Initial checkin of a simple loop unswitching pass. It still needs work,

but it's a start, and seems to do it's basic job.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13068 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-04-19 18:07:02 +00:00
parent 74332be31e
commit 18f16094c2

View File

@ -0,0 +1,357 @@
//===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass transforms loops that contain branches on loop-invariant conditions
// to have multiple loops. For example, it turns the left into the right code:
//
// for (...) if (lic)
// A for (...)
// if (lic) A; B; C
// B else
// C for (...)
// A; C
//
// This can increase the size of the code exponentially (doubling it every time
// a loop is unswitched) so we only unswitch if the resultant code will be
// smaller than a threshold.
//
// This pass expects LICM to be run before it to hoist invariant conditions out
// of the loop, to make the unswitching opportunity obvious.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "loop-unswitch"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "Support/Debug.h"
#include "Support/Statistic.h"
using namespace llvm;
namespace {
Statistic<> NumUnswitched("loop-unswitch", "Number of loops unswitched");
class LoopUnswitch : public FunctionPass {
LoopInfo *LI; // Loop information
DominatorSet *DS;
public:
virtual bool runOnFunction(Function &F);
bool visitLoop(Loop *L);
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG...
///
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequiredID(LoopSimplifyID);
//AU.addRequired<DominatorSet>();
AU.addRequired<LoopInfo>();
AU.addPreserved<LoopInfo>();
}
private:
void VersionLoop(Value *LIC, Loop *L);
BasicBlock *SplitBlock(BasicBlock *BB, bool SplitAtTop);
void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, bool Val);
};
RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
}
FunctionPass *createLoopUnswitchPass() { return new LoopUnswitch(); }
bool LoopUnswitch::runOnFunction(Function &F) {
bool Changed = false;
LI = &getAnalysis<LoopInfo>();
DS = 0; //&getAnalysis<DominatorSet>();
// Transform all the top-level loops. Copy the loop list so that the child
// can update the loop tree if it needs to delete the loop.
std::vector<Loop*> SubLoops(LI->begin(), LI->end());
for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
Changed |= visitLoop(SubLoops[i]);
return Changed;
}
bool LoopUnswitch::visitLoop(Loop *L) {
bool Changed = false;
// Recurse through all subloops before we process this loop. Copy the loop
// list so that the child can update the loop tree if it needs to delete the
// loop.
std::vector<Loop*> SubLoops(L->begin(), L->end());
for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
Changed |= visitLoop(SubLoops[i]);
// Loop over all of the basic blocks in the loop. If we find an interior
// block that is branching on a loop-invariant condition, we can unswitch this
// loop.
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I) {
TerminatorInst *TI = (*I)->getTerminator();
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
DEBUG(std::cerr << "Can't unswitching 'switch' loop %"
<< L->getHeader()->getName() << ", cost = "
<< L->getBlocks().size() << "\n" << **I);
} else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
if (BI->isConditional() && !isa<Constant>(BI->getCondition()) &&
L->isLoopInvariant(BI->getCondition())) {
// Check to see if it would be profitable to unswitch this loop.
if (L->getBlocks().size() > 10) {
DEBUG(std::cerr << "NOT unswitching loop %"
<< L->getHeader()->getName() << ", cost too high: "
<< L->getBlocks().size() << "\n");
} else {
// FIXME: check for profitability.
//std::cerr << "BEFORE:\n"; LI->dump();
VersionLoop(BI->getCondition(), L);
//std::cerr << "AFTER:\n"; LI->dump();
return true;
}
}
}
return Changed;
}
/// SplitBlock - Split the specified basic block into two pieces. If SplitAtTop
/// is false, this splits the block so the second half only has an unconditional
/// branch. If SplitAtTop is true, it makes it so the first half of the block
/// only has an unconditional branch in it.
///
/// This method updates the LoopInfo for this function to correctly reflect the
/// CFG changes made.
BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *BB, bool SplitAtTop) {
BasicBlock::iterator SplitPoint;
if (!SplitAtTop)
SplitPoint = BB->getTerminator();
else {
SplitPoint = BB->begin();
while (isa<PHINode>(SplitPoint)) ++SplitPoint;
}
BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
// New now lives in whichever loop that BB used to.
if (Loop *L = LI->getLoopFor(BB))
L->addBasicBlockToLoop(New, *LI);
return SplitAtTop ? BB : New;
}
// RemapInstruction - Convert the instruction operands from referencing the
// current values into those specified by ValueMap.
//
static inline void RemapInstruction(Instruction *I,
std::map<const Value *, Value*> &ValueMap) {
for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
Value *Op = I->getOperand(op);
std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
if (It != ValueMap.end()) Op = It->second;
I->setOperand(op, Op);
}
}
/// CloneLoop - Recursively clone the specified loop and all of its children,
/// mapping the blocks with the specified map.
static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
LoopInfo *LI) {
Loop *New = new Loop();
if (PL)
PL->addChildLoop(New);
else
LI->addTopLevelLoop(New);
// Add all of the blocks in L to the new loop.
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I)
if (LI->getLoopFor(*I) == L)
New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
// Add all of the subloops to the new loop.
for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
CloneLoop(*I, New, VM, LI);
return New;
}
/// InsertPHINodesForUsesOutsideLoop - If this instruction is used outside of
/// the specified loop, insert a PHI node in the appropriate exit block to merge
/// the values in the two different loop versions.
///
/// Most values are not used outside of the loop they are defined in, so be
/// efficient for this case.
///
static AllocaInst *
