llvm-6502/lib/Transforms/Utils/LCSSA.cpp
2011-05-16 22:05:03 +00:00

280 lines
9.9 KiB
C++

//===-- 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.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "lcssa"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Pass.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/PredIteratorCache.h"
using namespace llvm;
STATISTIC(NumLCSSA, "Number of live out of a loop variables");
namespace {
struct LCSSA : public LoopPass {
static char ID; // Pass identification, replacement for typeid
LCSSA() : LoopPass(ID) {
initializeLCSSAPass(*PassRegistry::getPassRegistry());
}
// Cached analysis information for the current function.
DominatorTree *DT;
std::vector<BasicBlock*> LoopBlocks;
PredIteratorCache PredCache;
Loop *L;
virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
/// 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.
///
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequired<DominatorTree>();
AU.addRequired<LoopInfo>();
AU.addPreservedID(LoopSimplifyID);
AU.addPreserved<ScalarEvolution>();
}
private:
bool ProcessInstruction(Instruction *Inst,
const SmallVectorImpl<BasicBlock*> &ExitBlocks);
/// verifyAnalysis() - Verify loop nest.
virtual void verifyAnalysis() const {
// Check the special guarantees that LCSSA makes.
assert(L->isLCSSAForm(*DT) && "LCSSA form not preserved!");
}
/// 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);
}
};
}
char LCSSA::ID = 0;
INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
Pass *llvm::createLCSSAPass() { return new LCSSA(); }
char &llvm::LCSSAID = LCSSA::ID;
/// 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 *TheLoop, LPPassManager &LPM) {
L = TheLoop;
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());
// 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;
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(*DT));
PredCache.clear();
return MadeChange;
}
/// 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) {
User *U = *UI;
BasicBlock *UserBB = cast<Instruction>(U)->getParent();
if (PHINode *PN = dyn_cast<PHINode>(U))
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
// 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;
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 (SmallVectorImpl<BasicBlock*>::const_iterator BBI = ExitBlocks.begin(),
BBE = ExitBlocks.end(); BBI != BBE; ++BBI) {
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(),
PredCache.GetNumPreds(ExitBB),
Inst->getName()+".lcssa",
ExitBB->begin());
// Add inputs from inside the loop for this PHI.
for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
PN->addIncoming(Inst, *PI);
// 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 (!inLoop(*PI))
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);
}
// 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, do full PHI insertion.
SSAUpdate.RewriteUse(*UsesToRewrite[i]);
}
// Remove PHI nodes that did not have any uses rewritten.
for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
if (AddedPHIs[i]->use_empty())
AddedPHIs[i]->eraseFromParent();
}
return true;
}