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improved support for branch folding, still not enabled.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26289 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2006-02-18 07:57:38 +00:00
parent 45af8fd8c2
commit a6fc94b71e
1 changed files with 269 additions and 143 deletions
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412
lib/Transforms/Scalar/LoopUnswitch.cpp
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@ -36,6 +36,7 @@
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/CommandLine.h"
#include <algorithm>
@ -57,6 +58,9 @@ namespace {
class LoopUnswitch : public FunctionPass {
LoopInfo *LI; // Loop information
// LoopProcessWorklist - List of loops we need to process.
std::vector<Loop*> LoopProcessWorklist;
public:
virtual bool runOnFunction(Function &F);
bool visitLoop(Loop *L);
@ -72,18 +76,30 @@ namespace {
}
private:
/// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
/// remove it.
void RemoveLoopFromWorklist(Loop *L) {
std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
LoopProcessWorklist.end(), L);
if (I != LoopProcessWorklist.end())
LoopProcessWorklist.erase(I);
}
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);
void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
bool EntersWhenTrue, BasicBlock *ExitBlock);
void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To);
BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt);
void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val,
bool isEqual);
void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
Constant *Val, bool isEqual);
void SimplifyCode(std::vector<Instruction*> &Worklist);
void RemoveBlockIfDead(BasicBlock *BB,
std::vector<Instruction*> &Worklist);
void RemoveLoopFromHierarchy(Loop *L);
};
RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
}
@ -94,12 +110,95 @@ 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]);
// Populate the worklist of loops to process in post-order.
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
for (po_iterator<Loop*> LI = po_begin(*I), E = po_end(*I); LI != E; ++LI)
LoopProcessWorklist.push_back(*LI);
// Process the loops in worklist order, this is a post-order visitation of
// the loops. We use a worklist of loops so that loops can be removed at any
// time if they are deleted (e.g. the backedge of a loop is removed).
while (!LoopProcessWorklist.empty()) {
Loop *L = LoopProcessWorklist.back();
LoopProcessWorklist.pop_back();
Changed |= visitLoop(L);
}
return Changed;
}
/// 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;
// 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;
}
@ -165,6 +264,9 @@ static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
return true;
}
/// isTrivialLoopExitBlock - Return true if the specified block unconditionally
/// leads to an exit from the specified loop, and has no side-effects in the
/// process. If so, return the block that is exited to, otherwise return null.
static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
std::set<BasicBlock*> Visited;
Visited.insert(L->getHeader()); // Branches to header are ok.
@ -266,88 +368,6 @@ unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
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.
@ -372,9 +392,6 @@ bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
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.
Constant *CondVal;
@ -383,16 +400,10 @@ bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal,
&EntersWhenTrue, &ExitBlock)) {
UnswitchTrivialCondition(L, LoopCond, CondVal, EntersWhenTrue, ExitBlock);
NewLoop1 = L;
} else {
VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2);
UnswitchNontrivialCondition(LoopCond, Val, L);
}
//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;
}
@ -549,6 +560,9 @@ void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
}
OrigPH->getTerminator()->eraseFromParent();
// We need to reprocess this loop, it could be unswitched again.
LoopProcessWorklist.push_back(L);
// 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.
@ -560,10 +574,9 @@ void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
/// 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) {
void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
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()
@ -675,13 +688,18 @@ void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L,
// Emit the new branch that selects between the two versions of this loop.
EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
OldBR->eraseFromParent();
LoopProcessWorklist.push_back(L);
LoopProcessWorklist.push_back(NewLoop);
// 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(L , LIC, Val, false);
// It's possible that simplifying one loop could cause the other to be
// deleted. If so, don't simplify it.
if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
}
/// RemoveFromWorklist - Remove all instances of I from the worklist vector
@ -723,39 +741,60 @@ static void ReplaceUsesOfWith(Instruction *I, Value *V,
///
void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
std::vector<Instruction*> &Worklist) {
if (pred_begin(BB) != pred_end(BB)) return; // not dead.
if (pred_begin(BB) != pred_end(BB)) {
// This block isn't dead, since an edge to BB was just removed, see if there
// are any easy simplifications we can do now.
if (BasicBlock *Pred = BB->getSinglePredecessor()) {
// If it has one pred, fold phi nodes in BB.
while (isa<PHINode>(BB->begin()))
ReplaceUsesOfWith(BB->begin(),
cast<PHINode>(BB->begin())->getIncomingValue(0),
Worklist);
// If this is the header of a loop and the only pred is the latch, we now
// have an unreachable loop.
if (Loop *L = LI->getLoopFor(BB))
if (L->getHeader() == BB && L->contains(Pred)) {
// Remove the branch from the latch to the header block, this makes
// the header dead, which will make the latch dead (because the header
// dominates the latch).
Pred->getTerminator()->eraseFromParent();
new UnreachableInst(Pred);
// The loop is now broken, remove it from LI.
RemoveLoopFromHierarchy(L);
// Reprocess the header, which now IS dead.
RemoveBlockIfDead(BB, Worklist);
return;
}
// If pred ends in a uncond branch, add uncond branch to worklist so that
// the two blocks will get merged.
if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
if (BI->isUnconditional())
Worklist.push_back(BI);
}
return;
}
DEBUG(std::cerr << "Nuking dead block: " << *BB);
// Remove the instructions in the basic block from the worklist.
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
RemoveFromWorklist(I, Worklist);
// Anything that uses the instructions in this basic block should have their
// uses replaced with undefs.
if (!I->use_empty())
I->replaceAllUsesWith(UndefValue::get(I->getType()));
}
// If this is the header block for a loop, remove the loop and all subloops.
