Add TargetLowering::prepareVolatileOrAtomicLoad

One unusual feature of the z architecture is that the result of a
previous load can be reused indefinitely for subsequent loads, even if
a cache-coherent store to that location is performed by another CPU.
A special serializing instruction must be used if you want to force
a load to be reattempted.

Since volatile loads are not supposed to be omitted in this way,
we should insert a serializing instruction before each such load.
The same goes for atomic loads.

The patch implements this at the IR->DAG boundary, in a similar way
to atomic fences.  It is a no-op for targets other than SystemZ.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196905 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 12 insertions, 0 deletions

diff --git a/include/llvm/Target/TargetLowering.h b/include/llvm/Target/TargetLowering.h
index 53eea3b50f..57e142139b 100644
--- a/include/llvm/Target/TargetLowering.h
+++ b/include/llvm/Target/TargetLowering.h
@@ -2082,6 +2082,18 @@ public:
     return NULL;
   }
 
+  /// This callback is used to prepare for a volatile or atomic load.
+  /// It takes a chain node as input and returns the chain for the load itself.
+  ///
+  /// Having a callback like this is necessary for targets like SystemZ,
+  /// which allows a CPU to reuse the result of a previous load indefinitely,
+  /// even if a cache-coherent store is performed by another CPU.  The default
+  /// implementation does nothing.
+  virtual SDValue prepareVolatileOrAtomicLoad(SDValue Chain, SDLoc DL,
+                                              SelectionDAG &DAG) const {
+    return Chain;
+  }
+
   /// This callback is invoked by the type legalizer to legalize nodes with an
   /// illegal operand type but legal result types.  It replaces the
   /// LowerOperation callback in the type Legalizer.  The reason we can not do