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355 lines
13 KiB
355 lines
13 KiB
//===- FixIrreducible.cpp - Convert irreducible control-flow into loops ---===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// An irreducible SCC is one which has multiple "header" blocks, i.e., blocks
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// with control-flow edges incident from outside the SCC. This pass converts a
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// irreducible SCC into a natural loop by applying the following transformation:
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//
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// 1. Collect the set of headers H of the SCC.
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// 2. Collect the set of predecessors P of these headers. These may be inside as
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// well as outside the SCC.
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// 3. Create block N and redirect every edge from set P to set H through N.
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//
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// This converts the SCC into a natural loop with N as the header: N is the only
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// block with edges incident from outside the SCC, and all backedges in the SCC
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// are incident on N, i.e., for every backedge, the head now dominates the tail.
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//
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// INPUT CFG: The blocks A and B form an irreducible loop with two headers.
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//
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// Entry
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// / \
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// v v
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// A ----> B
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// ^ /|
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// `----' |
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// v
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// Exit
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//
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// OUTPUT CFG: Edges incident on A and B are now redirected through a
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// new block N, forming a natural loop consisting of N, A and B.
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//
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// Entry
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// |
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// v
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// .---> N <---.
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// / / \ \
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// | / \ |
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// \ v v /
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// `-- A B --'
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// |
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// v
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// Exit
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//
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// The transformation is applied to every maximal SCC that is not already
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// recognized as a loop. The pass operates on all maximal SCCs found in the
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// function body outside of any loop, as well as those found inside each loop,
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// including inside any newly created loops. This ensures that any SCC hidden
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// inside a maximal SCC is also transformed.
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//
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// The actual transformation is handled by function CreateControlFlowHub, which
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// takes a set of incoming blocks (the predecessors) and outgoing blocks (the
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// headers). The function also moves every PHINode in an outgoing block to the
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// hub. Since the hub dominates all the outgoing blocks, each such PHINode
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// continues to dominate its uses. Since every header in an SCC has at least two
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// predecessors, every value used in the header (or later) but defined in a
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// predecessor (or earlier) is represented by a PHINode in a header. Hence the
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// above handling of PHINodes is sufficient and no further processing is
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// required to restore SSA.
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//
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// Limitation: The pass cannot handle switch statements and indirect
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// branches. Both must be lowered to plain branches first.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/FixIrreducible.h"
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#include "llvm/ADT/SCCIterator.h"
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#include "llvm/Analysis/LoopIterator.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Transforms/Utils.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#define DEBUG_TYPE "fix-irreducible"
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using namespace llvm;
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namespace {
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struct FixIrreducible : public FunctionPass {
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static char ID;
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FixIrreducible() : FunctionPass(ID) {
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initializeFixIrreduciblePass(*PassRegistry::getPassRegistry());
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequiredID(LowerSwitchID);
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addPreservedID(LowerSwitchID);
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addPreserved<LoopInfoWrapperPass>();
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}
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bool runOnFunction(Function &F) override;
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};
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} // namespace
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char FixIrreducible::ID = 0;
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FunctionPass *llvm::createFixIrreduciblePass() { return new FixIrreducible(); }
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INITIALIZE_PASS_BEGIN(FixIrreducible, "fix-irreducible",
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"Convert irreducible control-flow into natural loops",
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false /* Only looks at CFG */, false /* Analysis Pass */)
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INITIALIZE_PASS_DEPENDENCY(LowerSwitchLegacyPass)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
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INITIALIZE_PASS_END(FixIrreducible, "fix-irreducible",
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"Convert irreducible control-flow into natural loops",
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false /* Only looks at CFG */, false /* Analysis Pass */)
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// When a new loop is created, existing children of the parent loop may now be
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// fully inside the new loop. Reconnect these as children of the new loop.
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static void reconnectChildLoops(LoopInfo &LI, Loop *ParentLoop, Loop *NewLoop,
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SetVector<BasicBlock *> &Blocks,
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SetVector<BasicBlock *> &Headers) {
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auto &CandidateLoops = ParentLoop ? ParentLoop->getSubLoopsVector()
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: LI.getTopLevelLoopsVector();
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// The new loop cannot be its own child, and any candidate is a
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// child iff its header is owned by the new loop. Move all the
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// children to a new vector.
