Imported Upstream version 5.18.0.205

Former-commit-id: 7f59f7e792705db773f1caecdaa823092f4e2927

external/llvm/lib/Transforms/Utils/LCSSA.cpp

@@ -0,0 +1,439 @@
+//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass transforms loops by placing phi nodes at the end of the loops for
+// all values that are live across the loop boundary.  For example, it turns
+// the left into the right code:
+// 
+// for (...)                for (...)
+//   if (c)                   if (c)
+//     X1 = ...                 X1 = ...
+//   else                     else
+//     X2 = ...                 X2 = ...
+//   X3 = phi(X1, X2)         X3 = phi(X1, X2)
+// ... = X3 + 4             X4 = phi(X3)
+//                          ... = X4 + 4
+//
+// This is still valid LLVM; the extra phi nodes are purely redundant, and will
+// be trivially eliminated by InstCombine.  The major benefit of this 
+// transformation is that it makes many other loop optimizations, such as 
+// LoopUnswitching, simpler.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/LCSSA.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/BasicAliasAnalysis.h"
+#include "llvm/Analysis/GlobalsModRef.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/PredIteratorCache.h"
+#include "llvm/Pass.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/LoopUtils.h"
+#include "llvm/Transforms/Utils/SSAUpdater.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "lcssa"
+
+STATISTIC(NumLCSSA, "Number of live out of a loop variables");
+
+#ifdef EXPENSIVE_CHECKS
+static bool VerifyLoopLCSSA = true;
+#else
+static bool VerifyLoopLCSSA = false;
+#endif
+static cl::opt<bool, true>
+    VerifyLoopLCSSAFlag("verify-loop-lcssa", cl::location(VerifyLoopLCSSA),
+                        cl::Hidden,
+                        cl::desc("Verify loop lcssa form (time consuming)"));
+
+/// Return true if the specified block is in the list.
+static bool isExitBlock(BasicBlock *BB,
+                        const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
+  return is_contained(ExitBlocks, BB);
+}
+
+/// For every instruction from the worklist, check to see if it has any uses
+/// that are outside the current loop.  If so, insert LCSSA PHI nodes and
+/// rewrite the uses.
+bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
+                                    DominatorTree &DT, LoopInfo &LI) {
+  SmallVector<Use *, 16> UsesToRewrite;
+  SmallSetVector<PHINode *, 16> PHIsToRemove;
+  PredIteratorCache PredCache;
+  bool Changed = false;
+
+  // Cache the Loop ExitBlocks across this loop.  We expect to get a lot of
+  // instructions within the same loops, computing the exit blocks is
+  // expensive, and we're not mutating the loop structure.
+  SmallDenseMap<Loop*, SmallVector<BasicBlock *,1>> LoopExitBlocks;
+
+  while (!Worklist.empty()) {
+    UsesToRewrite.clear();
+
+    Instruction *I = Worklist.pop_back_val();
+    assert(!I->getType()->isTokenTy() && "Tokens shouldn't be in the worklist");
+    BasicBlock *InstBB = I->getParent();
+    Loop *L = LI.getLoopFor(InstBB);
+    assert(L && "Instruction belongs to a BB that's not part of a loop");
+    if (!LoopExitBlocks.count(L))
+      L->getExitBlocks(LoopExitBlocks[L]);
+    assert(LoopExitBlocks.count(L));
+    const SmallVectorImpl<BasicBlock *> &ExitBlocks = LoopExitBlocks[L];
+
+    if (ExitBlocks.empty())
+      continue;
+
+    for (Use &U : I->uses()) {
+      Instruction *User = cast<Instruction>(U.getUser());
+      BasicBlock *UserBB = User->getParent();
+      if (auto *PN = dyn_cast<PHINode>(User))
+        UserBB = PN->getIncomingBlock(U);
+
+      if (InstBB != UserBB && !L->contains(UserBB))
+        UsesToRewrite.push_back(&U);
+    }
+
+    // If there are no uses outside the loop, exit with no change.
+    if (UsesToRewrite.empty())
+      continue;
+
+    ++NumLCSSA; // We are applying the transformation
+
+    // Invoke instructions are special in that their result value is not
+    // available along their unwind edge. The code below tests to see whether
+    // DomBB dominates the value, so adjust DomBB to the normal destination
+    // block, which is effectively where the value is first usable.
