llvm-6502/lib/Transforms/Scalar/LoopUnswitch.cpp

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//===-- 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/LoopInfo.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/CommandLine.h"
#include <algorithm>
#include <iostream>
#include <set>
using namespace llvm;
namespace {
Statistic<> NumBranches("loop-unswitch", "Number of branches unswitched");
Statistic<> NumSwitches("loop-unswitch", "Number of switches unswitched");
Statistic<> NumSelects ("loop-unswitch", "Number of selects unswitched");
Statistic<> NumTrivial ("loop-unswitch",
"Number of unswitches that are trivial");
cl::opt<unsigned>
Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
cl::init(10), cl::Hidden);
class LoopUnswitch : public FunctionPass {
LoopInfo *LI; // Loop information
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.addPreservedID(LoopSimplifyID);
AU.addRequired<LoopInfo>();
AU.addPreserved<LoopInfo>();
}
private:
bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L);
unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
void VersionLoop(Value *LIC, Constant *OnVal,
Loop *L, Loop *&Out1, Loop *&Out2);
BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To);
void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val,
bool isEqual);
void UnswitchTrivialCondition(Loop *L, Value *Cond, bool EntersLoopOnCond,
BasicBlock *ExitBlock);
};
RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
}
FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
bool LoopUnswitch::runOnFunction(Function &F) {
bool Changed = false;
LI = &getAnalysis<LoopInfo>();
// 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;
}
/// LoopValuesUsedOutsideLoop - Return true if there are any values defined in
/// the loop that are used by instructions outside of it.
static bool LoopValuesUsedOutsideLoop(Loop *L) {
// We will be doing lots of "loop contains block" queries. Loop::contains is
// linear time, use a set to speed this up.
std::set<BasicBlock*> LoopBlocks;
for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
BB != E; ++BB)
LoopBlocks.insert(*BB);
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 (!LoopBlocks.count(UserBB))
return true;
}
}
return false;
}
/// FindTrivialLoopExitBlock - We know that we have a branch from the loop
/// header to the specified latch block. See if one of the successors of the
/// latch block is an exit, and if so what block it is.
static BasicBlock *FindTrivialLoopExitBlock(Loop *L, BasicBlock *Latch) {
BasicBlock *Header = L->getHeader();
BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator());
if (!LatchBranch || !LatchBranch->isConditional()) return 0;
// Simple case, the latch block is a conditional branch. The target that
// doesn't go to the loop header is our block if it is not in the loop.
if (LatchBranch->getSuccessor(0) == Header) {
if (L->contains(LatchBranch->getSuccessor(1))) return false;
return LatchBranch->getSuccessor(1);
} else {
assert(LatchBranch->getSuccessor(1) == Header);
if (L->contains(LatchBranch->getSuccessor(0))) return false;
return LatchBranch->getSuccessor(0);
}
}
/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
/// trivial: that is, that the condition controls whether or not the loop does
/// anything at all. If this is a trivial condition, unswitching produces no
/// code duplications (equivalently, it produces a simpler loop and a new empty
/// loop, which gets deleted).
///
/// If this is a trivial condition, return ConstantBool::True if the loop body
/// runs when the condition is true, False if the loop body executes when the
/// condition is false. Otherwise, return null to indicate a complex condition.
static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond,
bool *CondEntersLoop = 0,
BasicBlock **LoopExit = 0) {
BasicBlock *Header = L->getHeader();
BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator());
// If the header block doesn't end with a conditional branch on Cond, we can't
// handle it.
if (!HeaderTerm || !HeaderTerm->isConditional() ||
HeaderTerm->getCondition() != Cond)
return false;
// Check to see if the conditional branch goes to the latch block. If not,
// it's not trivial. This also determines the value of Cond that will execute
// the loop.
BasicBlock *Latch = L->getLoopLatch();
if (HeaderTerm->getSuccessor(1) == Latch) {
if (CondEntersLoop) *CondEntersLoop = true;
} else if (HeaderTerm->getSuccessor(0) == Latch)
if (CondEntersLoop) *CondEntersLoop = false;
else
return false; // Doesn't branch to latch block.
