lib/Transforms/Utils/LCSSA.cpp - native_client/pnacl-llvm - Git at Google

 //===-- 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/ADT/STLExtras.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/LoopPass.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/PredIteratorCache.h"
 #include "llvm/Transforms/Utils/SSAUpdater.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;
     LoopInfo *LI;
     ScalarEvolution *SE;
     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>();
   LI = &getAnalysis<LoopInfo>();
   SE = getAnalysisIfAvailable<ScalarEvolution>();

   // 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);
     }
   }

   // 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 && MadeChange)
     SE->forgetLoop(L);

   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)) {
       // Tell the VHs that the uses changed. This updates SCEV's caches.
       if (UsesToRewrite[i]->get()->hasValueHandle())
         ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
       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;
 }
	//===-- 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/ADT/STLExtras.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Analysis/Dominators.h"
	#include "llvm/Analysis/LoopPass.h"
	#include "llvm/Analysis/ScalarEvolution.h"
	#include "llvm/IR/Constants.h"
	#include "llvm/IR/Function.h"
	#include "llvm/IR/Instructions.h"
	#include "llvm/Pass.h"
	#include "llvm/Support/PredIteratorCache.h"
	#include "llvm/Transforms/Utils/SSAUpdater.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;
	LoopInfo *LI;
	ScalarEvolution *SE;
	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>();
	LI = &getAnalysis<LoopInfo>();
	SE = getAnalysisIfAvailable<ScalarEvolution>();

	// 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);
	}
	}

	// 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 && MadeChange)
	SE->forgetLoop(L);

	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)) {
	// Tell the VHs that the uses changed. This updates SCEV's caches.
	if (UsesToRewrite[i]->get()->hasValueHandle())
	ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
	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;
	}