+    BasicBlock *DomBB = InstBB;
+    if (auto *Inv = dyn_cast<InvokeInst>(I))
+      DomBB = Inv->getNormalDest();
+
+    DomTreeNode *DomNode = DT.getNode(DomBB);
+
+    SmallVector<PHINode *, 16> AddedPHIs;
+    SmallVector<PHINode *, 8> PostProcessPHIs;
+
+    SmallVector<PHINode *, 4> InsertedPHIs;
+    SSAUpdater SSAUpdate(&InsertedPHIs);
+    SSAUpdate.Initialize(I->getType(), I->getName());
+
+    // Insert the LCSSA phi's into all of the exit blocks dominated by the
+    // value, and add them to the Phi's map.
+    for (BasicBlock *ExitBB : ExitBlocks) {
+      if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
+        continue;
+
+      // If we already inserted something for this BB, don't reprocess it.
+      if (SSAUpdate.HasValueForBlock(ExitBB))
+        continue;
+
+      PHINode *PN = PHINode::Create(I->getType(), PredCache.size(ExitBB),
+                                    I->getName() + ".lcssa", &ExitBB->front());
+
+      // Add inputs from inside the loop for this PHI.
+      for (BasicBlock *Pred : PredCache.get(ExitBB)) {
+        PN->addIncoming(I, Pred);
+
+        // If the exit block has a predecessor not within the loop, arrange for
+        // the incoming value use corresponding to that predecessor to be
+        // rewritten in terms of a different LCSSA PHI.
+        if (!L->contains(Pred))
+          UsesToRewrite.push_back(
+              &PN->getOperandUse(PN->getOperandNumForIncomingValue(
+                  PN->getNumIncomingValues() - 1)));
+      }
+
+      AddedPHIs.push_back(PN);
+
+      // Remember that this phi makes the value alive in this block.
+      SSAUpdate.AddAvailableValue(ExitBB, PN);
+
+      // LoopSimplify might fail to simplify some loops (e.g. when indirect
+      // branches are involved). In such situations, it might happen that an
+      // exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we
+      // create PHIs in such an exit block, we are also inserting PHIs into L2's
+      // header. This could break LCSSA form for L2 because these inserted PHIs
+      // can also have uses outside of L2. Remember all PHIs in such situation
+      // as to revisit than later on. FIXME: Remove this if indirectbr support
+      // into LoopSimplify gets improved.
+      if (auto *OtherLoop = LI.getLoopFor(ExitBB))
+        if (!L->contains(OtherLoop))
+          PostProcessPHIs.push_back(PN);
+    }
+
+    // Rewrite all uses outside the loop in terms of the new PHIs we just
+    // inserted.
+    for (Use *UseToRewrite : UsesToRewrite) {
+      // If this use is in an exit block, rewrite to use the newly inserted PHI.
+      // This is required for correctness because SSAUpdate doesn't handle uses
+      // in the same block.  It assumes the PHI we inserted is at the end of the
+      // block.
+      Instruction *User = cast<Instruction>(UseToRewrite->getUser());
+      BasicBlock *UserBB = User->getParent();
+      if (auto *PN = dyn_cast<PHINode>(User))
+        UserBB = PN->getIncomingBlock(*UseToRewrite);
+
+      if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
+        // Tell the VHs that the uses changed. This updates SCEV's caches.
+        if (UseToRewrite->get()->hasValueHandle())
+          ValueHandleBase::ValueIsRAUWd(*UseToRewrite, &UserBB->front());
+        UseToRewrite->set(&UserBB->front());
+        continue;
+      }
+
+      // Otherwise, do full PHI insertion.
+      SSAUpdate.RewriteUse(*UseToRewrite);
+    }
+
+    // SSAUpdater might have inserted phi-nodes inside other loops. We'll need
+    // to post-process them to keep LCSSA form.
+    for (PHINode *InsertedPN : InsertedPHIs) {
+      if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent()))
+        if (!L->contains(OtherLoop))
+          PostProcessPHIs.push_back(InsertedPN);
+    }
+
+    // Post process PHI instructions that were inserted into another disjoint
+    // loop and update their exits properly.
