[C11-atomic] new test: limit precomputing values across acquire barriers

[C11-atomic] new test: limit precomputing values across acquire barriers

[Aldy Hernandez]

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This is a similar test to the previous acquire test.  Here we are 
incorrectly caching 'x' and failing to reload it after the __ATOMIC_ACQUIRE.

+  i = x + y;
+
+  if (__atomic_load_n (&flag, __ATOMIC_ACQUIRE))
+    {
+      /* x here should not be reused from above.  */
+      k = x;
+    }

Note that there is technically a data race on the load of x+y.  See the 
explanation on my previous testcase.

OK for branch?

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Index: testsuite/gcc.dg/simulate-thread/atomic-hoist-2.c
===================================================================
--- testsuite/gcc.dg/simulate-thread/atomic-hoist-2.c	(revision 0)
+++ testsuite/gcc.dg/simulate-thread/atomic-hoist-2.c	(revision 0)
@@ -0,0 +1,60 @@
+/* { dg-do link } */
+/* { dg-require-effective-target sync_int_long } */
+/* { dg-final { simulate-thread } } */
+
+/* Test that a load is not precomputed before an acquire.  */
+
+#include <stdio.h>
+#include "simulate-thread.h"
+
+int flag=0;
+int x = 0, y = 10, i = 0, k = -1;
+
+__attribute__((noinline))
+void simulate_thread_main()
+{
+  /* Test that the first load of x is not cached and reused in the second
+     load of x.  */
+
+  /* Note: Technically this first load of x/y is a data race.  See
+     note on atomic-hoist-1.c.  */
+  i = x + y;
+
+  if (__atomic_load_n (&flag, __ATOMIC_ACQUIRE))
+    {
+      /* x here should not be reused from above.  */
+      k = x;
+    }
+}
+
+void simulate_thread_other_threads ()
+{
+  /* Once i has been calculated in thread 1, change the value of x.  */
+  if (i != 0)
+    {
+      x = -1;
+      flag = 1;
+    }
+}
+
+int simulate_thread_step_verify ()
+{
+  return 0;
+}
+
+int simulate_thread_final_verify ()
+{
+  if (k != -1)
+    {
+      printf("FAIL: k != -1\n");
+      return 1;
+    }
+  return 0;
+}
+
+main()
+{
+  simulate_thread_main ();
+  simulate_thread_done ();
+  return 0;
+}