IfConversion: Rescan diamonds.

The cost of predicating a diamond is only the instructions that are not shared
between the two branches. Additionally If a predicate clobbering instruction
occurs in the shared portion of the branches (e.g. a cond move), it may still
be possible to if convert the sub-cfg. This change handles these two facts by
rescanning the non-shared portion of a diamond sub-cfg to recalculate both the
predication cost and whether both blocks are pred-clobbering.

llvm-svn: 279167

llvm/lib/CodeGen/IfConversion.cpp

@@ -149,11 +149,15 @@ namespace {
     struct IfcvtToken {
       BBInfo &BBI;
       IfcvtKind Kind;
-      bool NeedSubsumption;
       unsigned NumDups;
       unsigned NumDups2;
-      IfcvtToken(BBInfo &b, IfcvtKind k, bool s, unsigned d, unsigned d2 = 0)
+      bool NeedSubsumption : 1;
-        : BBI(b), Kind(k), NeedSubsumption(s), NumDups(d), NumDups2(d2) {}
+      bool TClobbersPred : 1;
+      bool FClobbersPred : 1;
+      IfcvtToken(BBInfo &b, IfcvtKind k, bool s, unsigned d, unsigned d2 = 0,
+                 bool tc = false, bool fc = false)
+        : BBI(b), Kind(k), NumDups(d), NumDups2(d2), NeedSubsumption(s),
+          TClobbersPred(tc), FClobbersPred(fc) {}
     };
 
     /// Results of if-conversion feasibility analysis indexed by basic block
@@ -203,15 +207,21 @@ namespace {
                        bool FalseBranch, unsigned &Dups,
                        BranchProbability Prediction) const;
     bool ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI,
-                      unsigned &Dups1, unsigned &Dups2) const;
+                      unsigned &Dups1, unsigned &Dups2,
+                      BBInfo &TrueBBICalc, BBInfo &FalseBBICalc) const;
     void AnalyzeBranches(BBInfo &BBI);
     void ScanInstructions(BBInfo &BBI,
                           MachineBasicBlock::iterator &Begin,
                           MachineBasicBlock::iterator &End) const;
+    bool RescanInstructions(
+        MachineBasicBlock::iterator &TIB, MachineBasicBlock::iterator &FIB,
+        MachineBasicBlock::iterator &TIE, MachineBasicBlock::iterator &FIE,
+        BBInfo &TrueBBI, BBInfo &FalseBBI) const;
     void AnalyzeBlock(MachineBasicBlock &MBB,
                       std::vector<std::unique_ptr<IfcvtToken>> &Tokens);
     bool FeasibilityAnalysis(BBInfo &BBI, SmallVectorImpl<MachineOperand> &Cond,
-                             bool isTriangle = false, bool RevBranch = false);
+                             bool isTriangle = false, bool RevBranch = false,
+                             bool hasCommonTail = false);
     void AnalyzeBlocks(MachineFunction &MF,
                        std::vector<std::unique_ptr<IfcvtToken>> &Tokens);
     void InvalidatePreds(MachineBasicBlock &MBB);
@@ -219,7 +229,8 @@ namespace {
     bool IfConvertSimple(BBInfo &BBI, IfcvtKind Kind);
     bool IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind);
     bool IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
-                          unsigned NumDups1, unsigned NumDups2);
+                          unsigned NumDups1, unsigned NumDups2,
+                          bool TClobbers, bool FClobbers);
     void PredicateBlock(BBInfo &BBI,
                         MachineBasicBlock::iterator E,
                         SmallVectorImpl<MachineOperand> &Cond,
@@ -406,7 +417,9 @@ bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
         DEBUG(dbgs() << "Ifcvt (Diamond): BB#" << BBI.BB->getNumber() << " (T:"
                      << BBI.TrueBB->getNumber() << ",F:"
                      << BBI.FalseBB->getNumber() << ") ");
-        RetVal = IfConvertDiamond(BBI, Kind, NumDups, NumDups2);
+        RetVal = IfConvertDiamond(BBI, Kind, NumDups, NumDups2,
+                                  Token->TClobbersPred,
+                                  Token->FClobbersPred);
         DEBUG(dbgs() << (RetVal ? "succeeded!" : "failed!") << "\n");
         if (RetVal) ++NumDiamonds;
         break;
@@ -677,10 +690,36 @@ static void countDuplicatedInstructions(
   }
 }
 