// The latch block must end with a conditional branch where one edge goes to
// the header (this much we know) and one edge goes OUT of the loop.
BasicBlock *LoopExitBlock = FindTrivialLoopExitBlock(L, Latch);
if (!LoopExitBlock) return 0;
if (LoopExit) *LoopExit = LoopExitBlock;
// We already know that nothing uses any scalar values defined inside of this
// loop. As such, we just have to check to see if this loop will execute any
// side-effecting instructions (e.g. stores, calls, volatile loads) in the
// part of the loop that the code *would* execute.
for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
if (I->mayWriteToMemory())
return false;
for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I)
if (I->mayWriteToMemory())
return false;
return true;
}
/// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
/// we choose to unswitch the specified loop on the specified value.
///
unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
// If the condition is trivial, always unswitch. There is no code growth for
// this case.
if (IsTrivialUnswitchCondition(L, LIC))
return 0;
unsigned Cost = 0;
// FIXME: this is brain dead. It should take into consideration code
// shrinkage.
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I) {
BasicBlock *BB = *I;
// Do not include empty blocks in the cost calculation. This happen due to
// loop canonicalization and will be removed.
if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
continue;
// Count basic blocks.
++Cost;
}
return Cost;
}
/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
/// invariant in the loop, or has an invariant piece, return the invariant.
/// Otherwise, return null.
static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
// Constants should be folded, not unswitched on!
if (isa<Constant>(Cond)) return false;
// TODO: Handle: br (VARIANT|INVARIANT).
// TODO: Hoist simple expressions out of loops.
if (L->isLoopInvariant(Cond)) return Cond;
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
if (BO->getOpcode() == Instruction::And ||
BO->getOpcode() == Instruction::Or) {
// If either the left or right side is invariant, we can unswitch on this,
// which will cause the branch to go away in one loop and the condition to
// simplify in the other one.
if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
return LHS;
if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
return RHS;
}
return 0;
}
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 (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
// If this isn't branching on an invariant condition, we can't unswitch
// it.
if (BI->isConditional()) {
// See if this, or some part of it, is loop invariant. If so, we can
// unswitch on it if we desire.
Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed);
if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) {
++NumBranches;
return true;
}
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
if (LoopCond && SI->getNumCases() > 1) {
// Find a value to unswitch on:
// FIXME: this should chose the most expensive case!
Constant *UnswitchVal = SI->getCaseValue(1);
if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) {
++NumSwitches;
return true;
}
}
}
// Scan the instructions to check for unswitchable values.
for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
BBI != E; ++BBI)
if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) {
++NumSelects;
return true;
}
}
}
return Changed;
}
/// UnswitchIfProfitable - We have found that we can unswitch L when
/// LoopCond == Val to simplify the loop. If we decide that this is profitable,
/// unswitch the loop, reprocess the pieces, then return true.
bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
// Check to see if it would be profitable to unswitch this loop.
if (getLoopUnswitchCost(L, LoopCond) > Threshold) {
// FIXME: this should estimate growth by the amount of code shared by the
// resultant unswitched loops.
//
DEBUG(std::cerr << "NOT unswitching loop %"
<< L->getHeader()->getName() << ", cost too high: "
<< L->getBlocks().size() << "\n");
return false;
}
// If this loop has live-out values, we can't unswitch it. We need something
// like loop-closed SSA form in order to know how to insert PHI nodes for
// these values.
if (LoopValuesUsedOutsideLoop(L)) {
DEBUG(std::cerr << "NOT unswitching loop %" << L->getHeader()->getName()
<< ", a loop value is used outside loop!\n");
return false;
}
//std::cerr << "BEFORE:\n"; LI->dump();
Loop *NewLoop1 = 0, *NewLoop2 = 0;
// If this is a trivial condition to unswitch (which results in no code
// duplication), do it now.