InsertPHINodesForUsesOutsideLoop(Instruction *OI, Instruction *NI,
DominatorSet &DS, Loop *OL, Loop *NL,
std::vector<BasicBlock*> &OldExitBlocks,
std::map<const Value*, Value*> &ValueMap) {
assert(OI->getType() == NI->getType() && OI->getOpcode() == NI->getOpcode() &&
"Hrm, should be mapping between identical instructions!");
for (Value::use_iterator UI = OI->use_begin(), E = OI->use_end(); UI != E;
++UI)
if (!OL->contains(cast<Instruction>(*UI)->getParent()) &&
!NL->contains(cast<Instruction>(*UI)->getParent()))
goto UsedOutsideOfLoop;
return 0;
UsedOutsideOfLoop:
// Okay, this instruction is used outside of the current loop. Insert a PHI
// nodes for the instruction merging the values together.
// FIXME: For now we just spill the object to the stack, assuming that a
// subsequent mem2reg pass will clean up after us. This should be improved in
// two ways:
// 1. If there is only one exit block, trivially insert the PHI nodes
// 2. Once we update domfrontier, we should do the promotion after everything
// is stable again.
AllocaInst *Result = DemoteRegToStack(*OI);
// Store to the stack location right after the new instruction.
BasicBlock::iterator InsertPoint = NI;
if (InvokeInst *II = dyn_cast<InvokeInst>(NI))
InsertPoint = II->getNormalDest()->begin();
else
++InsertPoint;
while (isa<PHINode>(InsertPoint)) ++InsertPoint;
new StoreInst(NI, Result, InsertPoint);
return Result;
}
/// VersionLoop - We determined that the loop is profitable to unswitch and
/// contains a branch on a loop invariant condition. Split it into loop
/// versions and test the condition outside of either loop.
void LoopUnswitch::VersionLoop(Value *LIC, Loop *L) {
Function *F = L->getHeader()->getParent();
DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
<< L->getHeader()->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function " << F->getName()
<< " on cond:" << *LIC << "\n");
std::vector<BasicBlock*> LoopBlocks;
// First step, split the preheader and exit blocks, and add these blocks to
// the LoopBlocks list.
BasicBlock *OrigPreheader = L->getLoopPreheader();
LoopBlocks.push_back(SplitBlock(OrigPreheader, false));
// We want the loop to come after the preheader, but before the exit blocks.
LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
std::vector<BasicBlock*> ExitBlocks;
L->getExitBlocks(ExitBlocks);
std::sort(ExitBlocks.begin(), ExitBlocks.end());
ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
ExitBlocks.end());
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
LoopBlocks.push_back(ExitBlocks[i] = SplitBlock(ExitBlocks[i], true));
// Next step, clone all of the basic blocks that make up the loop (including
// the loop preheader and exit blocks), keeping track of the mapping between
// the instructions and blocks.
std::vector<BasicBlock*> NewBlocks;
NewBlocks.reserve(LoopBlocks.size());
std::map<const Value*, Value*> ValueMap;
for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
NewBlocks.push_back(CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F));
ValueMap[LoopBlocks[i]] = NewBlocks.back(); // Keep the BB mapping.
}
// Splice the newly inserted blocks into the function right before the
// original preheader.
F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
NewBlocks[0], F->end());
// Now we create the new Loop object for the versioned loop.
Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
if (Loop *Parent = L->getParentLoop()) {
// Make sure to add the cloned preheader and exit blocks to the parent loop
// as well.
Parent->addBasicBlockToLoop(NewBlocks[0], *LI);
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
Parent->addBasicBlockToLoop(cast<BasicBlock>(ValueMap[ExitBlocks[i]]),
*LI);
}
// Rewrite the code to refer to itself.
for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
for (BasicBlock::iterator I = NewBlocks[i]->begin(),
E = NewBlocks[i]->end(); I != E; ++I)
RemapInstruction(I, ValueMap);
// If the instructions are used outside of the loop, insert a PHI node in any
// exit blocks dominated by the instruction.
for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
for (BasicBlock::iterator OI = LoopBlocks[i]->begin(),
E = LoopBlocks[i]->end(); OI != E; ++OI)
if (!OI->use_empty()) {
std::map<const Value*,Value*>::iterator OII = ValueMap.find(OI);
// The PHINode rewriting stuff can insert stores that are not in the
// mapping. Don't mess around with them.
if (OII != ValueMap.end()) {
Instruction *NI = cast<Instruction>(OII->second);
InsertPHINodesForUsesOutsideLoop(OI, NI, *DS, L, NewLoop,
ExitBlocks, ValueMap);
}
}
// Rewrite the original preheader to select between versions of the loop.
assert(isa<BranchInst>(OrigPreheader->getTerminator()) &&
cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
"Preheader splitting did not work correctly!");
// Remove the unconditional branch to LoopBlocks[0].
OrigPreheader->getInstList().pop_back();
// Insert a conditional branch on LIC to the two preheaders. The original
// code is the true version and the new code is the false version.
new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);
// Now we rewrite the original code to know that the condition is true and the
// new code to know that the condition is false.
RewriteLoopBodyWithConditionConstant(L, LIC, true);
RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
++NumUnswitched;
// Try to unswitch each of our new loops now!
visitLoop(L);
visitLoop(NewLoop);
}
// RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
// the value specified by Val in the specified loop. Rewrite any uses of LIC or
// of properties correlated to it.
void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
bool Val) {
// FIXME: Support correlated properties, like:
// for (...)
// if (li1 < li2)
// ...
// if (li1 > li2)
// ...
ConstantBool *BoolVal = ConstantBool::get(Val);
std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
for (unsigned i = 0, e = Users.size(); i != e; ++i)
if (Instruction *U = dyn_cast<Instruction>(Users[i]))
if (L->contains(U->getParent()))
U->replaceUsesOfWith(LIC, BoolVal);
}