offset < ref->offset
+	&& use_ref.offset + use_ref.size > ref->offset)
+      || (use_ref.offset >= ref->offset
+	  && use_ref.offset < ref->offset + ref->size))
+    {
+      normalize_ref (&use_ref, ref);
+
+      /* If USE_REF covers all of REF, then it will hit one or more
+	 live bytes.   This avoids useless iteration over the bitmap
+	 below.  */
+      if (use_ref.offset == ref->offset && use_ref.size == ref->size)
+	return true;
+
+      /* Now iterate over what's left in USE_REF and see if any of
+	 those bits are i LIVE.  */
+      for (int i = (use_ref.offset - ref->offset) / BITS_PER_UNIT;
+	   i < (use_ref.offset + use_ref.size) / BITS_PER_UNIT; i++)
+	if (bitmap_bit_p (live, i))
+	  return true;
+      return false;
+    }
+  return true;
+}
+
 /* A helper of dse_optimize_stmt.
    Given a GIMPLE_ASSIGN in STMT that writes to REF, find a candidate
    statement *USE_STMT that may prove STMT to be dead.
@@ -554,6 +589,41 @@ dse_classify_store (ao_ref *ref, gimple *stmt, gimple **use_stmt,
 	  /* If the statement is a use the store is not dead.  */
 	  else if (ref_maybe_used_by_stmt_p (use_stmt, ref))
 	    {
+	      /* Handle common cases where we can easily build a ao_ref
+		 structure for USE_STMT and in doing so we find that the
+		 references hit non-live bytes and thus can be ignored.  */
+	      if (live_bytes)
+		{
+		  if (is_gimple_assign (use_stmt))
+		    {
+		      /* Other cases were noted as non-aliasing by
+			 the call to ref_maybe_used_by_stmt_p.  */
+		      ao_ref use_ref;
+		      ao_ref_init (&use_ref, gimple_assign_rhs1 (use_stmt));
+		      if (valid_ao_ref_for_dse (&use_ref)
+			  && use_ref.base == ref->base
+			  && use_ref.size == use_ref.max_size
+			  && !live_bytes_read (use_ref, ref, live_bytes))
+			{
+			  if (gimple_vdef (use_stmt))
+			    {
+			      /* If we have already seen a store and
+			         this is also a store, then we have to
+			         fail.  */
+			      if (temp)
+			        {
+			          fail = true;
+			          BREAK_FROM_IMM_USE_STMT (ui);
+			        }
+
+			      /* Otherwise walk through this store.  */
+			      temp = use_stmt;
+			    }
+			  continue;
+			}
+		    }
+		}
+
 	      fail = true;
 	      BREAK_FROM_IMM_USE_STMT (ui);
 	    }