diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 9310c455addd3d374f92b8d31f376a05f63df395..c4edde6f55b586eba6c37198e00723de9557a7c1 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,11 @@
+2009-11-21  Alexandre Oliva  <aoliva@redhat.com>
+
+	PR tree-optimization/42078
+	* gimple.h (gimple_replace_lhs): New declaration.
+	* gimple.c (gimple_replace_lhs): New function.
+	* tree-ssa-math-opts.c (execute_cse_reciprocals): Call it before
+	modifying the call.
+
 2009-11-20  Sebastian Pop  <sebastian.pop@amd.com>
 
 	* config/i386/sse.md (*xop_pmacsdql_mem): Don't call reg_mentioned_p.
diff --git a/gcc/gimple.c b/gcc/gimple.c
index 9cec865a728b9156da2f93b7ed1e076519b99333..f84a20cd6c1a03e5ee3f10c72e06a0ae19580224 100644
--- a/gcc/gimple.c
+++ b/gcc/gimple.c
@@ -1981,6 +1981,39 @@ gimple_set_lhs (gimple stmt, tree lhs)
     gcc_unreachable();
 }
 
+/* Replace the LHS of STMT, an assignment, either a GIMPLE_ASSIGN or a
+   GIMPLE_CALL, with NLHS, in preparation for modifying the RHS to an
+   expression with a different value.
+
+   This will update any annotations (say debug bind stmts) referring
+   to the original LHS, so that they use the RHS instead.  This is
+   done even if NLHS and LHS are the same, for it is understood that
+   the RHS will be modified afterwards, and NLHS will not be assigned
+   an equivalent value.
+
+   Adjusting any non-annotation uses of the LHS, if needed, is a
+   responsibility of the caller.
+
+   The effect of this call should be pretty much the same as that of
+   inserting a copy of STMT before STMT, and then removing the
+   original stmt, at which time gsi_remove() would have update
+   annotations, but using this function saves all the inserting,
+   copying and removing.  */
+
+void
+gimple_replace_lhs (gimple stmt, tree nlhs)
+{
+  if (MAY_HAVE_DEBUG_STMTS)
+    {
+      tree lhs = gimple_get_lhs (stmt);
+
+      gcc_assert (SSA_NAME_DEF_STMT (lhs) == stmt);
+
+      insert_debug_temp_for_var_def (NULL, lhs);
+    }
+
+  gimple_set_lhs (stmt, nlhs);
+}
 
 /* Return a deep copy of statement STMT.  All the operands from STMT
    are reallocated and copied using unshare_expr.  The DEF, USE, VDEF
diff --git a/gcc/gimple.h b/gcc/gimple.h
index e956370bcd19708b19a3b2c1cc2ffc9f5e4cbbe0..f355ab1ba356672ed3c6ad1471514535090ac8bc 100644
--- a/gcc/gimple.h
+++ b/gcc/gimple.h
@@ -843,6 +843,7 @@ void gimple_assign_set_rhs_with_ops (gimple_stmt_iterator *, enum tree_code,
 				     tree, tree);
 tree gimple_get_lhs (const_gimple);
 void gimple_set_lhs (gimple, tree);
+void gimple_replace_lhs (gimple, tree);
 gimple gimple_copy (gimple);
 bool is_gimple_operand (const_tree);
 void gimple_set_modified (gimple, bool);
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index f88e1a44d262124087172645aa45ed09c708ae35..bb722a77ac3d14251015de381725fe9538c42259 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2009-11-21  Jakub Jelinek  <jakub@redhat.com>
+
+	PR tree-optimization/42078
+	* gcc.dg/pr42078.c: New test.
+
 2009-11-19  Andy Hutchinson  <hutchinsonandy@gcc.gnu.org>
 
 	PR Testsuite/42114
diff --git a/gcc/testsuite/gcc.dg/pr42078.c b/gcc/testsuite/gcc.dg/pr42078.c
new file mode 100644
index 0000000000000000000000000000000000000000..8107ff5557a5d86e3be40258bc928c6b806b82e5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr42078.c
@@ -0,0 +1,22 @@
+/* PR tree-optimization/42078 */
+/* { dg-do compile } */
+/* { dg-options "-g -O -ffast-math" } */
+
+double sqrt (double x);
+
+float
+foo (float x)
+{
+  float y = sqrt (x);
+  return x / y;
+}
+
+inline float
+bar (float x)
+{
+  float y = sqrt (x);
+  float a = y;
+  float b = y;
+  float c = y;
+  return x / y;
+}
diff --git a/gcc/tree-ssa-math-opts.c b/gcc/tree-ssa-math-opts.c
index c0ddc8afa305c90734e5ea1d01fa88ec49c42a0e..948707eb1a0e2f9091f3a167522ca99f373f5f85 100644
--- a/gcc/tree-ssa-math-opts.c
+++ b/gcc/tree-ssa-math-opts.c
@@ -563,6 +563,7 @@ execute_cse_reciprocals (void)
 		  if (fail)
 		    continue;
 
+		  gimple_replace_lhs (stmt1, arg1);
 		  gimple_call_set_fndecl (stmt1, fndecl);
 		  update_stmt (stmt1);