forked from OSchip/llvm-project
931 lines
35 KiB
C++
931 lines
35 KiB
C++
//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
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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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// This pass performs several transformations to transform natural loops into a
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// simpler form, which makes subsequent analyses and transformations simpler and
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// more effective.
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//
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// Loop pre-header insertion guarantees that there is a single, non-critical
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// entry edge from outside of the loop to the loop header. This simplifies a
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// number of analyses and transformations, such as LICM.
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//
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// Loop exit-block insertion guarantees that all exit blocks from the loop
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// (blocks which are outside of the loop that have predecessors inside of the
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// loop) only have predecessors from inside of the loop (and are thus dominated
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// by the loop header). This simplifies transformations such as store-sinking
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// that are built into LICM.
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//
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// This pass also guarantees that loops will have exactly one backedge.
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//
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// Indirectbr instructions introduce several complications. If the loop
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// contains or is entered by an indirectbr instruction, it may not be possible
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// to transform the loop and make these guarantees. Client code should check
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// that these conditions are true before relying on them.
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//
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// Similar complications arise from callbr instructions, particularly in
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// asm-goto where blockaddress expressions are used.
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//
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// Note that the simplifycfg pass will clean up blocks which are split out but
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// end up being unnecessary, so usage of this pass should not pessimize
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// generated code.
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//
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// This pass obviously modifies the CFG, but updates loop information and
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// dominator information.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/LoopSimplify.h"
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#include "llvm/ADT/DepthFirstIterator.h"
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#include "llvm/ADT/SetOperations.h"
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#include "llvm/ADT/SetVector.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/AssumptionCache.h"
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#include "llvm/Analysis/BasicAliasAnalysis.h"
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#include "llvm/Analysis/BranchProbabilityInfo.h"
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#include "llvm/Analysis/DependenceAnalysis.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/InstructionSimplify.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/MemorySSA.h"
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#include "llvm/Analysis/MemorySSAUpdater.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/LLVMContext.h"
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#include "llvm/IR/Module.h"
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#include "llvm/IR/Type.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Utils.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/LoopUtils.h"
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using namespace llvm;
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#define DEBUG_TYPE "loop-simplify"
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STATISTIC(NumNested , "Number of nested loops split out");
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// If the block isn't already, move the new block to right after some 'outside
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// block' block. This prevents the preheader from being placed inside the loop
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// body, e.g. when the loop hasn't been rotated.
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static void placeSplitBlockCarefully(BasicBlock *NewBB,
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SmallVectorImpl<BasicBlock *> &SplitPreds,
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Loop *L) {
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// Check to see if NewBB is already well placed.
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Function::iterator BBI = --NewBB->getIterator();
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for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
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if (&*BBI == SplitPreds[i])
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return;
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}
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// If it isn't already after an outside block, move it after one. This is
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// always good as it makes the uncond branch from the outside block into a
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// fall-through.
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// Figure out *which* outside block to put this after. Prefer an outside
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// block that neighbors a BB actually in the loop.
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BasicBlock *FoundBB = nullptr;
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for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
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Function::iterator BBI = SplitPreds[i]->getIterator();
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if (++BBI != NewBB->getParent()->end() && L->contains(&*BBI)) {
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FoundBB = SplitPreds[i];
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break;
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}
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}
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// If our heuristic for a *good* bb to place this after doesn't find
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// anything, just pick something. It's likely better than leaving it within
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// the loop.
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if (!FoundBB)
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FoundBB = SplitPreds[0];
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NewBB->moveAfter(FoundBB);
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}
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/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
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/// preheader, this method is called to insert one. This method has two phases:
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/// preheader insertion and analysis updating.
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///
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BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, DominatorTree *DT,
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LoopInfo *LI, MemorySSAUpdater *MSSAU,
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bool PreserveLCSSA) {
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BasicBlock *Header = L->getHeader();
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// Compute the set of predecessors of the loop that are not in the loop.
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SmallVector<BasicBlock*, 8> OutsideBlocks;
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for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
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PI != PE; ++PI) {
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BasicBlock *P = *PI;
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if (!L->contains(P)) { // Coming in from outside the loop?
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// If the loop is branched to from an indirect terminator, we won't
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// be able to fully transform the loop, because it prohibits
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// edge splitting.
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if (P->getTerminator()->isIndirectTerminator())
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return nullptr;
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// Keep track of it.
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OutsideBlocks.push_back(P);
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}
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}
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// Split out the loop pre-header.
