[CHR] Add a threshold for the code duplication

ControlHeightReduction (CHR) clones the code region to reduce the
branches in the hot code path. The number of clones is linear to the
depth of the region.

Currently it does not have control over the code size increase. We are
seeing one ~9000 BB functions get expanded to ~250000 BBs, an 25x
increase. This creates a big compile time issue for the downstream
optimizations.

This patch adds a cap for number of clones for one region.

Differential Revision: https://reviews.llvm.org/D138333

1 files changed, 51 insertions, 15 deletions

diff --git a/llvm/lib/Transforms/Instrumentation/ControlHeightReduction.cpp b/llvm/lib/Transforms/Instrumentation/ControlHeightReduction.cpp
index 8ecf2cf..aee9718 100644
--- a/llvm/lib/Transforms/Instrumentation/ControlHeightReduction.cpp
+++ b/llvm/lib/Transforms/Instrumentation/ControlHeightReduction.cpp
@@ -47,6 +47,9 @@ using namespace llvm;
 
 #define CHR_DEBUG(X) LLVM_DEBUG(X)
 
+static cl::opt<bool> DisableCHR("disable-chr", cl::init(false), cl::Hidden,
+                                cl::desc("Disable CHR for all functions"));
+
 static cl::opt<bool> ForceCHR("force-chr", cl::init(false), cl::Hidden,
                               cl::desc("Apply CHR for all functions"));
 
@@ -66,6 +69,10 @@ static cl::opt<std::string> CHRFunctionList(
     "chr-function-list", cl::init(""), cl::Hidden,
     cl::desc("Specify file to retrieve the list of functions to apply CHR to"));
 
+static cl::opt<unsigned> CHRDupThreshsold(
+    "chr-dup-threshold", cl::init(3), cl::Hidden,
+    cl::desc("Max number of duplications by CHR for a region"));
+
 static StringSet<> CHRModules;
 static StringSet<> CHRFunctions;
 
@@ -339,23 +346,27 @@ class CHR {
                                  BasicBlock *EntryBlock,
                                  BasicBlock *NewEntryBlock,
                                  ValueToValueMapTy &VMap);
-  void fixupBranchesAndSelects(CHRScope *Scope,
-                               BasicBlock *PreEntryBlock,
-                               BranchInst *MergedBR,
-                               uint64_t ProfileCount);
-  void fixupBranch(Region *R,
-                   CHRScope *Scope,
-                   IRBuilder<> &IRB,
+  void fixupBranchesAndSelects(CHRScope *Scope, BasicBlock *PreEntryBlock,
+                               BranchInst *MergedBR, uint64_t ProfileCount);
+  void fixupBranch(Region *R, CHRScope *Scope, IRBuilder<> &IRB,
                    Value *&MergedCondition, BranchProbability &CHRBranchBias);
-  void fixupSelect(SelectInst* SI,
-                   CHRScope *Scope,
-                   IRBuilder<> &IRB,
+  void fixupSelect(SelectInst *SI, CHRScope *Scope, IRBuilder<> &IRB,
                    Value *&MergedCondition, BranchProbability &CHRBranchBias);
   void addToMergedCondition(bool IsTrueBiased, Value *Cond,
-                            Instruction *BranchOrSelect,
-                            CHRScope *Scope,
-                            IRBuilder<> &IRB,
-                            Value *&MergedCondition);
+                            Instruction *BranchOrSelect, CHRScope *Scope,
+                            IRBuilder<> &IRB, Value *&MergedCondition);
+  unsigned getRegionDuplicationCount(const Region *R) {
+    unsigned Count = 0;
+    // Find out how many times region R is cloned. Note that if the parent
+    // of R is cloned, R is also cloned, but R's clone count is not updated
+    // from the clone of the parent. We need to accumlate all the counts
+    // from the ancestors to get the clone count.
+    while (R) {
+      Count += DuplicationCount[R];
+      R = R->getParent();
+    }
+    return Count;
+  }
 
   Function &F;
   BlockFrequencyInfo &BFI;
@@ -379,6 +390,8 @@ class CHR {
   DenseMap<SelectInst *, BranchProbability> SelectBiasMap;
   // All the scopes.
   DenseSet<CHRScope *> Scopes;
+  // This maps records how many times this region is cloned.
+  DenseMap<const Region *, unsigned> DuplicationCount;
 };
 
 } // end anonymous namespace
@@ -396,7 +409,10 @@ raw_ostream &operator<<(raw_ostream &OS, const CHRScope &Scope) {
   return OS;
 }
 
-static bool shouldApply(Function &F, ProfileSummaryInfo& PSI) {
+static bool shouldApply(Function &F, ProfileSummaryInfo &PSI) {
+  if (DisableCHR)
+    return false;
+
   if (ForceCHR)
     return true;
 
@@ -1666,6 +1682,26 @@ void CHR::transformScopes(CHRScope *Scope, DenseSet<PHINode *> &TrivialPHIs) {
   CHR_DEBUG(dbgs() << "transformScopes " << *Scope << "\n");
 
   assert(Scope->RegInfos.size() >= 1 && "Should have at least one Region");
+
+  for (RegInfo &RI : Scope->RegInfos) {
+    const Region *R = RI.R;
+    unsigned Duplication = getRegionDuplicationCount(R);
+    dbgs() << "Dup count for R=" << R << "  is " << Duplication << "\n";
+    if (Duplication >= CHRDupThreshsold) {
+      CHR_DEBUG(dbgs() << "Reached the dup threshold of " << Duplication
+                       << " for this region");
+      ORE.emit([&]() {
+        return OptimizationRemarkMissed(DEBUG_TYPE, "DupThresholdReached",
+                                        R->getEntry()->getTerminator())
+               << "Reached the duplication threshold for the region";
+      });
+      return;
+    }
+  }
+  for (RegInfo &RI : Scope->RegInfos) {
+    DuplicationCount[RI.R]++;
+  }
+
   Region *FirstRegion = Scope->RegInfos[0].R;
   BasicBlock *EntryBlock = FirstRegion->getEntry();
   Region *LastRegion = Scope->RegInfos[Scope->RegInfos.size() - 1].R;