MachineBlockPlacement: Strengthen the source order bias when picking an exit block.

[oota-llvm.git] / lib / CodeGen / MachineBlockPlacement.cpp

diff --git a/lib/CodeGen/MachineBlockPlacement.cpp b/lib/CodeGen/MachineBlockPlacement.cpp
index 4b0f7f38fcc9ecef0426a370936d92e3dd659092..00a309972a52caef715a6408e5eaee5543ccab4e 100644 (file)
--- a/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/lib/CodeGen/MachineBlockPlacement.cpp
@@ -58,6 +58,13 @@ static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
                                                 "blocks in the function."),
                                        cl::init(0), cl::Hidden);
 
+// FIXME: Find a good default for this flag and remove the flag.
+static cl::opt<unsigned>
+ExitBlockBias("block-placement-exit-block-bias",
+              cl::desc("Block frequency percentage a loop exit block needs "
+                       "over the original exit to be considered the new exit."),
+              cl::init(0), cl::Hidden);
+
 namespace {
 class BlockChain;
 /// \brief Type for our function-wide basic block -> block chain mapping.
@@ -691,13 +698,15 @@ MachineBlockPlacement::findBestLoopExit(MachineFunction &F,
       DEBUG(dbgs() << "    exiting: " << getBlockName(*I) << " -> "
                    << getBlockName(*SI) << " [L:" << SuccLoopDepth
                    << "] (" << ExitEdgeFreq << ")\n");
-      // Note that we slightly bias this toward an existing layout successor to
-      // retain incoming order in the absence of better information.
-      // FIXME: Should we bias this more strongly? It's pretty weak.
+      // Note that we bias this toward an existing layout successor to retain
+      // incoming order in the absence of better information. The exit must have
+      // a frequency higher than the current exit before we consider breaking
+      // the layout.
+      BranchProbability Bias(100 - ExitBlockBias, 100);
       if (!ExitingBB || BestExitLoopDepth < SuccLoopDepth ||
           ExitEdgeFreq > BestExitEdgeFreq ||
           ((*I)->isLayoutSuccessor(*SI) &&
-           !(ExitEdgeFreq < BestExitEdgeFreq))) {
+           !(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
         BestExitEdgeFreq = ExitEdgeFreq;
         ExitingBB = *I;
       }