llvm-6502/lib/Transforms/Utils/LCSSA.cpp
2008-09-04 17:05:41 +00:00

274 lines
10 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/ADT/SetVector.h"
#include "llvm/ADT/Statistic.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 <algorithm>
#include <map>
using namespace llvm;
STATISTIC(NumLCSSA, "Number of live out of a loop variables");
namespace {
struct VISIBILITY_HIDDEN LCSSA : public LoopPass {
static char ID; // Pass identification, replacement for typeid
LCSSA() : LoopPass(&ID) {}
// Cached analysis information for the current function.
LoopInfo *LI;
DominatorTree *DT;
std::vector<BasicBlock*> LoopBlocks;
virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
void ProcessInstruction(Instruction* Instr,
const SmallVector<BasicBlock*, 8>& exitBlocks);
/// 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.addRequiredID(LoopSimplifyID);
AU.addPreservedID(LoopSimplifyID);
AU.addRequired<LoopInfo>();
AU.addPreserved<LoopInfo>();
AU.addRequired<DominatorTree>();
AU.addPreserved<ScalarEvolution>();
AU.addPreserved<DominatorTree>();
// Request DominanceFrontier now, even though LCSSA does
// not use it. This allows Pass Manager to schedule Dominance
// Frontier early enough such that one LPPassManager can handle
// multiple loop transformation passes.
AU.addRequired<DominanceFrontier>();
AU.addPreserved<DominanceFrontier>();
}
private:
void getLoopValuesUsedOutsideLoop(Loop *L,
SetVector<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) {
return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
}
};
}
char LCSSA::ID = 0;
static RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
LoopPass *llvm::createLCSSAPass() { return new LCSSA(); }
const PassInfo *const llvm::LCSSAID = &X;
/// runOnFunction - Process all loops in the function, inner-most out.
bool LCSSA::runOnLoop(Loop *L, LPPassManager &LPM) {
LI = &LPM.getAnalysis<LoopInfo>();
DT = &getAnalysis<DominatorTree>();
// Speed up queries by creating a sorted list of blocks
LoopBlocks.clear();
LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
std::sort(LoopBlocks.begin(), LoopBlocks.end());
SetVector<Instruction*> AffectedValues;
getLoopValuesUsedOutsideLoop(L, AffectedValues);
// 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 (SetVector<Instruction*>::iterator I = AffectedValues.begin(),
E = AffectedValues.end(); I != E; ++I)
ProcessInstruction(*I, exitBlocks);
assert(L->isLCSSAForm());
return true;
}
/// 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) {
++NumLCSSA; // We are applying the transformation
// Keep track of the blocks that have the value available already.
DenseMap<DomTreeNode*, Value*> Phis;
DomTreeNode *InstrNode = DT->getNode(Instr->getParent());
// 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(),
BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
BasicBlock *BB = *BBI;
DomTreeNode *ExitBBNode = DT->getNode(BB);
Value *&Phi = Phis[ExitBBNode];
if (!Phi && DT->dominates(InstrNode, ExitBBNode)) {
PHINode *PN = PHINode::Create(Instr->getType(), Instr->getName()+".lcssa",
BB->begin());
PN->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
// Remember that this phi makes the value alive in this block.
Phi = PN;
// Add inputs from inside the loop for this PHI.
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
PN->addIncoming(Instr, *PI);
}
}
// 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)) {
unsigned OperandNo = UI.getOperandNo();
UserBB = P->getIncomingBlock(OperandNo/2);
}
// If the user is in the loop, don't rewrite it!
if (UserBB == Instr->getParent() || inLoop(UserBB)) {
++UI;
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,
SetVector<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(), E = L->block_end();
BB != E; ++BB) {
for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
++UI) {
BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
if (PHINode* p = dyn_cast<PHINode>(*UI)) {
unsigned OperandNo = UI.getOperandNo();
UserBB = p->getIncomingBlock(OperandNo/2);
}
if (*BB != UserBB && !inLoop(UserBB)) {
AffectedValues.insert(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;
}
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(std::distance(pred_begin(BBN), pred_end(BBN)));
Phis.insert(std::make_pair(BB, PN));
// Fill in the incoming values for the block.
for (pred_iterator PI = pred_begin(BBN), E = pred_end(BBN); PI != E; ++PI)
PN->addIncoming(GetValueForBlock(DT->getNode(*PI), OrigInst, Phis), *PI);
return PN;
}