[llvm-commits] [llvm] r79708 - /llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

[Owen Anderson]

Author: resistor
Date: Fri Aug 21 19:29:12 2009
New Revision: 79708

URL: http://llvm.org/viewvc/llvm-project?rev=79708&view=rev
Log:
Ease contention on this lock by noticing that all writes to the VTs array will
be of (dynamically) constant values, so races on it are immaterial.  We just need
to ensure that at least one write has completed before return the pointer into it.

With this change, parllc exhibits essentially no overhead on 403.gcc.

Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp?rev=79708&r1=79707&r2=79708&view=diff

==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp Fri Aug 21 19:29:12 2009
@@ -5022,12 +5022,15 @@
 /// getValueTypeList - Return a pointer to the specified value type.
 ///
 const EVT *SDNode::getValueTypeList(EVT VT) {
-  sys::SmartScopedLock<true> Lock(*VTMutex);
   if (VT.isExtended()) {
     return &(*EVTs->insert(VT).first);
   } else {
+    // All writes to this location will have the same value, so it's ok
+    // to race on it.  We only need to ensure that at least one write has
+    // succeeded before we return the pointer into the array.
     VTs[VT.getSimpleVT().SimpleTy] = VT;
-    return &VTs[VT.getSimpleVT().SimpleTy];
+    sys::MemoryFence();
+    return VTs + VT.getSimpleVT().SimpleTy;
   }
 }