Using branch probability to guide critical edge splitting.

Summary:
The original heuristic to break critical edge during machine sink is relatively conservertive: when there is only one instruction sinkable to the critical edge, it is likely that the machine sink pass will not break the critical edge. This leads to many speculative instructions executed at runtime. However, with profile info, we could model the splitting benefits: if the critical edge has 50% taken rate, it would always be beneficial to split the critical edge to avoid the speculated runtime instructions. This patch uses profile to guide critical edge splitting in machine sink pass.

The performance impact on speccpu2006 on Intel sandybridge machines:

spec/2006/fp/C++/444.namd                  25.3  +0.26%
spec/2006/fp/C++/447.dealII               45.96  -0.10%
spec/2006/fp/C++/450.soplex               41.97  +1.49%
spec/2006/fp/C++/453.povray               36.83  -0.96%
spec/2006/fp/C/433.milc                   23.81  +0.32%
spec/2006/fp/C/470.lbm                    41.17  +0.34%
spec/2006/fp/C/482.sphinx3                48.13  +0.69%
spec/2006/int/C++/471.omnetpp             22.45  +3.25%
spec/2006/int/C++/473.astar               21.35  -2.06%
spec/2006/int/C++/483.xalancbmk           36.02  -2.39%
spec/2006/int/C/400.perlbench              33.7  -0.17%
spec/2006/int/C/401.bzip2                  22.9  +0.52%
spec/2006/int/C/403.gcc                   32.42  -0.54%
spec/2006/int/C/429.mcf                   39.59  +0.19%
spec/2006/int/C/445.gobmk                 26.98  -0.00%
spec/2006/int/C/456.hmmer                 24.52  -0.18%
spec/2006/int/C/458.sjeng                 28.26  +0.02%
spec/2006/int/C/462.libquantum            55.44  +3.74%
spec/2006/int/C/464.h264ref               46.67  -0.39%

geometric mean                                   +0.20%

Manually checked 473 and 471 to verify the diff is in the noise range.

Reviewers: rengolin, davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D24818

llvm-svn: 284541

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineSink.cpp b/llvm/lib/CodeGen/MachineSink.cpp
index d2650dbb9af..690ecc6baf2 100644
--- a/llvm/lib/CodeGen/MachineSink.cpp
+++ b/llvm/lib/CodeGen/MachineSink.cpp
@@ -24,6 +24,7 @@
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
+#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
@@ -60,6 +61,15 @@ UseBlockFreqInfo("machine-sink-bfi",
            cl::desc("Use block frequency info to find successors to sink"),
            cl::init(true), cl::Hidden);
 
+static cl::opt<unsigned> SplitEdgeProbabilityThreshold(
+    "machine-sink-split-probability-threshold",
+    cl::desc(
+        "Percentage threshold for splitting single-instruction critical edge. "
+        "If the branch threshold is higher than this threshold, we allow "
+        "speculative execution of up to 1 instruction to avoid branching to "
+        "splitted critical edge"),
+    cl::init(40), cl::Hidden);
+
 STATISTIC(NumSunk,      "Number of machine instructions sunk");
 STATISTIC(NumSplit,     "Number of critical edges split");
 STATISTIC(NumCoalesces, "Number of copies coalesced");
@@ -74,6 +84,7 @@ namespace {
     MachinePostDominatorTree *PDT; // Machine post dominator tree
     MachineLoopInfo *LI;
     const MachineBlockFrequencyInfo *MBFI;
+    const MachineBranchProbabilityInfo *MBPI;
     AliasAnalysis *AA;
 
     // Remember which edges have been considered for breaking.
@@ -105,6 +116,7 @@ namespace {
       AU.addRequired<MachineDominatorTree>();
       AU.addRequired<MachinePostDominatorTree>();
       AU.addRequired<MachineLoopInfo>();
+      AU.addRequired<MachineBranchProbabilityInfo>();
       AU.addPreserved<MachineDominatorTree>();
       AU.addPreserved<MachinePostDominatorTree>();
       AU.addPreserved<MachineLoopInfo>();
@@ -283,6 +295,7 @@ bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
   PDT = &getAnalysis<MachinePostDominatorTree>();
   LI = &getAnalysis<MachineLoopInfo>();
   MBFI = UseBlockFreqInfo ? &getAnalysis<MachineBlockFrequencyInfo>() : nullptr;
+  MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
   AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
 
   bool EverMadeChange = false;
@@ -383,6 +396,10 @@ bool MachineSinking::isWorthBreakingCriticalEdge(MachineInstr &MI,
   if (!MI.isCopy() && !TII->isAsCheapAsAMove(MI))
     return true;
 
+  if (MBPI->getEdgeProbability(From, To) <=
+      BranchProbability(SplitEdgeProbabilityThreshold, 100))
+    return true;
+
   // MI is cheap, we probably don't want to break the critical edge for it.
   // However, if this would allow some definitions of its source operands
   // to be sunk then it's probably worth it.