Add a target-specific DAG combine on X86 to fold the common pattern of

fence-atomic-fence down to just the atomic op.  This is possible thanks to
X86's relatively strong memory model, which guarantees that locked instructions
(which are used to implement atomics) are implicit fences.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74435 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 54 insertions, 0 deletions

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index c78c9c6341..746a1ad959 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -917,6 +917,7 @@ X86TargetLowering::X86TargetLowering(X86TargetMachine &TM)
   setTargetDAGCombine(ISD::SRA);
   setTargetDAGCombine(ISD::SRL);
   setTargetDAGCombine(ISD::STORE);
+  setTargetDAGCombine(ISD::MEMBARRIER);
   if (Subtarget->is64Bit())
     setTargetDAGCombine(ISD::MUL);
 
@@ -8566,6 +8567,58 @@ static SDValue PerformVZEXT_MOVLCombine(SDNode *N, SelectionDAG &DAG) {
   return SDValue();
 }
 
+// On X86 and X86-64, atomic operations are lowered to locked instructions.
+// Locked instructions, in turn, have implicit fence semantics (all memory
+// operations are flushed before issuing the locked instruction, and the
+// are not buffered), so we can fold away the common pattern of 
+// fence-atomic-fence.
+static SDValue PerformMEMBARRIERCombine(SDNode* N, SelectionDAG &DAG) {
+  SDValue atomic = N->getOperand(0);
+  switch (atomic.getOpcode()) {
+    case ISD::ATOMIC_CMP_SWAP:
+    case ISD::ATOMIC_SWAP:
+    case ISD::ATOMIC_LOAD_ADD:
+    case ISD::ATOMIC_LOAD_SUB:
+    case ISD::ATOMIC_LOAD_AND:
+    case ISD::ATOMIC_LOAD_OR:
+    case ISD::ATOMIC_LOAD_XOR:
+    case ISD::ATOMIC_LOAD_NAND:
+    case ISD::ATOMIC_LOAD_MIN:
+    case ISD::ATOMIC_LOAD_MAX:
+    case ISD::ATOMIC_LOAD_UMIN:
+    case ISD::ATOMIC_LOAD_UMAX:
+      break;
+    default:
+      return SDValue();
+  }
+  
+  SDValue fence = atomic.getOperand(0);
+  if (fence.getOpcode() != ISD::MEMBARRIER)
+    return SDValue();
+  
+  switch (atomic.getOpcode()) {
+    case ISD::ATOMIC_CMP_SWAP:
+      return DAG.UpdateNodeOperands(atomic, fence.getOperand(0),
+                                    atomic.getOperand(1), atomic.getOperand(2),
+                                    atomic.getOperand(3));
+    case ISD::ATOMIC_SWAP:
+    case ISD::ATOMIC_LOAD_ADD:
+    case ISD::ATOMIC_LOAD_SUB:
+    case ISD::ATOMIC_LOAD_AND:
+    case ISD::ATOMIC_LOAD_OR:
+    case ISD::ATOMIC_LOAD_XOR:
+    case ISD::ATOMIC_LOAD_NAND:
+    case ISD::ATOMIC_LOAD_MIN:
+    case ISD::ATOMIC_LOAD_MAX:
+    case ISD::ATOMIC_LOAD_UMIN:
+    case ISD::ATOMIC_LOAD_UMAX:
+      return DAG.UpdateNodeOperands(atomic, fence.getOperand(0),
+                                    atomic.getOperand(1), atomic.getOperand(2));
+    default:
+      return SDValue();
+  }
+}
+
 SDValue X86TargetLowering::PerformDAGCombine(SDNode *N,
                                              DAGCombinerInfo &DCI) const {
   SelectionDAG &DAG = DCI.DAG;
@@ -8584,6 +8637,7 @@ SDValue X86TargetLowering::PerformDAGCombine(SDNode *N,
   case X86ISD::FAND:        return PerformFANDCombine(N, DAG);
   case X86ISD::BT:          return PerformBTCombine(N, DAG, DCI);
   case X86ISD::VZEXT_MOVL:  return PerformVZEXT_MOVLCombine(N, DAG);
+  case ISD::MEMBARRIER:     return PerformMEMBARRIERCombine(N, DAG);
   }
 
   return SDValue();