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BasicBlock *PreheaderBB;
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PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", DT,
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LI, MSSAU, PreserveLCSSA);
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if (!PreheaderBB)
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return nullptr;
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LLVM_DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
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<< PreheaderBB->getName() << "\n");
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// Make sure that NewBB is put someplace intelligent, which doesn't mess up
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// code layout too horribly.
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placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
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return PreheaderBB;
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}
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/// Add the specified block, and all of its predecessors, to the specified set,
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/// if it's not already in there. Stop predecessor traversal when we reach
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/// StopBlock.
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static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
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std::set<BasicBlock*> &Blocks) {
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SmallVector<BasicBlock *, 8> Worklist;
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Worklist.push_back(InputBB);
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do {
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BasicBlock *BB = Worklist.pop_back_val();
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if (Blocks.insert(BB).second && BB != StopBlock)
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// If BB is not already processed and it is not a stop block then
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// insert its predecessor in the work list
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for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
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BasicBlock *WBB = *I;
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Worklist.push_back(WBB);
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}
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} while (!Worklist.empty());
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}
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/// The first part of loop-nestification is to find a PHI node that tells
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/// us how to partition the loops.
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static PHINode *findPHIToPartitionLoops(Loop *L, DominatorTree *DT,
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AssumptionCache *AC) {
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const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
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for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
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PHINode *PN = cast<PHINode>(I);
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++I;
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if (Value *V = SimplifyInstruction(PN, {DL, nullptr, DT, AC})) {
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// This is a degenerate PHI already, don't modify it!
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PN->replaceAllUsesWith(V);
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PN->eraseFromParent();
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continue;
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}
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// Scan this PHI node looking for a use of the PHI node by itself.
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) == PN &&
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L->contains(PN->getIncomingBlock(i)))
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// We found something tasty to remove.
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return PN;
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}
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return nullptr;
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}
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/// If this loop has multiple backedges, try to pull one of them out into
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/// a nested loop.
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///
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/// This is important for code that looks like
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/// this:
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///
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/// Loop:
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/// ...
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/// br cond, Loop, Next
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/// ...
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/// br cond2, Loop, Out
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///
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/// To identify this common case, we look at the PHI nodes in the header of the
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/// loop. PHI nodes with unchanging values on one backedge correspond to values
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/// that change in the "outer" loop, but not in the "inner" loop.
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///
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/// If we are able to separate out a loop, return the new outer loop that was
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/// created.
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///
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static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
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DominatorTree *DT, LoopInfo *LI,
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ScalarEvolution *SE, bool PreserveLCSSA,
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AssumptionCache *AC, MemorySSAUpdater *MSSAU) {
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// Don't try to separate loops without a preheader.
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if (!Preheader)
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return nullptr;
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// The header is not a landing pad; preheader insertion should ensure this.
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BasicBlock *Header = L->getHeader();
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assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
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PHINode *PN = findPHIToPartitionLoops(L, DT, AC);
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if (!PN) return nullptr; // No known way to partition.
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// Pull out all predecessors that have varying values in the loop. This
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// handles the case when a PHI node has multiple instances of itself as
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// arguments.
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SmallVector<BasicBlock*, 8> OuterLoopPreds;
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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if (PN->getIncomingValue(i) != PN ||
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!L->contains(PN->getIncomingBlock(i))) {
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// We can't split indirect control flow edges.
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if (PN->getIncomingBlock(i)->getTerminator()->isIndirectTerminator())
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return nullptr;
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OuterLoopPreds.push_back(PN->getIncomingBlock(i));
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}
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}
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LLVM_DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
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// If ScalarEvolution is around and knows anything about values in
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// this loop, tell it to forget them, because we're about to
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// substantially change it.
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if (SE)
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SE->forgetLoop(L);
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BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer",
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DT, LI, MSSAU, PreserveLCSSA);
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// Make sure that NewBB is put someplace intelligent, which doesn't mess up
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// code layout too horribly.
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placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
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// Create the new outer loop.
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Loop *NewOuter = LI->AllocateLoop();
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// Change the parent loop to use the outer loop as its child now.
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if (Loop *Parent = L->getParentLoop())
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Parent->replaceChildLoopWith(L, NewOuter);
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else
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LI->changeTopLevelLoop(L, NewOuter);
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// L is now a subloop of our outer loop.
