[PATCH 4/4] Add warning for non-constant memory models

[Andi Kleen]

From: Andi Kleen <a...@linux.intel.com>

Non constant memory models drop HLE attribute bits. Since we had
problems with this even in gcc code itself, like libstdc++ <atomic>
passing variables when not optimizing, add a warning for this case.

Passed bootstrap and test on x86_64-linux.

gcc/:
2013-03-15  Andi Kleen  <a...@my.domain.org>

        PR target/55947
        * gcc/builtins.c (get_memmodel): Warn for non constant
        memory model.
---
 gcc/builtins.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/gcc/builtins.c b/gcc/builtins.c
index 68b6a2c..c4efc4c 100644
--- a/gcc/builtins.c
+++ b/gcc/builtins.c
@@ -5280,10 +5280,14 @@ get_memmodel (tree exp)
   rtx op;
   unsigned HOST_WIDE_INT val;
 
-  /* If the parameter is not a constant, it's a run time value so we'll just
-     convert it to MEMMODEL_SEQ_CST to avoid annoying runtime checking.  */
+  /* Warn about non constant memory models, as we drop target specific
+     bits (like HLE). */
   if (TREE_CODE (exp) != INTEGER_CST)
-    return MEMMODEL_SEQ_CST;
+    {
+      warning (OPT_Winvalid_memory_model,
+              "Non constant memory model: Assuming __ATOMIC_SEQ_CST");
+      return MEMMODEL_SEQ_CST;
+    }
 
   op = expand_normal (exp);
 
-- 
1.7.10.4