[CaptureTracking] Avoid long compilation time on large basic blocks

CaptureTracking becomes very expensive in large basic blocks while
calling PointerMayBeCaptured. PointerMayBeCaptured scans the BB the
number of times equal to the number of uses of 'BeforeHere', which is
currently capped at 20 and bails out with Tracker->tooManyUses().

The bottleneck here is the number of calls to PointerMayBeCaptured * the
basic block scan. In a testcase with a 82k instruction BB,
PointerMayBeCaptured is called 130k times, leading to 'shouldExplore'
taking 527k runs, this currently takes ~12min.

To fix this we locally (within PointerMayBeCaptured) number the
instructions in the basic block using a DenseMap to cache instruction
positions/numbers. We build the cache incrementally every time we need
to scan an unexplored part of the BB, improving compile time to only
take ~2min.

This triggers in the flow: DeadStoreElimination -> MepDepAnalysis ->
CaptureTracking.

Side note: after multiple runs in the test-suite I've seen no
performance nor compile time regressions, but could note a couple of
compile time improvements:

Performance Improvements - Compile Time Delta Previous  Current StdDev
SingleSource/Benchmarks/Misc-C++/bigfib -4.48%  0.8547  0.8164  0.0022
MultiSource/Benchmarks/TSVC/LoopRerolling-dbl/LoopRerolling-dbl -1.47% 1.3912  1.3707  0.0056

Differential Revision: http://reviews.llvm.org/D7010

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240560 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/CFG.h

@@ -78,6 +78,17 @@ bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To,
                             const DominatorTree *DT = nullptr,
                             const LoopInfo *LI = nullptr);
 
+/// \brief Determine whether there is at least one path from a block in
+/// 'Worklist' to 'StopBB', returning true if uncertain.
+///
+/// Determine whether there is a path from at least one block in Worklist to
+/// StopBB within a single function. Returns false only if we can prove that
+/// once any block in 'Worklist' has been reached then 'StopBB' can not be
+/// executed. Conservatively returns true.
+bool isPotentiallyReachableFromMany(SmallVectorImpl<BasicBlock *> &Worklist,
+                                    BasicBlock *StopBB,
+                                    const DominatorTree *DT = nullptr,
+                                    const LoopInfo *LI = nullptr);
 } // End llvm namespace
 
 #endif

lib/Analysis/CFG.cpp

@@ -126,10 +126,9 @@ static bool loopContainsBoth(const LoopInfo *LI,
   return L1 != nullptr && L1 == L2;
 }
 
-static bool isPotentiallyReachableInner(SmallVectorImpl<BasicBlock *> &Worklist,
-                                        BasicBlock *StopBB,
-                                        const DominatorTree *DT,
-                                        const LoopInfo *LI) {
+bool llvm::isPotentiallyReachableFromMany(
+    SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
+    const DominatorTree *DT, const LoopInfo *LI) {
   // When the stop block is unreachable, it's dominated from everywhere,
   // regardless of whether there's a path between the two blocks.
   if (DT && !DT->isReachableFromEntry(StopBB))
@@ -179,8 +178,8 @@ bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
   SmallVector<BasicBlock*, 32> Worklist;
   Worklist.push_back(const_cast<BasicBlock*>(A));
 
-  return isPotentiallyReachableInner(Worklist, const_cast<BasicBlock*>(B),
-                                     DT, LI);
+  return isPotentiallyReachableFromMany(Worklist, const_cast<BasicBlock *>(B),
+                                        DT, LI);
 }
 
 bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
@@ -230,7 +229,6 @@ bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
   if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
     return false;
 
-  return isPotentiallyReachableInner(Worklist,
-                                     const_cast<BasicBlock*>(B->getParent()),
-                                     DT, LI);
+  return isPotentiallyReachableFromMany(
+      Worklist, const_cast<BasicBlock *>(B->getParent()), DT, LI);
 }

lib/Analysis/CaptureTracking.cpp

@@ -52,56 +52,151 @@ namespace {
     bool Captured;
   };
 
