[llvm] r328884 - [BlockPlacement] Disable block placement tail duplciation in structured CFG.

[Tim Shen via llvm-commits]

Author: timshen
Date: Fri Mar 30 10:51:00 2018
New Revision: 328884

URL: http://llvm.org/viewvc/llvm-project?rev=328884&view=rev
Log:
[BlockPlacement] Disable block placement tail duplciation in structured CFG.

Summary:
Tail duplication easily breaks the structure of CFG, e.g. duplicating on
a region entry. If the structure is intended to be preserved, then we
may want to configure tail duplication, or disable it for structured
CFG. From our benchmark results disabling it doesn't cause performance
regression.

Notice that this currently affects AMDGPU backend. In the next patch, I
also plan to turn on requiresStructuredCFG for NVPTX.

All unit tests still pass.

Reviewers: jlebar, arsenm

Subscribers: jholewinski, sanjoy, wdng, tpr, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D45008

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

Modified: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp?rev=328884&r1=328883&r2=328884&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Fri Mar 30 10:51:00 2018
@@ -513,6 +513,11 @@ public:
 
   bool runOnMachineFunction(MachineFunction &F) override;
 
+  bool allowTailDupPlacement() const {
+    assert(F);
+    return TailDupPlacement && !F->getTarget().requiresStructuredCFG();
+  }
+
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<MachineBranchProbabilityInfo>();
     AU.addRequired<MachineBlockFrequencyInfo>();
@@ -1018,7 +1023,7 @@ MachineBlockPlacement::getBestTrellisSuc
     MachineBasicBlock *Succ1 = BestA.Dest;
     MachineBasicBlock *Succ2 = BestB.Dest;
     // Check to see if tail-duplication would be profitable.
-    if (TailDupPlacement && shouldTailDuplicate(Succ2) &&
+    if (allowTailDupPlacement() && shouldTailDuplicate(Succ2) &&
         canTailDuplicateUnplacedPreds(BB, Succ2, Chain, BlockFilter) &&
         isProfitableToTailDup(BB, Succ2, MBPI->getEdgeProbability(BB, Succ1),
                               Chain, BlockFilter)) {
@@ -1044,7 +1049,7 @@ MachineBlockPlacement::getBestTrellisSuc
   return Result;
 }
 
-/// When the option TailDupPlacement is on, this method checks if the
+/// When the option allowTailDupPlacement() is on, this method checks if the
 /// fallthrough candidate block \p Succ (of block \p BB) can be tail-duplicated
 /// into all of its unplaced, unfiltered predecessors, that are not BB.
 bool MachineBlockPlacement::canTailDuplicateUnplacedPreds(
@@ -1493,7 +1498,7 @@ MachineBlockPlacement::selectBestSuccess
     if (hasBetterLayoutPredecessor(BB, Succ, SuccChain, SuccProb, RealSuccProb,
                                    Chain, BlockFilter)) {
       // If tail duplication would make Succ profitable, place it.
-      if (TailDupPlacement && shouldTailDuplicate(Succ))
+      if (allowTailDupPlacement() && shouldTailDuplicate(Succ))
         DupCandidates.push_back(std::make_tuple(SuccProb, Succ));
       continue;
     }
@@ -1702,7 +1707,7 @@ void MachineBlockPlacement::buildChain(
     auto Result = selectBestSuccessor(BB, Chain, BlockFilter);
     MachineBasicBlock* BestSucc = Result.BB;
     bool ShouldTailDup = Result.ShouldTailDup;
-    if (TailDupPlacement)
+    if (allowTailDupPlacement())
       ShouldTailDup |= (BestSucc && shouldTailDuplicate(BestSucc));
 
     // If an immediate successor isn't available, look for the best viable
@@ -1724,7 +1729,7 @@ void MachineBlockPlacement::buildChain(
 
     // Placement may have changed tail duplication opportunities.
     // Check for that now.
-    if (TailDupPlacement && BestSucc && ShouldTailDup) {
+    if (allowTailDupPlacement() && BestSucc && ShouldTailDup) {
       // If the chosen successor was duplicated into all its predecessors,
       // don't bother laying it out, just go round the loop again with BB as
       // the chain end.
@@ -2758,7 +2763,7 @@ bool MachineBlockPlacement::runOnMachine
       TailDupSize = TailDupPlacementAggressiveThreshold;
   }
 
-  if (TailDupPlacement) {
+  if (allowTailDupPlacement()) {
     MPDT = &getAnalysis<MachinePostDominatorTree>();
     if (MF.getFunction().optForSize())
       TailDupSize = 1;