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NewOuter->addChildLoop(L);
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for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
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I != E; ++I)
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NewOuter->addBlockEntry(*I);
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// Now reset the header in L, which had been moved by
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// SplitBlockPredecessors for the outer loop.
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L->moveToHeader(Header);
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// Determine which blocks should stay in L and which should be moved out to
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// the Outer loop now.
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std::set<BasicBlock*> BlocksInL;
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for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
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BasicBlock *P = *PI;
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if (DT->dominates(Header, P))
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addBlockAndPredsToSet(P, Header, BlocksInL);
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}
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// Scan all of the loop children of L, moving them to OuterLoop if they are
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// not part of the inner loop.
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const std::vector<Loop*> &SubLoops = L->getSubLoops();
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for (size_t I = 0; I != SubLoops.size(); )
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if (BlocksInL.count(SubLoops[I]->getHeader()))
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++I; // Loop remains in L
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else
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NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
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SmallVector<BasicBlock *, 8> OuterLoopBlocks;
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OuterLoopBlocks.push_back(NewBB);
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// Now that we know which blocks are in L and which need to be moved to
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// OuterLoop, move any blocks that need it.
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for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
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BasicBlock *BB = L->getBlocks()[i];
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if (!BlocksInL.count(BB)) {
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// Move this block to the parent, updating the exit blocks sets
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L->removeBlockFromLoop(BB);
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if ((*LI)[BB] == L) {
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LI->changeLoopFor(BB, NewOuter);
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OuterLoopBlocks.push_back(BB);
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}
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--i;
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}
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}
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// Split edges to exit blocks from the inner loop, if they emerged in the
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// process of separating the outer one.
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formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA);
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if (PreserveLCSSA) {
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// Fix LCSSA form for L. Some values, which previously were only used inside
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// L, can now be used in NewOuter loop. We need to insert phi-nodes for them
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// in corresponding exit blocks.
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// We don't need to form LCSSA recursively, because there cannot be uses
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// inside a newly created loop of defs from inner loops as those would
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// already be a use of an LCSSA phi node.
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formLCSSA(*L, *DT, LI, SE);
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assert(NewOuter->isRecursivelyLCSSAForm(*DT, *LI) &&
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"LCSSA is broken after separating nested loops!");
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}
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return NewOuter;
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}
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/// This method is called when the specified loop has more than one
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/// backedge in it.
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///
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/// If this occurs, revector all of these backedges to target a new basic block
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/// and have that block branch to the loop header. This ensures that loops
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/// have exactly one backedge.
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static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
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DominatorTree *DT, LoopInfo *LI,
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MemorySSAUpdater *MSSAU) {
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assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
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// Get information about the loop
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BasicBlock *Header = L->getHeader();
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Function *F = Header->getParent();
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// Unique backedge insertion currently depends on having a preheader.
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if (!Preheader)
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return nullptr;
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// The header is not an EH pad; preheader insertion should ensure this.
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assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
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// Figure out which basic blocks contain back-edges to the loop header.
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std::vector<BasicBlock*> BackedgeBlocks;
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for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
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BasicBlock *P = *I;
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// Indirect edges cannot be split, so we must fail if we find one.
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if (P->getTerminator()->isIndirectTerminator())
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return nullptr;
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if (P != Preheader) BackedgeBlocks.push_back(P);
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}
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// Create and insert the new backedge block...
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BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
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Header->getName() + ".backedge", F);
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BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
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BETerminator->setDebugLoc(Header->getFirstNonPHI()->getDebugLoc());
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LLVM_DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
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<< BEBlock->getName() << "\n");
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// Move the new backedge block to right after the last backedge block.
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Function::iterator InsertPos = ++BackedgeBlocks.back()->getIterator();
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F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
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// Now that the block has been inserted into the function, create PHI nodes in
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// the backedge block which correspond to any PHI nodes in the header block.
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for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
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PHINode *PN = cast<PHINode>(I);
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PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
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PN->getName()+".be", BETerminator);
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// Loop over the PHI node, moving all entries except the one for the
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// preheader over to the new PHI node.
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unsigned PreheaderIdx = ~0U;
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bool HasUniqueIncomingValue = true;
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Value *UniqueValue = nullptr;
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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BasicBlock *IBB = PN->getIncomingBlock(i);
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Value *IV = PN->getIncomingValue(i);
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if (IBB == Preheader) {
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PreheaderIdx = i;
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} else {
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NewPN->addIncoming(IV, IBB);
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if (HasUniqueIncomingValue) {
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if (!UniqueValue)
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UniqueValue = IV;
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else if (UniqueValue != IV)
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HasUniqueIncomingValue = false;
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}
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}
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}
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// Delete all of the incoming values from the old PN except the preheader's
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assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
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if (PreheaderIdx != 0) {
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PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
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PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
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}
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// Nuke all entries except the zero'th.