+/// RescanInstructions - Run ScanInstructions on a pair of blocks.
+/// @param TIB - True Iterator Begin, points to first non-shared instruction
+/// @param FIB - False Iterator Begin, points to first non-shared instruction
+/// @param TIE - True Iterator End, points past last non-shared instruction
+/// @param FIE - False Iterator End, points past last non-shared instruction
+/// @param TrueBBI  - BBInfo to update for the true block.
+/// @param FalseBBI - BBInfo to update for the false block.
+/// @returns - false if either block cannot be predicated or if both blocks end
+///   with a predicate-clobbering instruction.
+bool IfConverter::RescanInstructions(
+    MachineBasicBlock::iterator &TIB, MachineBasicBlock::iterator &FIB,
+    MachineBasicBlock::iterator &TIE, MachineBasicBlock::iterator &FIE,
+    BBInfo &TrueBBI, BBInfo &FalseBBI) const {
+  ScanInstructions(TrueBBI, TIB, TIE);
+  if (TrueBBI.IsUnpredicable)
+    return false;
+  ScanInstructions(FalseBBI, FIB, FIE);
+  if (FalseBBI.IsUnpredicable)
+    return false;
+  if (TrueBBI.ClobbersPred && FalseBBI.ClobbersPred)
+    return false;
+  return true;
+}
+
 /// ValidDiamond - Returns true if the 'true' and 'false' blocks (along
 /// with their common predecessor) forms a valid diamond shape for ifcvt.
-bool IfConverter::ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI,
+bool IfConverter::ValidDiamond(
-                               unsigned &Dups1, unsigned &Dups2) const {
+    BBInfo &TrueBBI, BBInfo &FalseBBI,
+    unsigned &Dups1, unsigned &Dups2,
+    BBInfo &TrueBBICalc, BBInfo &FalseBBICalc) const {
   Dups1 = Dups2 = 0;
   if (TrueBBI.IsBeingAnalyzed || TrueBBI.IsDone ||
       FalseBBI.IsBeingAnalyzed || FalseBBI.IsDone)
@@ -701,8 +740,7 @@ bool IfConverter::ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI,
     return false;
 
   // FIXME: Allow true block to have an early exit?
-  if (TrueBBI.FalseBB || FalseBBI.FalseBB ||
+  if (TrueBBI.FalseBB || FalseBBI.FalseBB)
-      (TrueBBI.ClobbersPred && FalseBBI.ClobbersPred))
     return false;
 
   // Count duplicate instructions at the beginning and end of the true and
@@ -714,6 +752,16 @@ bool IfConverter::ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI,
   countDuplicatedInstructions(TIB, FIB, TIE, FIE, Dups1, Dups2,
                               *TrueBBI.BB, *FalseBBI.BB,
                               /* SkipConditionalBranches */ true);
+
+  TrueBBICalc.BB = TrueBBI.BB;
+  FalseBBICalc.BB = FalseBBI.BB;
+  if (!RescanInstructions(TIB, FIB, TIE, FIE, TrueBBICalc, FalseBBICalc))
+    return false;
+  // The size is used to decide whether to if-convert, and the shared portions
+  // are subtracted off. Because of the subtraction, we just use the size that
+  // was calculated by the original ScanInstructions, as it is correct.
+  TrueBBICalc.NonPredSize = TrueBBI.NonPredSize;
+  FalseBBICalc.NonPredSize = FalseBBI.NonPredSize;
   return true;
 }
 
@@ -841,11 +889,22 @@ void IfConverter::ScanInstructions(BBInfo &BBI,
 
 /// Determine if the block is a suitable candidate to be predicated by the
 /// specified predicate.
+/// @param BBI BBInfo for the block to check
+/// @param Pred Predicate array for the branch that leads to BBI
+/// @param isTriangle true if the Analysis is for a triangle
+/// @param RevBranch true if Reverse(Pred) leads to BBI (e.g. BBI is the false
+///        case
+/// @param hasCommonTail true if BBI shares a tail with a sibling block that
+///        contains any instruction that would make the block unpredicable.
 bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
                                       SmallVectorImpl<MachineOperand> &Pred,
-                                      bool isTriangle, bool RevBranch) {
+                                      bool isTriangle, bool RevBranch,
+                                      bool hasCommonTail) {
   // If the block is dead or unpredicable, then it cannot be predicated.
-  if (BBI.IsDone || BBI.IsUnpredicable)
+  // Two blocks may share a common unpredicable tail, but this doesn't prevent
+  // them from being if-converted. The non-shared portion is assumed to have
+  // been checked
+  if (BBI.IsDone || (BBI.IsUnpredicable && !hasCommonTail))
     return false;
 