+  struct NumberedInstCache {
+    SmallDenseMap<const Instruction *, unsigned, 32> NumberedInsts;
+    BasicBlock::const_iterator LastInstFound;
+    unsigned LastInstPos;
+    const BasicBlock *BB;
+
+    NumberedInstCache(const BasicBlock *BasicB) : LastInstPos(0), BB(BasicB) {
+      LastInstFound = BB->end();
+    }
+
+    /// \brief Find the first instruction 'A' or 'B' in 'BB'. Number out
+    /// instruction while walking 'BB'.
+    const Instruction *find(const Instruction *A, const Instruction *B) {
+      const Instruction *Inst = nullptr;
+      assert(!(LastInstFound == BB->end() && LastInstPos != 0) &&
+             "Instruction supposed to be in NumberedInsts");
+
+      // Start the search with the instruction found in the last lookup round.
+      auto II = BB->begin();
+      auto IE = BB->end();
+      if (LastInstFound != IE)
+        II = std::next(LastInstFound);
+
+      // Number all instructions up to the point where we find 'A' or 'B'.
+      for (++LastInstPos; II != IE; ++II, ++LastInstPos) {
+        Inst = cast<Instruction>(II);
+        NumberedInsts[Inst] = LastInstPos;
+        if (Inst == A || Inst == B)
+          break;
+      }
+
+      assert(II != IE && "Instruction not found?");
+      LastInstFound = II;
+      return Inst;
+    }
+
+    /// \brief Find out whether 'A' dominates 'B', meaning whether 'A'
+    /// comes before 'B' in 'BB'. This is a simplification that considers
+    /// cached instruction positions and ignores other basic blocks, being
+    /// only relevant to compare relative instructions positions inside 'BB'.
+    bool dominates(const Instruction *A, const Instruction *B) {
+      assert(A->getParent() == B->getParent() &&
+             "Instructions must be in the same basic block!");
+
+      unsigned NA = NumberedInsts.lookup(A);
+      unsigned NB = NumberedInsts.lookup(B);
+      if (NA && NB)
+        return NA < NB;
+      if (NA)
+        return true;
+      if (NB)
+        return false;
+
+      return A == find(A, B);
+    }
+  };
+
   /// Only find pointer captures which happen before the given instruction. Uses
   /// the dominator tree to determine whether one instruction is before another.
   /// Only support the case where the Value is defined in the same basic block
   /// as the given instruction and the use.
   struct CapturesBefore : public CaptureTracker {
+
     CapturesBefore(bool ReturnCaptures, const Instruction *I, DominatorTree *DT,
                    bool IncludeI)
-      : BeforeHere(I), DT(DT), ReturnCaptures(ReturnCaptures),
-        IncludeI(IncludeI), Captured(false) {}
+      : LocalInstCache(I->getParent()), BeforeHere(I), DT(DT),
+        ReturnCaptures(ReturnCaptures), IncludeI(IncludeI), Captured(false) {}
 
     void tooManyUses() override { Captured = true; }
 
-    bool shouldExplore(const Use *U) override {
-      Instruction *I = cast<Instruction>(U->getUser());
-      if (BeforeHere == I && !IncludeI)
-        return false;
-
+    bool isSafeToPrune(Instruction *I) {
       BasicBlock *BB = I->getParent();
       // We explore this usage only if the usage can reach "BeforeHere".
       // If use is not reachable from entry, there is no need to explore.
       if (BeforeHere != I && !DT->isReachableFromEntry(BB))
+        return true;
+
+      // Compute the case where both instructions are inside the same basic
+      // block. Since instructions in the same BB as BeforeHere are numbered in
+      // 'LocalInstCache', avoid using 'dominates' and 'isPotentiallyReachable'
+      // which are very expensive for large basic blocks.
+      if (BB == BeforeHere->getParent()) {
+        // 'I' dominates 'BeforeHere' => not safe to prune.
+        //
+        // The value defined by an invoke dominates an instruction only if it
+        // dominates every instruction in UseBB. A PHI is dominated only if
+        // the instruction dominates every possible use in the UseBB. Since
+        // UseBB == BB, avoid pruning.
+        if (isa<InvokeInst>(BeforeHere) || isa<PHINode>(I) || I == BeforeHere)
+          return false;
+        if (!LocalInstCache.dominates(BeforeHere, I))
+          return false;
+
+        // 'BeforeHere' comes before 'I', it's safe to prune if we also
+        // guarantee that 'I' never reaches 'BeforeHere' through a back-edge or
+        // by its successors, i.e, prune if:
+        //
+        //  (1) BB is an entry block or have no sucessors.
+        //  (2) There's no path coming back through BB sucessors.
+        if (BB == &BB->getParent()->getEntryBlock() ||
+            !BB->getTerminator()->getNumSuccessors())
+          return true;
+
+        SmallVector<BasicBlock*, 32> Worklist;
+        Worklist.append(succ_begin(BB), succ_end(BB));
+        if (!isPotentiallyReachableFromMany(Worklist, BB, DT))
+          return true;
+
         return false;
+      }
+
       // If the value is defined in the same basic block as use and BeforeHere,
       // there is no need to explore the use if BeforeHere dominates use.
       // Check whether there is a path from I to BeforeHere.
       if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
           !isPotentiallyReachable(I, BeforeHere, DT))
+        return true;
+
+      return false;
+    }
+
+    bool shouldExplore(const Use *U) override {
+      Instruction *I = cast<Instruction>(U->getUser());
+
+      if (BeforeHere == I && !IncludeI)
+        return false;
+
+      if (isSafeToPrune(I))
         return false;
+
       return true;
     }
 
     bool captured(const Use *U) override {
       if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
         return false;
 
-      Instruction *I = cast<Instruction>(U->getUser());
-      if (BeforeHere == I && !IncludeI)
+      if (!shouldExplore(U))
         return false;
 
-      BasicBlock *BB = I->getParent();
-      // Same logic as in shouldExplore.
-      if (BeforeHere != I && !DT->isReachableFromEntry(BB))
-        return false;
-      if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
-          !isPotentiallyReachable(I, BeforeHere, DT))
-        return false;
       Captured = true;
       return true;
     }
 
+    NumberedInstCache LocalInstCache;
     const Instruction *BeforeHere;
     DominatorTree *DT;