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for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
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PN->removeIncomingValue(e-i, false);
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// Finally, add the newly constructed PHI node as the entry for the BEBlock.
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PN->addIncoming(NewPN, BEBlock);
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// As an optimization, if all incoming values in the new PhiNode (which is a
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// subset of the incoming values of the old PHI node) have the same value,
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// eliminate the PHI Node.
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if (HasUniqueIncomingValue) {
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NewPN->replaceAllUsesWith(UniqueValue);
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BEBlock->getInstList().erase(NewPN);
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}
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}
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// Now that all of the PHI nodes have been inserted and adjusted, modify the
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// backedge blocks to jump to the BEBlock instead of the header.
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// If one of the backedges has llvm.loop metadata attached, we remove
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// it from the backedge and add it to BEBlock.
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unsigned LoopMDKind = BEBlock->getContext().getMDKindID("llvm.loop");
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MDNode *LoopMD = nullptr;
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for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
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Instruction *TI = BackedgeBlocks[i]->getTerminator();
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if (!LoopMD)
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LoopMD = TI->getMetadata(LoopMDKind);
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TI->setMetadata(LoopMDKind, nullptr);
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TI->replaceSuccessorWith(Header, BEBlock);
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}
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BEBlock->getTerminator()->setMetadata(LoopMDKind, LoopMD);
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//===--- Update all analyses which we must preserve now -----------------===//
|
|
|
|
// Update Loop Information - we know that this block is now in the current
|
|
// loop and all parent loops.
|
|
L->addBasicBlockToLoop(BEBlock, *LI);
|
|
|
|
// Update dominator information
|
|
DT->splitBlock(BEBlock);
|
|
|
|
if (MSSAU)
|
|
MSSAU->updatePhisWhenInsertingUniqueBackedgeBlock(Header, Preheader,
|
|
BEBlock);
|
|
|
|
return BEBlock;
|
|
}
|
|
|
|
/// Simplify one loop and queue further loops for simplification.
|
|
static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
|
|
DominatorTree *DT, LoopInfo *LI,
|
|
ScalarEvolution *SE, AssumptionCache *AC,
|
|
MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
|
|
bool Changed = false;
|
|
if (MSSAU && VerifyMemorySSA)
|
|
MSSAU->getMemorySSA()->verifyMemorySSA();
|
|
|
|
ReprocessLoop:
|
|
|
|
// Check to see that no blocks (other than the header) in this loop have
|
|
// predecessors that are not in the loop. This is not valid for natural
|
|
// loops, but can occur if the blocks are unreachable. Since they are
|
|
// unreachable we can just shamelessly delete those CFG edges!
|
|
for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
|
|
BB != E; ++BB) {
|
|
if (*BB == L->getHeader()) continue;
|
|
|
|
SmallPtrSet<BasicBlock*, 4> BadPreds;
|
|
for (pred_iterator PI = pred_begin(*BB),
|
|
PE = pred_end(*BB); PI != PE; ++PI) {
|
|
BasicBlock *P = *PI;
|
|
if (!L->contains(P))
|
|
BadPreds.insert(P);
|
|
}
|
|
|
|
// Delete each unique out-of-loop (and thus dead) predecessor.
|
|
for (BasicBlock *P : BadPreds) {
|
|
|
|
LLVM_DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
|
|
<< P->getName() << "\n");
|
|
|
|
// Zap the dead pred's terminator and replace it with unreachable.
|
|
Instruction *TI = P->getTerminator();
|
|
changeToUnreachable(TI, /*UseLLVMTrap=*/false, PreserveLCSSA,
|
|
/*DTU=*/nullptr, MSSAU);
|
|
Changed = true;
|
|
}
|
|
}
|
|
|
|
if (MSSAU && VerifyMemorySSA)
|
|
MSSAU->getMemorySSA()->verifyMemorySSA();
|
|
|
|
// If there are exiting blocks with branches on undef, resolve the undef in
|
|
// the direction which will exit the loop. This will help simplify loop
|
|
// trip count computations.
