[DFAJumpThreading] Avoid exploring the paths that never come back (#85505)

This patch does:
- Preserve loop info when unfolding selects.
- Reduce the search space for loop paths.

Author: XChy

llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp

@@ -131,7 +131,7 @@ class SelectInstToUnfold {
   explicit operator bool() const { return SI && SIUse; }
 };
 
-void unfold(DomTreeUpdater *DTU, SelectInstToUnfold SIToUnfold,
+void unfold(DomTreeUpdater *DTU, LoopInfo *LI, SelectInstToUnfold SIToUnfold,
             std::vector<SelectInstToUnfold> *NewSIsToUnfold,
             std::vector<BasicBlock *> *NewBBs);
 
@@ -142,6 +142,7 @@ class DFAJumpThreading {
       : AC(AC), DT(DT), LI(LI), TTI(TTI), ORE(ORE) {}
 
   bool run(Function &F);
+  bool LoopInfoBroken;
 
 private:
   void
@@ -157,7 +158,7 @@ class DFAJumpThreading {
 
       std::vector<SelectInstToUnfold> NewSIsToUnfold;
       std::vector<BasicBlock *> NewBBs;
-      unfold(&DTU, SIToUnfold, &NewSIsToUnfold, &NewBBs);
+      unfold(&DTU, LI, SIToUnfold, &NewSIsToUnfold, &NewBBs);
 
       // Put newly discovered select instructions into the work list.
       for (const SelectInstToUnfold &NewSIToUnfold : NewSIsToUnfold)
@@ -201,7 +202,7 @@ void createBasicBlockAndSinkSelectInst(
 /// created basic blocks into \p NewBBs.
 ///
 /// TODO: merge it with CodeGenPrepare::optimizeSelectInst() if possible.
-void unfold(DomTreeUpdater *DTU, SelectInstToUnfold SIToUnfold,
+void unfold(DomTreeUpdater *DTU, LoopInfo *LI, SelectInstToUnfold SIToUnfold,
             std::vector<SelectInstToUnfold> *NewSIsToUnfold,
             std::vector<BasicBlock *> *NewBBs) {
   SelectInst *SI = SIToUnfold.getInst();
@@ -307,6 +308,12 @@ void unfold(DomTreeUpdater *DTU, SelectInstToUnfold SIToUnfold,
   DTU->applyUpdates({{DominatorTree::Insert, StartBlock, TT},
                      {DominatorTree::Insert, StartBlock, FT}});
 
+  // Preserve loop info
+  if (Loop *L = LI->getLoopFor(SI->getParent())) {
+    for (BasicBlock *NewBB : *NewBBs)
+      L->addBasicBlockToLoop(NewBB, *LI);
+  }
+
   // The select is now dead.
   assert(SI->use_empty() && "Select must be dead now");
   SI->eraseFromParent();
@@ -522,9 +529,10 @@ struct MainSwitch {
 };
 
 struct AllSwitchPaths {
-  AllSwitchPaths(const MainSwitch *MSwitch, OptimizationRemarkEmitter *ORE)
-      : Switch(MSwitch->getInstr()), SwitchBlock(Switch->getParent()),
-        ORE(ORE) {}
+  AllSwitchPaths(const MainSwitch *MSwitch, OptimizationRemarkEmitter *ORE,
+                 LoopInfo *LI)
+      : Switch(MSwitch->getInstr()), SwitchBlock(Switch->getParent()), ORE(ORE),
+        LI(LI) {}
 
   std::vector<ThreadingPath> &getThreadingPaths() { return TPaths; }
   unsigned getNumThreadingPaths() { return TPaths.size(); }
@@ -596,6 +604,12 @@ struct AllSwitchPaths {
 
     Visited.insert(BB);
 
+    // Stop if we have reached the BB out of loop, since its successors have no
+    // impact on the DFA.
+    // TODO: Do we need to stop exploring if BB is the outer loop of the switch?
+    if (!LI->getLoopFor(BB))
+      return Res;
+
     // Some blocks have multiple edges to the same successor, and this set
     // is used to prevent a duplicate path from being generated
     SmallSet<BasicBlock *, 4> Successors;
@@ -737,6 +751,7 @@ struct AllSwitchPaths {
   BasicBlock *SwitchBlock;
   OptimizationRemarkEmitter *ORE;
   std::vector<ThreadingPath> TPaths;
+  LoopInfo *LI;
 };
 
 struct TransformDFA {
@@ -1283,6 +1298,7 @@ bool DFAJumpThreading::run(Function &F) {
 
   SmallVector<AllSwitchPaths, 2> ThreadableLoops;
   bool MadeChanges = false;
+  LoopInfoBroken = false;
 
   for (BasicBlock &BB : F) {
     auto *SI = dyn_cast<SwitchInst>(BB.getTerminator());
@@ -1304,7 +1320,7 @@ bool DFAJumpThreading::run(Function &F) {
     if (!Switch.getSelectInsts().empty())
       MadeChanges = true;
 
-    AllSwitchPaths SwitchPaths(&Switch, ORE);
+    AllSwitchPaths SwitchPaths(&Switch, ORE, LI);
     SwitchPaths.run();
 
     if (SwitchPaths.getNumThreadingPaths() > 0) {
@@ -1315,10 +1331,15 @@ bool DFAJumpThreading::run(Function &F) {
       // strict requirement but it can cause buggy behavior if there is an
       // overlap of blocks in different opportunities. There is a lot of room to
       // experiment with catching more opportunities here.
+      // NOTE: To release this contraint, we must handle LoopInfo invalidation
       break;
     }
   }
 
+#ifdef NDEBUG
+  LI->verify(*DT);
+#endif
+
   SmallPtrSet<const Value *, 32> EphValues;
   if (ThreadableLoops.size() > 0)
     CodeMetrics::collectEphemeralValues(&F, AC, EphValues);
@@ -1327,6 +1348,7 @@ bool DFAJumpThreading::run(Function &F) {
     TransformDFA Transform(&SwitchPaths, DT, AC, TTI, ORE, EphValues);
     Transform.run();
     MadeChanges = true;
+    LoopInfoBroken = true;
   }
 
 #ifdef EXPENSIVE_CHECKS
@@ -1347,11 +1369,13 @@ PreservedAnalyses DFAJumpThreadingPass::run(Function &F,
   LoopInfo &LI = AM.getResult<LoopAnalysis>(F);
   TargetTransformInfo &TTI = AM.getResult<TargetIRAnalysis>(F);
   OptimizationRemarkEmitter ORE(&F);
-
-  if (!DFAJumpThreading(&AC, &DT, &LI, &TTI, &ORE).run(F))
+  DFAJumpThreading ThreadImpl(&AC, &DT, &LI, &TTI, &ORE);
+  if (!ThreadImpl.run(F))
     return PreservedAnalyses::all();
 
   PreservedAnalyses PA;
   PA.preserve<DominatorTreeAnalysis>();
+  if (!ThreadImpl.LoopInfoBroken)
+    PA.preserve<LoopAnalysis>();
   return PA;
 }