// We leave all of the blocks that used to be part of this loop in the parent
// loop if any.
// If this is the edge to the header block for a loop, remove the loop and
// promote all subloops.
if (Loop *BBLoop = LI->getLoopFor(BB)) {
if (BBLoop->getHeader() == BB) {
if (Loop *ParentLoop = BBLoop->getParentLoop()) { // not a top-level loop.
for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();;
++I) {
assert(I != E && "Couldn't find loop");
if (*I == BBLoop) {
ParentLoop->removeChildLoop(I);
break;
}
}
} else {
for (LoopInfo::iterator I = LI->begin(), E = LI->end();; ++I) {
assert(I != E && "Couldn't find loop");
if (*I == BBLoop) {
LI->removeLoop(I);
break;
}
}
}
delete BBLoop;
}
if (BBLoop->getLoopLatch() == BB)
RemoveLoopFromHierarchy(BBLoop);
}
// Remove the block from the loop info, which removes it from any loops it
@ -771,17 +810,94 @@ void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
TI->getSuccessor(i)->removePredecessor(BB);
}
// Unique the successors.
// Unique the successors, remove anything with multiple uses.
std::sort(Succs.begin(), Succs.end());
Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
// Remove the basic block, including all of the instructions contained in it.
BB->eraseFromParent();
// Remove successor blocks here that are not dead, so that we know we only
// have dead blocks in this list. Nondead blocks have a way of becoming dead,
// then getting removed before we revisit them, which is badness.
//
for (unsigned i = 0; i != Succs.size(); ++i)
if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
// One exception is loop headers. If this block was the preheader for a
// loop, then we DO want to visit the loop so the loop gets deleted.
// We know that if the successor is a loop header, that this loop had to
// be the preheader: the case where this was the latch block was handled
// above and headers can only have two predecessors.
if (!LI->isLoopHeader(Succs[i])) {
Succs.erase(Succs.begin()+i);
--i;
}
}
for (unsigned i = 0, e = Succs.size(); i != e; ++i)
RemoveBlockIfDead(Succs[i], Worklist);
}
/// RemoveLoopFromHierarchy - We have discovered that the specified loop has
/// become unwrapped, either because the backedge was deleted, or because the
/// edge into the header was removed. If the edge into the header from the
/// latch block was removed, the loop is unwrapped but subloops are still alive,
/// so they just reparent loops. If the loops are actually dead, they will be
/// removed later.
void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
if (Loop *ParentLoop = L->getParentLoop()) { // Not a top-level loop.
// Reparent all of the blocks in this loop. Since BBLoop had a parent,
// they are now all in it.
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I)
if (LI->getLoopFor(*I) == L) // Don't change blocks in subloops.
LI->changeLoopFor(*I, ParentLoop);
// Remove the loop from its parent loop.
for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();;
++I) {
assert(I != E && "Couldn't find loop");
if (*I == L) {
ParentLoop->removeChildLoop(I);
break;
}
}
// Move all subloops into the parent loop.
while (L->begin() != L->end())
ParentLoop->addChildLoop(L->removeChildLoop(L->end()-1));
} else {
// Reparent all of the blocks in this loop. Since BBLoop had no parent,
// they no longer in a loop at all.
for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
// Don't change blocks in subloops.
if (LI->getLoopFor(L->getBlocks()[i]) == L) {
LI->removeBlock(L->getBlocks()[i]);
--i;
}
}
// Remove the loop from the top-level LoopInfo object.
for (LoopInfo::iterator I = LI->begin(), E = LI->end();; ++I) {
assert(I != E && "Couldn't find loop");
if (*I == L) {
LI->removeLoop(I);
break;
}
}
// Move all of the subloops to the top-level.
while (L->begin() != L->end())
LI->addTopLevelLoop(L->removeChildLoop(L->end()-1));
}
delete L;
RemoveLoopFromWorklist(L);
}
// 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.
@ -805,9 +921,8 @@ void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
// FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
// selects, switches.
std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
std::vector<Instruction*> Worklist;
// If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
// in the loop with the appropriate one directly.
if (IsEqual || NotVal) {
@ -849,10 +964,21 @@ void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
// LIC == Val -> false.
}
}
// Okay, now that we have simplified some instructions in the loop, walk over
// it and constant prop, dce, and fold control flow where possible. Note that
// this is effectively a very simple loop-structure-aware optimizer.
SimplifyCode(Worklist);
}
/// SimplifyCode - Okay, now that we have simplified some instructions in the
/// loop, walk over it and constant prop, dce, and fold control flow where
/// possible. Note that this is effectively a very simple loop-structure-aware
/// optimizer. During processing of this loop, L could very well be deleted, so
/// it must not be used.
///
/// FIXME: When the loop optimizer is more mature, separate this out to a new
/// pass.
///
void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist) {
Worklist.back()->getParent()->getParent()->viewCFG();
while (!Worklist.empty()) {
Instruction *I = Worklist.back();
Worklist.pop_back();
@ -940,7 +1066,7 @@ void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
Succ->eraseFromParent();
++NumSimplify;
} else if (ConstantBool *CB = dyn_cast<ConstantBool>(BI->getCondition())){
break; // FIXME: disabled
break; // FIXME: Enable.
// Conditional branch. Turn it into an unconditional branch, then
// remove dead blocks.
DEBUG(std::cerr << "Folded branch: " << *BI);