|
|
SmallVector<BasicBlock*, 8> ExitingBlocks;
|
|
L->getExitingBlocks(ExitingBlocks);
|
|
for (BasicBlock *ExitingBlock : ExitingBlocks)
|
|
if (BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()))
|
|
if (BI->isConditional()) {
|
|
if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
|
|
|
|
LLVM_DEBUG(dbgs()
|
|
<< "LoopSimplify: Resolving \"br i1 undef\" to exit in "
|
|
<< ExitingBlock->getName() << "\n");
|
|
|
|
BI->setCondition(ConstantInt::get(Cond->getType(),
|
|
!L->contains(BI->getSuccessor(0))));
|
|
|
|
Changed = true;
|
|
}
|
|
}
|
|
|
|
// Does the loop already have a preheader? If so, don't insert one.
|
|
BasicBlock *Preheader = L->getLoopPreheader();
|
|
if (!Preheader) {
|
|
Preheader = InsertPreheaderForLoop(L, DT, LI, MSSAU, PreserveLCSSA);
|
|
if (Preheader)
|
|
Changed = true;
|
|
}
|
|
|
|
// Next, check to make sure that all exit nodes of the loop only have
|
|
// predecessors that are inside of the loop. This check guarantees that the
|
|
// loop preheader/header will dominate the exit blocks. If the exit block has
|
|
// predecessors from outside of the loop, split the edge now.
|
|
if (formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA))
|
|
Changed = true;
|
|
|
|
if (MSSAU && VerifyMemorySSA)
|
|
MSSAU->getMemorySSA()->verifyMemorySSA();
|
|
|
|
// If the header has more than two predecessors at this point (from the
|
|
// preheader and from multiple backedges), we must adjust the loop.
|
|
BasicBlock *LoopLatch = L->getLoopLatch();
|
|
if (!LoopLatch) {
|
|
// If this is really a nested loop, rip it out into a child loop. Don't do
|
|
// this for loops with a giant number of backedges, just factor them into a
|
|
// common backedge instead.
|
|
if (L->getNumBackEdges() < 8) {
|
|
if (Loop *OuterL = separateNestedLoop(L, Preheader, DT, LI, SE,
|
|
PreserveLCSSA, AC, MSSAU)) {
|
|
++NumNested;
|
|
// Enqueue the outer loop as it should be processed next in our
|
|
// depth-first nest walk.
|
|
Worklist.push_back(OuterL);
|
|
|
|
// This is a big restructuring change, reprocess the whole loop.
|
|
Changed = true;
|
|
// GCC doesn't tail recursion eliminate this.
|
|
// FIXME: It isn't clear we can't rely on LLVM to TRE this.
|
|
goto ReprocessLoop;
|
|
}
|
|
}
|
|
|
|
// If we either couldn't, or didn't want to, identify nesting of the loops,
|
|
// insert a new block that all backedges target, then make it jump to the
|
|
// loop header.
|
|
LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI, MSSAU);
|
|
if (LoopLatch)
|
|
Changed = true;
|
|
}
|
|
|
|
if (MSSAU && VerifyMemorySSA)
|
|
MSSAU->getMemorySSA()->verifyMemorySSA();
|
|
|
|
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
|
|
|
|
// Scan over the PHI nodes in the loop header. Since they now have only two
|
|
// incoming values (the loop is canonicalized), we may have simplified the PHI
|
|
// down to 'X = phi [X, Y]', which should be replaced with 'Y'.
|
|
PHINode *PN;
|
|
for (BasicBlock::iterator I = L->getHeader()->begin();
|
|
(PN = dyn_cast<PHINode>(I++)); )
|
|
if (Value *V = SimplifyInstruction(PN, {DL, nullptr, DT, AC})) {
|
|
if (SE) SE->forgetValue(PN);
|
|
if (!PreserveLCSSA || LI->replacementPreservesLCSSAForm(PN, V)) {
|
|
PN->replaceAllUsesWith(V);
|
|
PN->eraseFromParent();
|
|
}
|
|
}
|
|
|
|
// If this loop has multiple exits and the exits all go to the same
|
|
// block, attempt to merge the exits. This helps several passes, such
|
|
// as LoopRotation, which do not support loops with multiple exits.
|
|
// SimplifyCFG also does this (and this code uses the same utility
|
|
// function), however this code is loop-aware, where SimplifyCFG is
|
|
// not. That gives it the advantage of being able to hoist
|
|
// loop-invariant instructions out of the way to open up more
|
|
// opportunities, and the disadvantage of having the responsibility
|
|
// to preserve dominator information.