bool EntersLoopOnCond;
BasicBlock *ExitBlock;
if (IsTrivialUnswitchCondition(L, LoopCond, &EntersLoopOnCond, &ExitBlock)){
UnswitchTrivialCondition(L, LoopCond, EntersLoopOnCond, ExitBlock);
NewLoop1 = L;
} else {
VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2);
}
//std::cerr << "AFTER:\n"; LI->dump();
// Try to unswitch each of our new loops now!
if (NewLoop1) visitLoop(NewLoop1);
if (NewLoop2) visitLoop(NewLoop2);
return true;
}
BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) {
TerminatorInst *LatchTerm = BB->getTerminator();
unsigned SuccNum = 0;
for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) {
assert(i != e && "Didn't find edge?");
if (LatchTerm->getSuccessor(i) == Succ) {
SuccNum = i;
break;
}
}
// If this is a critical edge, let SplitCriticalEdge do it.
if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this))
return LatchTerm->getSuccessor(SuccNum);
// If the edge isn't critical, then BB has a single successor or Succ has a
// single pred. Split the block.
BasicBlock *BlockToSplit;
BasicBlock::iterator SplitPoint;
if (BasicBlock *SP = Succ->getSinglePredecessor()) {
// If the successor only has a single pred, split the top of the successor
// block.
assert(SP == BB && "CFG broken");
BlockToSplit = Succ;
SplitPoint = Succ->begin();
} else {
// Otherwise, if BB has a single successor, split it at the bottom of the
// block.
assert(BB->getTerminator()->getNumSuccessors() == 1 &&
"Should have a single succ!");
BlockToSplit = BB;
SplitPoint = BB->getTerminator();
}
BasicBlock *New =
BlockToSplit->splitBasicBlock(SplitPoint,
BlockToSplit->getName()+".tail");
// New now lives in whichever loop that BB used to.
if (Loop *L = LI->getLoopFor(BlockToSplit))
L->addBasicBlockToLoop(New, *LI);
return 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;
}
/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
/// condition in it (a cond branch from its header block to its latch block,
/// where the path through the loop that doesn't execute its body has no
/// side-effects), unswitch it. This doesn't involve any code duplication, just
/// moving the conditional branch outside of the loop and updating loop info.
void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
bool EnterOnCond,
BasicBlock *ExitBlock) {
DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
<< L->getHeader()->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function " << L->getHeader()->getParent()->getName()
<< " on cond:" << *Cond << "\n");
// First step, split the preheader, so that we know that there is a safe place
// to insert the conditional branch. We will change 'OrigPH' to have a
// conditional branch on Cond.
BasicBlock *OrigPH = L->getLoopPreheader();
BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader());
// Now that we have a place to insert the conditional branch, create a place
// to branch to: this is the exit block out of the loop that we should
// short-circuit to.
// Split this edge now, so that the loop maintains its exit block.
assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
BasicBlock *NewExit = SplitEdge(L->getLoopLatch(), ExitBlock);
assert(NewExit != ExitBlock && "Edge not split!");
// Okay, now we have a position to branch from and a position to branch to,
// insert the new conditional branch.
new BranchInst(EnterOnCond ? NewPH : NewExit, EnterOnCond ? NewExit : NewPH,
Cond, OrigPH->getTerminator());
OrigPH->getTerminator()->eraseFromParent();
// Now that we know that the loop is never entered when this condition is a
// particular value, rewrite the loop with this info. We know that this will
// at least eliminate the old branch.
RewriteLoopBodyWithConditionConstant(L, Cond, ConstantBool::get(EnterOnCond),
true);
++NumTrivial;
}
/// VersionLoop - We determined that the loop is profitable to unswitch when LIC
/// equal Val. Split it into loop versions and test the condition outside of
/// either loop. Return the loops created as Out1/Out2.
void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L,
Loop *&Out1, Loop *&Out2) {
Function *F = L->getHeader()->getParent();
DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
<< L->getHeader()->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function " << F->getName()
<< " when '" << *Val << "' == " << *LIC << "\n");
// LoopBlocks contains all of the basic blocks of the loop, including the
// preheader of the loop, the body of the loop, and the exit blocks of the
// loop, in that order.
