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, 6 insertions, 1 deletions

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 34b7df59acd..c63a88b9d77 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -3400,7 +3400,7 @@ void SelectionDAGBuilder::visitLoad(const LoadInst &I) {
 
   SDValue Root;
   bool ConstantMemory = false;
-  if (I.isVolatile() || NumValues > MaxParallelChains)
+  if (isVolatile || NumValues > MaxParallelChains)
     // Serialize volatile loads with other side effects.
     Root = getRoot();
   else if (AA->pointsToConstantMemory(
@@ -3413,6 +3413,10 @@ void SelectionDAGBuilder::visitLoad(const LoadInst &I) {
     Root = DAG.getRoot();
   }
 
+  const TargetLowering *TLI = TM.getTargetLowering();
+  if (isVolatile)
+    Root = TLI->prepareVolatileOrAtomicLoad(Root, getCurSDLoc(), DAG);
+
   SmallVector<SDValue, 4> Values(NumValues);
   SmallVector<SDValue, 4> Chains(std::min(unsigned(MaxParallelChains),
                                           NumValues));
@@ -3637,6 +3641,7 @@ void SelectionDAGBuilder::visitAtomicLoad(const LoadInst &I) {
   if (I.getAlignment() < VT.getSizeInBits() / 8)
     report_fatal_error("Cannot generate unaligned atomic load");
 
+  InChain = TLI->prepareVolatileOrAtomicLoad(InChain, dl, DAG);
   SDValue L =
     DAG.getAtomic(ISD::ATOMIC_LOAD, dl, VT, VT, InChain,
                   getValue(I.getPointerOperand()),