|
|
auto HasUniqueExitBlock = [&]() {
|
|
BasicBlock *UniqueExit = nullptr;
|
|
for (auto *ExitingBB : ExitingBlocks)
|
|
for (auto *SuccBB : successors(ExitingBB)) {
|
|
if (L->contains(SuccBB))
|
|
continue;
|
|
|
|
if (!UniqueExit)
|
|
UniqueExit = SuccBB;
|
|
else if (UniqueExit != SuccBB)
|
|
return false;
|
|
}
|
|
|
|
return true;
|
|
};
|
|
if (HasUniqueExitBlock()) {
|
|
for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
|
|
BasicBlock *ExitingBlock = ExitingBlocks[i];
|
|
if (!ExitingBlock->getSinglePredecessor()) continue;
|
|
BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
|
|
if (!BI || !BI->isConditional()) continue;
|
|
CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
|
|
if (!CI || CI->getParent() != ExitingBlock) continue;
|
|
|
|
// Attempt to hoist out all instructions except for the
|
|
// comparison and the branch.
|
|
bool AllInvariant = true;
|
|
bool AnyInvariant = false;
|
|
for (auto I = ExitingBlock->instructionsWithoutDebug().begin(); &*I != BI; ) {
|
|
Instruction *Inst = &*I++;
|
|
if (Inst == CI)
|
|
continue;
|
|
if (!L->makeLoopInvariant(
|
|
Inst, AnyInvariant,
|
|
Preheader ? Preheader->getTerminator() : nullptr, MSSAU)) {
|
|
AllInvariant = false;
|
|
break;
|
|
}
|
|
}
|
|
if (AnyInvariant) {
|
|
Changed = true;
|
|
// The loop disposition of all SCEV expressions that depend on any
|
|
// hoisted values have also changed.
|
|
if (SE)
|
|
SE->forgetLoopDispositions(L);
|
|
}
|
|
if (!AllInvariant) continue;
|
|
|
|
// The block has now been cleared of all instructions except for
|
|
// a comparison and a conditional branch. SimplifyCFG may be able
|
|
// to fold it now.
|
|
if (!FoldBranchToCommonDest(BI, MSSAU))
|
|
continue;
|
|
|
|
// Success. The block is now dead, so remove it from the loop,
|
|
// update the dominator tree and delete it.
|
|
LLVM_DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
|
|
<< ExitingBlock->getName() << "\n");
|
|
|
|
assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
|
|
Changed = true;
|
|
LI->removeBlock(ExitingBlock);
|
|
|
|
DomTreeNode *Node = DT->getNode(ExitingBlock);
|
|
const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
|
|
Node->getChildren();
|
|
while (!Children.empty()) {
|
|
DomTreeNode *Child = Children.front();
|
|
DT->changeImmediateDominator(Child, Node->getIDom());
|
|
}
|
|
DT->eraseNode(ExitingBlock);
|
|
if (MSSAU) {
|
|
SmallSetVector<BasicBlock *, 8> ExitBlockSet;
|
|
ExitBlockSet.insert(ExitingBlock);
|
|
MSSAU->removeBlocks(ExitBlockSet);
|
|
}
|
|
|
|
BI->getSuccessor(0)->removePredecessor(
|
|
ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
|
|
BI->getSuccessor(1)->removePredecessor(
|
|
ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
|
|
ExitingBlock->eraseFromParent();
|
|
}
|
|
}
|
|
|
|
// Changing exit conditions for blocks may affect exit counts of this loop and
|
|
// any of its paretns, so we must invalidate the entire subtree if we've made
|
|
// any changes.
|
|
if (Changed && SE)
|
|
SE->forgetTopmostLoop(L);
|
|
|
|
if (MSSAU && VerifyMemorySSA)
|
|
MSSAU->getMemorySSA()->verifyMemorySSA();
|
|
|
|
return Changed;
|
|
}
|
|
|
|
bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,
|
|
ScalarEvolution *SE, AssumptionCache *AC,
|
|
MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
|
|
bool Changed = false;
|
|
|
|
#ifndef NDEBUG
|
|
// If we're asked to preserve LCSSA, the loop nest needs to start in LCSSA
|
|
// form.
