[llvm-commits] CVS: llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
Chris Lattner
lattner at cs.uiuc.edu
Fri Feb 17 23:57:50 PST 2006
Changes in directory llvm/lib/Transforms/Scalar:
LoopUnswitch.cpp updated: 1.32 -> 1.33
---
Log message:
improved support for branch folding, still not enabled.
---
Diffs of the changes: (+269 -143)
LoopUnswitch.cpp | 412 +++++++++++++++++++++++++++++++++++--------------------
1 files changed, 269 insertions(+), 143 deletions(-)
Index: llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
diff -u llvm/lib/Transforms/Scalar/LoopUnswitch.cpp:1.32 llvm/lib/Transforms/Scalar/LoopUnswitch.cpp:1.33
--- llvm/lib/Transforms/Scalar/LoopUnswitch.cpp:1.32 Fri Feb 17 20:42:34 2006
+++ llvm/lib/Transforms/Scalar/LoopUnswitch.cpp Sat Feb 18 01:57:38 2006
@@ -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 @@
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 @@
}
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 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 @@
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 @@
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 @@
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 @@
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 @@
}
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 @@
/// 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 @@
// 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 @@
///
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 @@
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 @@
// 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 @@
// 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 @@
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);
More information about the llvm-commits
mailing list