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rewrite LCSSA to use SSAUpdate, to only return true if it modifies

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the IR, and to implement the FIXME'd optimization.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83748 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2009-10-11 02:53:37 +00:00
parent 39b0c3d613
commit 8c1db67a4c
1 changed files with 130 additions and 154 deletions
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284
lib/Transforms/Utils/LCSSA.cpp
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@ -33,22 +33,19 @@
#include "llvm/Pass.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/LLVMContext.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/PredIteratorCache.h"
#include <map>
using namespace llvm;
STATISTIC(NumLCSSA, "Number of live out of a loop variables");
namespace {
struct VISIBILITY_HIDDEN LCSSA : public LoopPass {
struct LCSSA : public LoopPass {
static char ID; // Pass identification, replacement for typeid
LCSSA() : LoopPass(&ID) {}
@ -83,8 +80,8 @@ namespace {
AU.addPreserved<DominanceFrontier>();
}
private:
void ProcessInstruction(Instruction* Instr,
const SmallVector<BasicBlock*, 8>& exitBlocks);
bool ProcessInstruction(Instruction *Inst,
const SmallVectorImpl<BasicBlock*> &ExitBlocks);
/// verifyAnalysis() - Verify loop nest.
virtual void verifyAnalysis() const {
@ -92,12 +89,6 @@ namespace {
assert(L->isLCSSAForm() && "LCSSA form not preserved!");
}
void getLoopValuesUsedOutsideLoop(Loop *L,
SmallVectorImpl<Instruction*> &AffectedValues);
Value *GetValueForBlock(DomTreeNode *BB, Instruction *OrigInst,
DenseMap<DomTreeNode*, Value*> &Phis);
/// inLoop - returns true if the given block is within the current loop
bool inLoop(BasicBlock *B) const {
return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
@ -111,178 +102,163 @@ static RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
Pass *llvm::createLCSSAPass() { return new LCSSA(); }
const PassInfo *const llvm::LCSSAID = &X;
/// BlockDominatesAnExit - Return true if the specified block dominates at least
/// one of the blocks in the specified list.
static bool BlockDominatesAnExit(BasicBlock *BB,
const SmallVectorImpl<BasicBlock*> &ExitBlocks,
DominatorTree *DT) {
DomTreeNode *DomNode = DT->getNode(BB);
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
if (DT->dominates(DomNode, DT->getNode(ExitBlocks[i])))
return true;
return false;
}
/// runOnFunction - Process all loops in the function, inner-most out.
bool LCSSA::runOnLoop(Loop *l, LPPassManager &LPM) {
L = l;
PredCache.clear();
bool LCSSA::runOnLoop(Loop *TheLoop, LPPassManager &LPM) {
L = TheLoop;
LI = &LPM.getAnalysis<LoopInfo>();
DT = &getAnalysis<DominatorTree>();
// Get the set of exiting blocks.
SmallVector<BasicBlock*, 8> ExitBlocks;
L->getExitBlocks(ExitBlocks);
if (ExitBlocks.empty())
return false;
// Speed up queries by creating a sorted vector of blocks.
LoopBlocks.clear();
LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
array_pod_sort(LoopBlocks.begin(), LoopBlocks.end());
SmallVector<Instruction*, 16> AffectedValues;
getLoopValuesUsedOutsideLoop(L, AffectedValues);
// Look at all the instructions in the loop, checking to see if they have uses
// outside the loop. If so, rewrite those uses.
bool MadeChange = false;
// If no values are affected, we can save a lot of work, since we know that
// nothing will be changed.
if (AffectedValues.empty())
return false;
SmallVector<BasicBlock*, 8> ExitBlocks;
L->getExitBlocks(ExitBlocks);
// Iterate over all affected values for this loop and insert Phi nodes
// for them in the appropriate exit blocks.