|
|
if (PreserveLCSSA) {
|
|
assert(DT && "DT not available.");
|
|
assert(LI && "LI not available.");
|
|
assert(L->isRecursivelyLCSSAForm(*DT, *LI) &&
|
|
"Requested to preserve LCSSA, but it's already broken.");
|
|
}
|
|
#endif
|
|
|
|
// Worklist maintains our depth-first queue of loops in this nest to process.
|
|
SmallVector<Loop *, 4> Worklist;
|
|
Worklist.push_back(L);
|
|
|
|
// Walk the worklist from front to back, pushing newly found sub loops onto
|
|
// the back. This will let us process loops from back to front in depth-first
|
|
// order. We can use this simple process because loops form a tree.
|
|
for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
|
|
Loop *L2 = Worklist[Idx];
|
|
Worklist.append(L2->begin(), L2->end());
|
|
}
|
|
|
|
while (!Worklist.empty())
|
|
Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE,
|
|
AC, MSSAU, PreserveLCSSA);
|
|
|
|
return Changed;
|
|
}
|
|
|
|
namespace {
|
|
struct LoopSimplify : public FunctionPass {
|
|
static char ID; // Pass identification, replacement for typeid
|
|
LoopSimplify() : FunctionPass(ID) {
|
|
initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
|
|
}
|
|
|
|
bool runOnFunction(Function &F) override;
|
|
|
|
void getAnalysisUsage(AnalysisUsage &AU) const override {
|
|
AU.addRequired<AssumptionCacheTracker>();
|
|
|
|
// We need loop information to identify the loops...
|
|
AU.addRequired<DominatorTreeWrapperPass>();
|
|
AU.addPreserved<DominatorTreeWrapperPass>();
|
|
|
|
AU.addRequired<LoopInfoWrapperPass>();
|
|
AU.addPreserved<LoopInfoWrapperPass>();
|
|
|
|
AU.addPreserved<BasicAAWrapperPass>();
|
|
AU.addPreserved<AAResultsWrapperPass>();
|
|
AU.addPreserved<GlobalsAAWrapperPass>();
|
|
AU.addPreserved<ScalarEvolutionWrapperPass>();
|
|
AU.addPreserved<SCEVAAWrapperPass>();
|
|
AU.addPreservedID(LCSSAID);
|
|
AU.addPreserved<DependenceAnalysisWrapperPass>();
|
|
AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
|
|
AU.addPreserved<BranchProbabilityInfoWrapperPass>();
|
|
if (EnableMSSALoopDependency)
|
|
AU.addPreserved<MemorySSAWrapperPass>();
|
|
}
|
|
|
|
/// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
|
|
void verifyAnalysis() const override;
|
|
};
|
|
}
|
|
|
|
char LoopSimplify::ID = 0;
|
|
INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
|
|
"Canonicalize natural loops", false, false)
|
|
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
|
|
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
|
|
INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
|
|
"Canonicalize natural loops", false, false)
|
|
|
|
// Publicly exposed interface to pass...
|
|
char &llvm::LoopSimplifyID = LoopSimplify::ID;
|
|
Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
|
|
|
|
/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
|
|
/// it in any convenient order) inserting preheaders...
|
|
///
|
|
bool LoopSimplify::runOnFunction(Function &F) {
|
|
bool Changed = false;
|
|
LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
|
|
DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
|
|
auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
|
|
ScalarEvolution *SE = SEWP ? &SEWP->getSE() : nullptr;
|
|
AssumptionCache *AC =
|
|
&getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
|
|
MemorySSA *MSSA = nullptr;
|
|
std::unique_ptr<MemorySSAUpdater> MSSAU;
|
|
if (EnableMSSALoopDependency) {
|
|
auto *MSSAAnalysis = getAnalysisIfAvailable<MemorySSAWrapperPass>();
|
|
if (MSSAAnalysis) {
|
|
MSSA = &MSSAAnalysis->getMSSA();
|
|
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
|
|
}
|
|
}
|
|
|
|
bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
|
|
|
|
// Simplify each loop nest in the function.