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auto FirstChild = std::partition(
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CandidateLoops.begin(), CandidateLoops.end(), [&](Loop *L) {
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return L == NewLoop || Blocks.count(L->getHeader()) == 0;
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});
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SmallVector<Loop *, 8> ChildLoops(FirstChild, CandidateLoops.end());
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CandidateLoops.erase(FirstChild, CandidateLoops.end());
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for (auto II = ChildLoops.begin(), IE = ChildLoops.end(); II != IE; ++II) {
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auto Child = *II;
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LLVM_DEBUG(dbgs() << "child loop: " << Child->getHeader()->getName()
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<< "\n");
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// TODO: A child loop whose header is also a header in the current
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// SCC gets destroyed since its backedges are removed. That may
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// not be necessary if we can retain such backedges.
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if (Headers.count(Child->getHeader())) {
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for (auto BB : Child->blocks()) {
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LI.changeLoopFor(BB, NewLoop);
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LLVM_DEBUG(dbgs() << "moved block from child: " << BB->getName()
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<< "\n");
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}
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LI.destroy(Child);
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LLVM_DEBUG(dbgs() << "subsumed child loop (common header)\n");
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continue;
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}
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Child->setParentLoop(nullptr);
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NewLoop->addChildLoop(Child);
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LLVM_DEBUG(dbgs() << "added child loop to new loop\n");
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}
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}
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// Given a set of blocks and headers in an irreducible SCC, convert it into a
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// natural loop. Also insert this new loop at its appropriate place in the
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// hierarchy of loops.
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static void createNaturalLoopInternal(LoopInfo &LI, DominatorTree &DT,
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Loop *ParentLoop,
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SetVector<BasicBlock *> &Blocks,
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SetVector<BasicBlock *> &Headers) {
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#ifndef NDEBUG
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// All headers are part of the SCC
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for (auto H : Headers) {
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assert(Blocks.count(H));
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}
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#endif
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SetVector<BasicBlock *> Predecessors;
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for (auto H : Headers) {
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for (auto P : predecessors(H)) {
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Predecessors.insert(P);
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}
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}
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LLVM_DEBUG(
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dbgs() << "Found predecessors:";
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for (auto P : Predecessors) {
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dbgs() << " " << P->getName();
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}
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dbgs() << "\n");
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// Redirect all the backedges through a "hub" consisting of a series
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// of guard blocks that manage the flow of control from the
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// predecessors to the headers.
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SmallVector<BasicBlock *, 8> GuardBlocks;
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DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
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CreateControlFlowHub(&DTU, GuardBlocks, Predecessors, Headers, "irr");
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#if defined(EXPENSIVE_CHECKS)
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assert(DT.verify(DominatorTree::VerificationLevel::Full));
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#else
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assert(DT.verify(DominatorTree::VerificationLevel::Fast));
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#endif
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// Create a new loop from the now-transformed cycle
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auto NewLoop = LI.AllocateLoop();
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if (ParentLoop) {
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ParentLoop->addChildLoop(NewLoop);
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} else {
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LI.addTopLevelLoop(NewLoop);
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}
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// Add the guard blocks to the new loop. The first guard block is
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// the head of all the backedges, and it is the first to be inserted
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// in the loop. This ensures that it is recognized as the
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// header. Since the new loop is already in LoopInfo, the new blocks
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// are also propagated up the chain of parent loops.
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for (auto G : GuardBlocks) {
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LLVM_DEBUG(dbgs() << "added guard block: " << G->getName() << "\n");
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NewLoop->addBasicBlockToLoop(G, LI);
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}
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// Add the SCC blocks to the new loop.
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for (auto BB : Blocks) {
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NewLoop->addBlockEntry(BB);
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if (LI.getLoopFor(BB) == ParentLoop) {
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LLVM_DEBUG(dbgs() << "moved block from parent: " << BB->getName()
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<< "\n");
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LI.changeLoopFor(BB, NewLoop);
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} else {
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LLVM_DEBUG(dbgs() << "added block from child: " << BB->getName() << "\n");
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}
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}
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LLVM_DEBUG(dbgs() << "header for new loop: "
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<< NewLoop->getHeader()->getName() << "\n");
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reconnectChildLoops(LI, ParentLoop, NewLoop, Blocks, Headers);
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NewLoop->verifyLoop();
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if (ParentLoop) {
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ParentLoop->verifyLoop();
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}
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#if defined(EXPENSIVE_CHECKS)
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LI.verify(DT);
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#endif // EXPENSIVE_CHECKS
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}
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namespace llvm {
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// Enable the graph traits required for traversing a Loop body.