for (SmallVectorImpl<Instruction*>::iterator I = AffectedValues.begin(),
E = AffectedValues.end(); I != E; ++I)
ProcessInstruction(*I, ExitBlocks);
for (Loop::block_iterator BBI = L->block_begin(), E = L->block_end();
BBI != E; ++BBI) {
BasicBlock *BB = *BBI;
// 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, ExitBlocks, DT))
continue;
for (BasicBlock::iterator I = BB->begin(), E = BB->end();
I != E; ++I) {
// 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->use_back()->getParent() == BB &&
!isa<PHINode>(I->use_back())))
continue;
MadeChange |= ProcessInstruction(I, ExitBlocks);
}
}
assert(L->isLCSSAForm());
return true;
PredCache.clear();
return MadeChange;
}
/// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
/// eliminate all out-of-loop uses.
void LCSSA::ProcessInstruction(Instruction *Instr,
const SmallVector<BasicBlock*, 8> &ExitBlocks) {
/// isExitBlock - Return true if the specified block is in the list.
static bool isExitBlock(BasicBlock *BB,
const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
if (ExitBlocks[i] == BB)
return true;
return false;
}
/// ProcessInstruction - 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.
bool LCSSA::ProcessInstruction(Instruction *Inst,
const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
SmallVector<Use*, 16> UsesToRewrite;
BasicBlock *InstBB = Inst->getParent();
for (Value::use_iterator UI = Inst->use_begin(), E = Inst->use_end();
UI != E; ++UI) {
BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
if (PHINode *PN = dyn_cast<PHINode>(*UI))
UserBB = PN->getIncomingBlock(UI);
if (InstBB != UserBB && !inLoop(UserBB))
UsesToRewrite.push_back(&UI.getUse());
}
// If there are no uses outside the loop, exit with no change.
if (UsesToRewrite.empty()) return false;
++NumLCSSA; // We are applying the transformation
// Keep track of the blocks that have the value available already.
DenseMap<DomTreeNode*, Value*> Phis;
BasicBlock *DomBB = Instr->getParent();
// 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.
if (InvokeInst *Inv = dyn_cast<InvokeInst>(Instr))
BasicBlock *DomBB = Inst->getParent();
if (InvokeInst *Inv = dyn_cast<InvokeInst>(Inst))
DomBB = Inv->getNormalDest();
DomTreeNode *DomNode = DT->getNode(DomBB);
// Insert the LCSSA phi's into the exit blocks (dominated by the value), and
// add them to the Phi's map.
for (SmallVector<BasicBlock*, 8>::const_iterator BBI = ExitBlocks.begin(),
SSAUpdater SSAUpdate;
SSAUpdate.Initialize(Inst);
// Insert the LCSSA phi's into all of the exit blocks dominated by the
// value., and add them to the Phi's map.
for (SmallVectorImpl<BasicBlock*>::const_iterator BBI = ExitBlocks.begin(),
BBE = ExitBlocks.end(); BBI != BBE; ++BBI) {
BasicBlock *BB = *BBI;
DomTreeNode *ExitBBNode = DT->getNode(BB);
Value *&Phi = Phis[ExitBBNode];
if (!Phi && DT->dominates(DomNode, ExitBBNode)) {
PHINode *PN = PHINode::Create(Instr->getType(), Instr->getName()+".lcssa",
BB->begin());
PN->reserveOperandSpace(PredCache.GetNumPreds(BB));
BasicBlock *ExitBB = *BBI;
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(), Inst->getName()+".lcssa",
ExitBB->begin());
PN->reserveOperandSpace(PredCache.GetNumPreds(ExitBB));
// Remember that this phi makes the value alive in this block.
Phi = PN;
// Add inputs from inside the loop for this PHI.
for (BasicBlock** PI = PredCache.GetPreds(BB); *PI; ++PI)
PN->addIncoming(Instr, *PI);
}
// Add inputs from inside the loop for this PHI.
for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI)
PN->addIncoming(Inst, *PI);
// Remember that this phi makes the value alive in this block.