|
|
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
|
|
Changed |= simplifyLoop(*I, DT, LI, SE, AC, MSSAU.get(), PreserveLCSSA);
|
|
|
|
#ifndef NDEBUG
|
|
if (PreserveLCSSA) {
|
|
bool InLCSSA = all_of(
|
|
*LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT, *LI); });
|
|
assert(InLCSSA && "LCSSA is broken after loop-simplify.");
|
|
}
|
|
#endif
|
|
return Changed;
|
|
}
|
|
|
|
PreservedAnalyses LoopSimplifyPass::run(Function &F,
|
|
FunctionAnalysisManager &AM) {
|
|
bool Changed = false;
|
|
LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);
|
|
DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
|
|
ScalarEvolution *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
|
|
AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
|
|
auto *MSSAAnalysis = AM.getCachedResult<MemorySSAAnalysis>(F);
|
|
std::unique_ptr<MemorySSAUpdater> MSSAU;
|
|
if (MSSAAnalysis) {
|
|
auto *MSSA = &MSSAAnalysis->getMSSA();
|
|
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
|
|
}
|
|
|
|
|
|
// Note that we don't preserve LCSSA in the new PM, if you need it run LCSSA
|
|
// after simplifying the loops. MemorySSA is preserved if it exists.
|
|
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
|
|
Changed |=
|
|
simplifyLoop(*I, DT, LI, SE, AC, MSSAU.get(), /*PreserveLCSSA*/ false);
|
|
|
|
if (!Changed)
|
|
return PreservedAnalyses::all();
|
|
|
|
PreservedAnalyses PA;
|
|
PA.preserve<DominatorTreeAnalysis>();
|
|
PA.preserve<LoopAnalysis>();
|
|
PA.preserve<BasicAA>();
|
|
PA.preserve<GlobalsAA>();
|
|
PA.preserve<SCEVAA>();
|
|
PA.preserve<ScalarEvolutionAnalysis>();
|
|
PA.preserve<DependenceAnalysis>();
|
|
if (MSSAAnalysis)
|
|
PA.preserve<MemorySSAAnalysis>();
|
|
// BPI maps conditional terminators to probabilities, LoopSimplify can insert
|
|
// blocks, but it does so only by splitting existing blocks and edges. This
|
|
// results in the interesting property that all new terminators inserted are
|
|
// unconditional branches which do not appear in BPI. All deletions are
|
|
// handled via ValueHandle callbacks w/in BPI.
|
|
PA.preserve<BranchProbabilityAnalysis>();
|
|
return PA;
|
|
}
|
|
|
|
// FIXME: Restore this code when we re-enable verification in verifyAnalysis
|
|
// below.
|
|
#if 0
|
|
static void verifyLoop(Loop *L) {
|
|
// Verify subloops.
|
|
for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
|
|
verifyLoop(*I);
|
|
|
|
// It used to be possible to just assert L->isLoopSimplifyForm(), however
|
|
// with the introduction of indirectbr, there are now cases where it's
|
|
// not possible to transform a loop as necessary. We can at least check
|
|
// that there is an indirectbr near any time there's trouble.
|
|
|
|
// Indirectbr can interfere with preheader and unique backedge insertion.
|
|
if (!L->getLoopPreheader() || !L->getLoopLatch()) {
|
|
bool HasIndBrPred = false;
|
|
for (pred_iterator PI = pred_begin(L->getHeader()),
|
|
PE = pred_end(L->getHeader()); PI != PE; ++PI)
|
|
if (isa<IndirectBrInst>((*PI)->getTerminator())) {
|
|
HasIndBrPred = true;
|
|
break;
|
|
}
|
|
assert(HasIndBrPred &&
|
|
"LoopSimplify has no excuse for missing loop header info!");
|
|
(void)HasIndBrPred;
|
|
}
|
|
|
|
// Indirectbr can interfere with exit block canonicalization.
|
|
if (!L->hasDedicatedExits()) {
|
|
bool HasIndBrExiting = false;
|
|
SmallVector<BasicBlock*, 8> ExitingBlocks;
|
|
L->getExitingBlocks(ExitingBlocks);
|
|
for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
|
|
if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
|
|
HasIndBrExiting = true;
|
|
break;
|
|
}
|
|
}
|
|
|
|
assert(HasIndBrExiting &&
|
|
"LoopSimplify has no excuse for missing exit block info!");
|
|
(void)HasIndBrExiting;
|
|
}
|
|
}
|
|
#endif
|
|
|
|
void LoopSimplify::verifyAnalysis() const {
|
|
// FIXME: This routine is being called mid-way through the loop pass manager
|
|
// as loop passes destroy this analysis. That's actually fine, but we have no
|
|
// way of expressing that here. Once all of the passes that destroy this are
|
|
// hoisted out of the loop pass manager we can add back verification here.
|
|
#if 0
|
|
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
|
|
verifyLoop(*I);
|
|
#endif
|
|
}
|