2 files changed, 52 insertions, 1 deletions
diff --git a/gcc/testsuite/gcc.target/s390/builtin-constant-p-threading.c b/gcc/testsuite/gcc.target/s390/builtin-constant-p-threading.c
new file mode 100644
index 0000000..5f0acdc
--- /dev/null
+++ b/gcc/testsuite/gcc.target/s390/builtin-constant-p-threading.c
@@ -0,0 +1,46 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=z196 -mzarch" } */
+
+typedef struct
+{
+  int counter;
+} atomic_t;
+
+static inline __attribute__ ((__gnu_inline__)) int
+__atomic_add (int val, int *ptr)
+{
+  int old;
+  asm volatile("laa %[old],%[val],%[ptr]\n"
+	       : [old] "=d" (old), [ptr] "+Q"(*ptr)
+	       : [val] "d" (val)
+	       : "cc", "memory");
+  return old;
+}
+
+static inline __attribute__ ((__gnu_inline__)) void
+__atomic_add_const (int val, int *ptr)
+{
+  asm volatile("asi %[ptr],%[val]\n"
+	       : [ptr] "+Q" (*ptr)
+	       : [val] "i" (val)
+	       : "cc", "memory");
+}
+
+static inline __attribute__ ((__gnu_inline__)) void
+atomic_add (int i, atomic_t *v)
+{
+  if (__builtin_constant_p (i) && (i > -129) && (i < 128))
+    {
+      __atomic_add_const (i, &v->counter);
+      return;
+    }
+  __atomic_add (i, &v->counter);
+}
+
+static atomic_t num_active_cpus = { (0) };
+
+void
+ledtrig_cpu (_Bool is_active)
+{
+  atomic_add (is_active ? 1 : -1, &num_active_cpus);
+}
diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 327628f..30f6926 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -259,8 +259,13 @@ thread_jumps::profitable_jump_thread_path (basic_block bbi, tree name,
 	       !gsi_end_p (gsi);
 	       gsi_next_nondebug (&gsi))
 	    {
+	      /* Do not allow OpenACC loop markers and __builtin_constant_p on
+		 threading paths.  The latter is disallowed, because an
+		 expression might be constant on two threading paths, and
+		 become non-constant (i.e.: phi) when they merge.  */
 	      gimple *stmt = gsi_stmt (gsi);
-	      if (gimple_call_internal_p (stmt, IFN_UNIQUE))
+	      if (gimple_call_internal_p (stmt, IFN_UNIQUE)
+		  || gimple_call_builtin_p (stmt, BUILT_IN_CONSTANT_P))
 		{
 		  m_path.pop ();
 		  return NULL;