+    for (auto *PostProcessPN : PostProcessPHIs)
+      if (!PostProcessPN->use_empty())
+        Worklist.push_back(PostProcessPN);
+
+    // Keep track of PHI nodes that we want to remove because they did not have
+    // any uses rewritten.
+    for (PHINode *PN : AddedPHIs)
+      if (PN->use_empty())
+        PHIsToRemove.insert(PN);
+
+    Changed = true;
+  }
+  // Remove PHI nodes that did not have any uses rewritten.
+  for (PHINode *PN : PHIsToRemove) {
+    assert (PN->use_empty() && "Trying to remove a phi with uses.");
+    PN->eraseFromParent();
+  }
+  return Changed;
+}
+
+// Compute the set of BasicBlocks in the loop `L` dominating at least one exit.
+static void computeBlocksDominatingExits(
+    Loop &L, DominatorTree &DT, SmallVector<BasicBlock *, 8> &ExitBlocks,
+    SmallSetVector<BasicBlock *, 8> &BlocksDominatingExits) {
+  SmallVector<BasicBlock *, 8> BBWorklist;
+
+  // We start from the exit blocks, as every block trivially dominates itself
+  // (not strictly).
+  for (BasicBlock *BB : ExitBlocks)
+    BBWorklist.push_back(BB);
+
+  while (!BBWorklist.empty()) {
+    BasicBlock *BB = BBWorklist.pop_back_val();
+
+    // Check if this is a loop header. If this is the case, we're done.
+    if (L.getHeader() == BB)
+      continue;
+
+    // Otherwise, add its immediate predecessor in the dominator tree to the
+    // worklist, unless we visited it already.
+    BasicBlock *IDomBB = DT.getNode(BB)->getIDom()->getBlock();
+
+    // Exit blocks can have an immediate dominator not beloinging to the
+    // loop. For an exit block to be immediately dominated by another block
+    // outside the loop, it implies not all paths from that dominator, to the
+    // exit block, go through the loop.
+    // Example:
+    //
+    // |---- A
+    // |     |
+    // |     B<--
+    // |     |  |
+    // |---> C --
+    //       |
+    //       D
+    //
+    // C is the exit block of the loop and it's immediately dominated by A,
+    // which doesn't belong to the loop.
+    if (!L.contains(IDomBB))
+      continue;
+
+    if (BlocksDominatingExits.insert(IDomBB))
+      BBWorklist.push_back(IDomBB);
+  }
+}
+
+bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
+                     ScalarEvolution *SE) {
+  bool Changed = false;
+
+  SmallVector<BasicBlock *, 8> ExitBlocks;
+  L.getExitBlocks(ExitBlocks);
+  if (ExitBlocks.empty())
+    return false;
+
+  SmallSetVector<BasicBlock *, 8> BlocksDominatingExits;
+
+  // We want to avoid use-scanning leveraging dominance informations.
+  // If a block doesn't dominate any of the loop exits, the none of the values
+  // defined in the loop can be used outside.
+  // We compute the set of blocks fullfilling the conditions in advance
+  // walking the dominator tree upwards until we hit a loop header.
+  computeBlocksDominatingExits(L, DT, ExitBlocks, BlocksDominatingExits);
+
+  SmallVector<Instruction *, 8> Worklist;
+
+  // Look at all the instructions in the loop, checking to see if they have uses
+  // outside the loop.  If so, put them into the worklist to rewrite those uses.
+  for (BasicBlock *BB : BlocksDominatingExits) {
+    for (Instruction &I : *BB) {
+      // Reject two common cases fast: instructions with no uses (like stores)
+      // and instructions with one use that is in the same block as this.
+      if (I.use_empty() ||
+          (I.hasOneUse() && I.user_back()->getParent() == BB &&
+           !isa<PHINode>(I.user_back())))
+        continue;
+
+      // Tokens cannot be used in PHI nodes, so we skip over them.
+      // We can run into tokens which are live out of a loop with catchswitch
+      // instructions in Windows EH if the catchswitch has one catchpad which
+      // is inside the loop and another which is not.
+      if (I.getType()->isTokenTy())
+        continue;
+
+      Worklist.push_back(&I);
+    }
+  }
+  Changed = formLCSSAForInstructions(Worklist, DT, *LI);
+
+  // If we modified the code, remove any caches about the loop from SCEV to
+  // avoid dangling entries.
