You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
351 lines
13 KiB
351 lines
13 KiB
//===-- 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");
|
|
|
|
/// Return true if the specified block is in the list.
|
|
static bool isExitBlock(BasicBlock *BB,
|
|
const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
|
|
return find(ExitBlocks, BB) != ExitBlocks.end();
|
|
}
|
|
|
|
/// Given an instruction in the loop, check to see if it has any uses that are
|
|
/// outside the current loop. If so, insert LCSSA PHI nodes and rewrite the
|
|
/// uses.
|
|
static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT,
|
|
const SmallVectorImpl<BasicBlock *> &ExitBlocks,
|
|
PredIteratorCache &PredCache, LoopInfo *LI) {
|
|
SmallVector<Use *, 16> UsesToRewrite;
|
|
|
|
// 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 (Inst.getType()->isTokenTy())
|
|
return false;
|
|
|
|
BasicBlock *InstBB = Inst.getParent();
|
|
|
|
for (Use &U : Inst.uses()) {
|
|
Instruction *User = cast<Instruction>(U.getUser());
|
|
BasicBlock *UserBB = User->getParent();
|
|
if (PHINode *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())
|
|
return false;
|
|
|
|
++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 = Inst.getParent();
|
|
if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst))
|
|
DomBB = Inv->getNormalDest();
|
|
|
|
DomTreeNode *DomNode = DT.getNode(DomBB);
|
|
|
|
SmallVector<PHINode *, 16> AddedPHIs;
|
|
SmallVector<PHINode *, 8> PostProcessPHIs;
|
|
|
|
SSAUpdater SSAUpdate;
|
|
SSAUpdate.Initialize(Inst.getType(), Inst.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(Inst.getType(), PredCache.size(ExitBB),
|
|
Inst.getName() + ".lcssa", &ExitBB->front());
|
|
|
|
// Add inputs from inside the loop for this PHI.
|
|
for (BasicBlock *Pred : PredCache.get(ExitBB)) {
|
|
PN->addIncoming(&Inst, 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 (PHINode *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);
|
|
}
|
|
|
|
// Post process PHI instructions that were inserted into another disjoint loop
|
|
// and update their exits properly.
|
|
for (auto *I : PostProcessPHIs) {
|
|
if (I->use_empty())
|
|
continue;
|
|
|
|
BasicBlock *PHIBB = I->getParent();
|
|
Loop *OtherLoop = LI->getLoopFor(PHIBB);
|
|
SmallVector<BasicBlock *, 8> EBs;
|
|
OtherLoop->getExitBlocks(EBs);
|
|
if (EBs.empty())
|
|
continue;
|
|
|
|
// Recurse and re-process each PHI instruction. FIXME: we should really
|
|
// convert this entire thing to a worklist approach where we process a
|
|
// vector of instructions...
|
|
processInstruction(*OtherLoop, *I, DT, EBs, PredCache, LI);
|
|
}
|
|
|
|
// Remove PHI nodes that did not have any uses rewritten.
|
|
for (PHINode *PN : AddedPHIs)
|
|
if (PN->use_empty())
|
|
PN->eraseFromParent();
|
|
|
|
return true;
|
|
}
|
|
|
|
/// Return true if the specified block dominates at least
|
|
/// one of the blocks in the specified list.
|
|
static bool
|
|
blockDominatesAnExit(BasicBlock *BB,
|
|
DominatorTree &DT,
|
|
const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
|
|
DomTreeNode *DomNode = DT.getNode(BB);
|
|
return llvm::any_of(ExitBlocks, [&](BasicBlock * EB) {
|
|
return DT.dominates(DomNode, DT.getNode(EB));
|
|
});
|
|
}
|
|
|
|
bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
|
|
ScalarEvolution *SE) {
|
|
bool Changed = false;
|
|
|
|
// Get the set of exiting blocks.
|
|
SmallVector<BasicBlock *, 8> ExitBlocks;
|
|
L.getExitBlocks(ExitBlocks);
|
|
|
|
if (ExitBlocks.empty())
|
|
return false;
|
|
|
|
PredIteratorCache PredCache;
|
|
|
|
// Look at all the instructions in the loop, checking to see if they have uses
|
|
// outside the loop. If so, rewrite those uses.
|
|
for (BasicBlock *BB : L.blocks()) {
|
|
// For large loops, avoid use-scanning by using dominance information: In
|
|
// particular, if a block does not dominate any of the loop exits, then none
|
|
// of the values defined in the block could be used outside the loop.
|
|
if (!blockDominatesAnExit(BB, DT, ExitBlocks))
|
|
continue;
|
|
|
|
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;
|
|
|
|
Changed |= processInstruction(L, I, DT, ExitBlocks, PredCache, 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;
|
|
|
|
/// 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>();
|
|
}
|
|
};
|
|
}
|
|
|
|
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_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, AnalysisManager<Function> &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();
|
|
|
|
// FIXME: This should also 'preserve the CFG'.
|
|
PreservedAnalyses PA;
|
|
PA.preserve<BasicAA>();
|
|
PA.preserve<GlobalsAA>();
|
|
PA.preserve<SCEVAA>();
|
|
PA.preserve<ScalarEvolutionAnalysis>();
|
|
return PA;
|
|
}
|