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(SplitEdge(OrigPreheader, L->getHeader()));
// 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());
// Split all of the edges from inside the loop to their exit blocks. This
// unswitching trivial: no phi nodes to update.
unsigned NumBlocks = L->getBlocks().size();
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
BasicBlock *ExitBlock = ExitBlocks[i];
std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock));
for (unsigned j = 0, e = Preds.size(); j != e; ++j) {
assert(L->contains(Preds[j]) &&
"All preds of loop exit blocks must be the same loop!");
SplitEdge(Preds[j], ExitBlock);
}
}
// The exit blocks may have been changed due to edge splitting, recompute.
ExitBlocks.clear();
L->getExitBlocks(ExitBlocks);
std::sort(ExitBlocks.begin(), ExitBlocks.end());
ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
ExitBlocks.end());
// Add exit blocks to the loop blocks.
LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
// 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) {
BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
NewBlocks.push_back(New);
ValueMap[LoopBlocks[i]] = New; // 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);
Loop *ParentLoop = L->getParentLoop();
if (ParentLoop) {
// Make sure to add the cloned preheader and exit blocks to the parent loop
// as well.
ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
}
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
if (ParentLoop)
ParentLoop->addBasicBlockToLoop(cast<BasicBlock>(NewExit), *LI);
assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
"Exit block should have been split to have one successor!");
BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
// If the successor of the exit block had PHI nodes, add an entry for
// NewExit.
PHINode *PN;
for (BasicBlock::iterator I = ExitSucc->begin();
(PN = dyn_cast<PHINode>(I)); ++I) {
Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
std::map<const Value *, Value*>::iterator It = ValueMap.find(V);
if (It != ValueMap.end()) V = It->second;
PN->addIncoming(V, NewExit);
}
}
// 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);
// 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!");
// 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.
Value *BranchVal = LIC;
if (!isa<ConstantBool>(BranchVal)) {
BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp",
OrigPreheader->getTerminator());
} else if (Val != ConstantBool::True) {
// We want to enter the new loop when the condition is true.
BranchVal = BinaryOperator::createNot(BranchVal, "tmp",
OrigPreheader->getTerminator());
}
// Remove the unconditional branch to LoopBlocks[0] and insert the new branch.
OrigPreheader->getInstList().pop_back();
new BranchInst(NewBlocks[0], LoopBlocks[0], BranchVal, 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, Val, false);
RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
Out1 = L;
Out2 = NewLoop;
}
// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
// the value specified by Val in the specified loop, or we know it does NOT have
// that value. Rewrite any uses of LIC or of properties correlated to it.
void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
Constant *Val,
bool IsEqual) {
assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
// FIXME: Support correlated properties, like:
// for (...)
// if (li1 < li2)
// ...
// if (li1 > li2)
// ...
// NotVal - If Val is a bool, this contains its inverse.
Constant *NotVal = 0;
if (ConstantBool *CB = dyn_cast<ConstantBool>(Val))
NotVal = ConstantBool::get(!CB->getValue());
// FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
// selects, switches.
std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
// Haha, this loop could be unswitched. Get it? The unswitch pass could
// unswitch itself. Amazing.
for (unsigned i = 0, e = Users.size(); i != e; ++i)
if (Instruction *U = cast<Instruction>(Users[i]))
if (L->contains(U->getParent()))
if (IsEqual) {
U->replaceUsesOfWith(LIC, Val);
} else if (NotVal) {
U->replaceUsesOfWith(LIC, NotVal);
} else {
// If we know that LIC is not Val, use this info to simplify code.
if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
if (SI->getCaseValue(i) == Val) {
// Found a dead case value. Don't remove PHI nodes in the
// successor if they become single-entry, those PHI nodes may
// be in the Users list.
SI->getSuccessor(i)->removePredecessor(SI->getParent(), true);
SI->removeCase(i);
break;
}
}
}
// TODO: We could simplify stuff like X == C.
}
}