+  // FIXME: This is a big hammer, can we clear the cache more selectively?
+  if (SE && Changed)
+    SE->forgetLoop(&L);
+
+  assert(L.isLCSSAForm(DT));
+
+  return Changed;
+}
+
+/// Process a loop nest depth first.
+bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI,
+                                ScalarEvolution *SE) {
+  bool Changed = false;
+
+  // Recurse depth-first through inner loops.
+  for (Loop *SubLoop : L.getSubLoops())
+    Changed |= formLCSSARecursively(*SubLoop, DT, LI, SE);
+
+  Changed |= formLCSSA(L, DT, LI, SE);
+  return Changed;
+}
+
+/// Process all loops in the function, inner-most out.
+static bool formLCSSAOnAllLoops(LoopInfo *LI, DominatorTree &DT,
+                                ScalarEvolution *SE) {
+  bool Changed = false;
+  for (auto &L : *LI)
+    Changed |= formLCSSARecursively(*L, DT, LI, SE);
+  return Changed;
+}
+
+namespace {
+struct LCSSAWrapperPass : public FunctionPass {
+  static char ID; // Pass identification, replacement for typeid
+  LCSSAWrapperPass() : FunctionPass(ID) {
+    initializeLCSSAWrapperPassPass(*PassRegistry::getPassRegistry());
+  }
+
+  // Cached analysis information for the current function.
+  DominatorTree *DT;
+  LoopInfo *LI;
+  ScalarEvolution *SE;
+
+  bool runOnFunction(Function &F) override;
+  void verifyAnalysis() const override {
+    // This check is very expensive. On the loop intensive compiles it may cause
+    // up to 10x slowdown. Currently it's disabled by default. LPPassManager
+    // always does limited form of the LCSSA verification. Similar reasoning
+    // was used for the LoopInfo verifier.
+    if (VerifyLoopLCSSA) {
+      assert(all_of(*LI,
+                    [&](Loop *L) {
+                      return L->isRecursivelyLCSSAForm(*DT, *LI);
+                    }) &&
+             "LCSSA form is broken!");
+    }
+  };
+
+  /// This transformation requires natural loop information & requires that
+  /// loop preheaders be inserted into the CFG.  It maintains both of these,
+  /// as well as the CFG.  It also requires dominator information.
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+
+    AU.addRequired<DominatorTreeWrapperPass>();
+    AU.addRequired<LoopInfoWrapperPass>();
+    AU.addPreservedID(LoopSimplifyID);
+    AU.addPreserved<AAResultsWrapperPass>();
+    AU.addPreserved<BasicAAWrapperPass>();
+    AU.addPreserved<GlobalsAAWrapperPass>();
+    AU.addPreserved<ScalarEvolutionWrapperPass>();
+    AU.addPreserved<SCEVAAWrapperPass>();
+
+    // This is needed to perform LCSSA verification inside LPPassManager
+    AU.addRequired<LCSSAVerificationPass>();
+    AU.addPreserved<LCSSAVerificationPass>();
+  }
+};
+}
+
+char LCSSAWrapperPass::ID = 0;
+INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
+                      false, false)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LCSSAVerificationPass)
+INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
+                    false, false)
+
+Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); }
+char &llvm::LCSSAID = LCSSAWrapperPass::ID;
+
+/// Transform \p F into loop-closed SSA form.
+bool LCSSAWrapperPass::runOnFunction(Function &F) {
+  LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+  auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
+  SE = SEWP ? &SEWP->getSE() : nullptr;
+
+  return formLCSSAOnAllLoops(LI, *DT, SE);
+}
+
+PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {
+  auto &LI = AM.getResult<LoopAnalysis>(F);
+  auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
+  auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
+  if (!formLCSSAOnAllLoops(&LI, DT, SE))
+    return PreservedAnalyses::all();
+
+  PreservedAnalyses PA;
+  PA.preserveSet<CFGAnalyses>();
+  PA.preserve<BasicAA>();
+  PA.preserve<GlobalsAA>();
+  PA.preserve<SCEVAA>();
+  PA.preserve<ScalarEvolutionAnalysis>();
+  return PA;
+}