   // If it is already predicated but we couldn't analyze its terminator, the
@@ -859,7 +918,7 @@ bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
   if (BBI.Predicate.size() && !TII->SubsumesPredicate(Pred, BBI.Predicate))
     return false;
 
-  if (BBI.BrCond.size()) {
+  if (!hasCommonTail && BBI.BrCond.size()) {
     if (!isTriangle)
       return false;
 
@@ -966,14 +1025,23 @@ void IfConverter::AnalyzeBlock(
 
     BranchProbability Prediction = MBPI->getEdgeProbability(BB, TrueBBI.BB);
 
-    if (CanRevCond && ValidDiamond(TrueBBI, FalseBBI, Dups, Dups2) &&
+    if (CanRevCond) {
+      BBInfo TrueBBICalc, FalseBBICalc;
+      if (ValidDiamond(TrueBBI, FalseBBI, Dups, Dups2,
+                       TrueBBICalc, FalseBBICalc) &&
         MeetIfcvtSizeLimit(*TrueBBI.BB, (TrueBBI.NonPredSize - (Dups + Dups2) +
-                                         TrueBBI.ExtraCost), TrueBBI.ExtraCost2,
+                                         TrueBBICalc.ExtraCost),
+                           TrueBBICalc.ExtraCost2,
                            *FalseBBI.BB, (FalseBBI.NonPredSize - (Dups + Dups2) +
-                                        FalseBBI.ExtraCost),FalseBBI.ExtraCost2,
+                                          FalseBBICalc.ExtraCost),
+                           FalseBBICalc.ExtraCost2,
                            Prediction) &&
-        FeasibilityAnalysis(TrueBBI, BBI.BrCond) &&
+        FeasibilityAnalysis(TrueBBI, BBI.BrCond,
-        FeasibilityAnalysis(FalseBBI, RevCond)) {
+                            /* IsTriangle */ false, /* RevCond */ false,
+                            /* hasCommonTail */ true) &&
+        FeasibilityAnalysis(FalseBBI, RevCond,
+                            /* IsTriangle */ false, /* RevCond */ false,
+                            /* hasCommonTail */ true)) {
         // Diamond:
         //   EBB
         //   / \_
@@ -983,9 +1051,12 @@ void IfConverter::AnalyzeBlock(
         //  TailBB
         // Note TailBB can be empty.
         Tokens.push_back(llvm::make_unique<IfcvtToken>(
-          BBI, ICDiamond, TNeedSub | FNeedSub, Dups, Dups2));
+                BBI, ICDiamond, TNeedSub | FNeedSub, Dups, Dups2,
+                (bool) TrueBBICalc.ClobbersPred,
+                (bool) FalseBBICalc.ClobbersPred));
         Enqueued = true;
       }
+    }
 
     if (ValidTriangle(TrueBBI, FalseBBI, false, Dups, Prediction) &&
         MeetIfcvtSizeLimit(*TrueBBI.BB, TrueBBI.NonPredSize + TrueBBI.ExtraCost,
@@ -1429,8 +1500,18 @@ bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
 }
 
 /// If convert a diamond sub-CFG.
+/// \p BBI is the head of the diamond
+/// \p NumDups1 - number of shared instructions at the beginning of TrueBBI and
+///               FalseBBI
+/// \p NumDups2 - number of shared instructions at the end of TrueBBI and
+///               FalseBBI
+/// \p TClobbersPred - True if the true block clobbers the predicate in the
+///                    non-shared portion.
+/// \p TClobbersPred - True if the false block clobbers the predicate in the
+///                    non-shared portion.
 bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
-                                   unsigned NumDups1, unsigned NumDups2) {
+                                   unsigned NumDups1, unsigned NumDups2,
+                                   bool TClobbersPred, bool FClobbersPred) {
   BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
   BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
   MachineBasicBlock *TailBB = TrueBBI.TrueBB;
@@ -1468,9 +1549,9 @@ bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
 
   // Figure out the more profitable ordering.
   bool DoSwap = false;
-  if (TrueBBI.ClobbersPred && !FalseBBI.ClobbersPred)
+  if (TClobbersPred && !FClobbersPred)
     DoSwap = true;
-  else if (TrueBBI.ClobbersPred == FalseBBI.ClobbersPred) {
+  else if (TClobbersPred == FClobbersPred) {
     if (TrueBBI.NonPredSize > FalseBBI.NonPredSize)
       DoSwap = true;
   }