diff --git a/gcc/testsuite/c-c++-common/tm/20111206.c b/gcc/testsuite/c-c++-common/tm/20111206.c
new file mode 100644
index 0000000..74a5519
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/tm/20111206.c
@@ -0,0 +1,53 @@
+/* { dg-do compile } */
+/* { dg-options "-fgnu-tm -O2" } */
+/* This test case triggered block sharing between the two transactions.  */
+
+void func1 (void) __attribute__ ((transaction_callable, used));
+long func2 (void) __attribute__ ((transaction_callable, used));
+unsigned long rand (void);
+
+void client_run (void)
+{
+  long types[100];
+  long i;
+
+  for (i = 0; i < 100; i++)
+    {
+      long action = rand ();
+
+      switch (action)
+	{
+	case 0:
+	  {
+	    __transaction_relaxed
+	    {
+	      long bill = func2 ();
+	      if (bill >= 0)
+		{
+		  func1 ();
+		}
+	    }
+	    break;
+	  }
+
+	case 1:
+	  {
+	    long n;
+	    __transaction_relaxed
+	    {
+	      for (n = 0; n < 100; n++)
+		{
+		  long t = types[n];
+		  switch (t)
+		    {
+		    case 0:
+		      func1 ();
+		      break;
+		    }
+		}
+	    }
+	    break;
+	  }
+	}
+    }
+}
diff --git a/gcc/tree-ssa-tail-merge.c b/gcc/tree-ssa-tail-merge.c
index a501b07..7452266 100644
--- a/gcc/tree-ssa-tail-merge.c
+++ b/gcc/tree-ssa-tail-merge.c
@@ -1051,6 +1051,14 @@ gimple_equal_p (same_succ same_succ, gimple s1, gimple s2)
       if (!gimple_call_same_target_p (s1, s2))
         return false;
 
+      /* Eventually, we'll significantly complicate the CFG by adding
+	 back edges to properly model the effects of transaction restart.
+	 For the bulk of optimization this does not matter, but what we
+	 cannot recover from is tail merging blocks between two separate
+	 transactions.  Avoid that by making commit not match.  */
+      if (gimple_call_builtin_p (s1, BUILT_IN_TM_COMMIT))
+	return false;
+
       equal = true;
       for (i = 0; i < gimple_call_num_args (s1); ++i)
 	{