SSAUpdate.AddAvailableValue(ExitBB, PN);
}
// Record all uses of Instr outside the loop. We need to rewrite these. The
// LCSSA phis won't be included because they use the value in the loop.
for (Value::use_iterator UI = Instr->use_begin(), E = Instr->use_end();
UI != E;) {
BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
if (PHINode *P = dyn_cast<PHINode>(*UI))
UserBB = P->getIncomingBlock(UI);
// If the user is in the loop, don't rewrite it!
if (UserBB == Instr->getParent() || inLoop(UserBB)) {
++UI;
// Rewrite all uses outside the loop in terms of the new PHIs we just
// inserted.
for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
// 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>(UsesToRewrite[i]->getUser());
BasicBlock *UserBB = User->getParent();
if (PHINode *PN = dyn_cast<PHINode>(User))
UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
if (isa<PHINode>(UserBB->begin()) &&
isExitBlock(UserBB, ExitBlocks)) {
UsesToRewrite[i]->set(UserBB->begin());
continue;
}
// Otherwise, patch up uses of the value with the appropriate LCSSA Phi,
// inserting PHI nodes into join points where needed.
Value *Val = GetValueForBlock(DT->getNode(UserBB), Instr, Phis);
// Preincrement the iterator to avoid invalidating it when we change the
// value.
Use &U = UI.getUse();
++UI;
U.set(Val);
}
}
/// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
/// are used by instructions outside of it.
void LCSSA::getLoopValuesUsedOutsideLoop(Loop *L,
SmallVectorImpl<Instruction*> &AffectedValues) {
// FIXME: For large loops, we may be able to avoid a lot of 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.
for (Loop::block_iterator BB = L->block_begin(), BE = L->block_end();
BB != BE; ++BB) {
for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
UI != UE; ++UI) {
BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
if (PHINode *PN = dyn_cast<PHINode>(*UI))
UserBB = PN->getIncomingBlock(UI);
if (*BB != UserBB && !inLoop(UserBB)) {
AffectedValues.push_back(I);
break;
}
}
}
}
/// GetValueForBlock - Get the value to use within the specified basic block.
/// available values are in Phis.
Value *LCSSA::GetValueForBlock(DomTreeNode *BB, Instruction *OrigInst,
DenseMap<DomTreeNode*, Value*> &Phis) {
// If there is no dominator info for this BB, it is unreachable.
if (BB == 0)
return UndefValue::get(OrigInst->getType());
// If we have already computed this value, return the previously computed val.
if (Phis.count(BB)) return Phis[BB];
DomTreeNode *IDom = BB->getIDom();
// Otherwise, there are two cases: we either have to insert a PHI node or we
// don't. We need to insert a PHI node if this block is not dominated by one
// of the exit nodes from the loop (the loop could have multiple exits, and
// though the value defined *inside* the loop dominated all its uses, each
// exit by itself may not dominate all the uses).
//
// The simplest way to check for this condition is by checking to see if the
// idom is in the loop. If so, we *know* that none of the exit blocks
// dominate this block. Note that we *know* that the block defining the
// original instruction is in the idom chain, because if it weren't, then the
// original value didn't dominate this use.
if (!inLoop(IDom->getBlock())) {
// Idom is not in the loop, we must still be "below" the exit block and must
// be fully dominated by the value live in the idom.
Value* val = GetValueForBlock(IDom, OrigInst, Phis);
Phis.insert(std::make_pair(BB, val));
return val;
// Otherwise, do full PHI insertion.
SSAUpdate.RewriteUse(*UsesToRewrite[i]);
}
BasicBlock *BBN = BB->getBlock();
// Otherwise, the idom is the loop, so we need to insert a PHI node. Do so
// now, then get values to fill in the incoming values for the PHI.
PHINode *PN = PHINode::Create(OrigInst->getType(),
OrigInst->getName() + ".lcssa", BBN->begin());
PN->reserveOperandSpace(PredCache.GetNumPreds(BBN));
Phis.insert(std::make_pair(BB, PN));
// Fill in the incoming values for the block.
for (BasicBlock **PI = PredCache.GetPreds(BBN); *PI; ++PI)
PN->addIncoming(GetValueForBlock(DT->getNode(*PI), OrigInst, Phis), *PI);
return PN;
return true;
}