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template <> struct GraphTraits<Loop> : LoopBodyTraits {};
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} // namespace llvm
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// Overloaded wrappers to go with the function template below.
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static BasicBlock *unwrapBlock(BasicBlock *B) { return B; }
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static BasicBlock *unwrapBlock(LoopBodyTraits::NodeRef &N) { return N.second; }
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static void createNaturalLoop(LoopInfo &LI, DominatorTree &DT, Function *F,
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SetVector<BasicBlock *> &Blocks,
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SetVector<BasicBlock *> &Headers) {
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createNaturalLoopInternal(LI, DT, nullptr, Blocks, Headers);
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}
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static void createNaturalLoop(LoopInfo &LI, DominatorTree &DT, Loop &L,
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SetVector<BasicBlock *> &Blocks,
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SetVector<BasicBlock *> &Headers) {
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createNaturalLoopInternal(LI, DT, &L, Blocks, Headers);
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}
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// Convert irreducible SCCs; Graph G may be a Function* or a Loop&.
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template <class Graph>
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static bool makeReducible(LoopInfo &LI, DominatorTree &DT, Graph &&G) {
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bool Changed = false;
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for (auto Scc = scc_begin(G); !Scc.isAtEnd(); ++Scc) {
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if (Scc->size() < 2)
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continue;
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SetVector<BasicBlock *> Blocks;
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LLVM_DEBUG(dbgs() << "Found SCC:");
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for (auto N : *Scc) {
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auto BB = unwrapBlock(N);
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LLVM_DEBUG(dbgs() << " " << BB->getName());
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Blocks.insert(BB);
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}
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LLVM_DEBUG(dbgs() << "\n");
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// Minor optimization: The SCC blocks are usually discovered in an order
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// that is the opposite of the order in which these blocks appear as branch
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// targets. This results in a lot of condition inversions in the control
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// flow out of the new ControlFlowHub, which can be mitigated if the orders
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// match. So we discover the headers using the reverse of the block order.
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SetVector<BasicBlock *> Headers;
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LLVM_DEBUG(dbgs() << "Found headers:");
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for (auto BB : reverse(Blocks)) {
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for (const auto P : predecessors(BB)) {
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// Skip unreachable predecessors.
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if (!DT.isReachableFromEntry(P))
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continue;
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if (!Blocks.count(P)) {
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LLVM_DEBUG(dbgs() << " " << BB->getName());
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Headers.insert(BB);
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break;
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}
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}
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}
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LLVM_DEBUG(dbgs() << "\n");
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if (Headers.size() == 1) {
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assert(LI.isLoopHeader(Headers.front()));
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LLVM_DEBUG(dbgs() << "Natural loop with a single header: skipped\n");
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continue;
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}
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createNaturalLoop(LI, DT, G, Blocks, Headers);
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Changed = true;
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}
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return Changed;
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}
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static bool FixIrreducibleImpl(Function &F, LoopInfo &LI, DominatorTree &DT) {
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LLVM_DEBUG(dbgs() << "===== Fix irreducible control-flow in function: "
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<< F.getName() << "\n");
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bool Changed = false;
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SmallVector<Loop *, 8> WorkList;
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LLVM_DEBUG(dbgs() << "visiting top-level\n");
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Changed |= makeReducible(LI, DT, &F);
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// Any SCCs reduced are now already in the list of top-level loops, so simply
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// add them all to the worklist.
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for (auto L : LI) {
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WorkList.push_back(L);
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}
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while (!WorkList.empty()) {
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auto L = WorkList.back();
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WorkList.pop_back();
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LLVM_DEBUG(dbgs() << "visiting loop with header "
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<< L->getHeader()->getName() << "\n");
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Changed |= makeReducible(LI, DT, *L);
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// Any SCCs reduced are now already in the list of child loops, so simply
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// add them all to the worklist.
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WorkList.append(L->begin(), L->end());
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}
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return Changed;
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}
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bool FixIrreducible::runOnFunction(Function &F) {
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auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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return FixIrreducibleImpl(F, LI, DT);
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}
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PreservedAnalyses FixIrreduciblePass::run(Function &F,
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FunctionAnalysisManager &AM) {
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auto &LI = AM.getResult<LoopAnalysis>(F);
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auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
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if (!FixIrreducibleImpl(F, LI, DT))
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return PreservedAnalyses::all();
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PreservedAnalyses PA;
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PA.preserve<LoopAnalysis>();
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PA.preserve<DominatorTreeAnalysis>();
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return PA;
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}
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