280 lines
9.9 KiB
C++
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;
|
||
|
|
}
|
||
|
|
|