[llvm-commits] CVS: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
Chris Lattner
lattner at cs.uiuc.edu
Wed Feb 23 22:18:05 PST 2005
Changes in directory llvm/lib/Transforms/Utils:
SimplifyCFG.cpp updated: 1.63 -> 1.64
---
Log message:
Implement Transforms/SimplifyCFG/switch_thread.ll
This does a simple form of "jump threading", which eliminates CFG edges that
are provably dead. This triggers 90 times in the external tests, and
eliminating CFG edges is always always a good thing! :)
---
Diffs of the changes: (+190 -4)
SimplifyCFG.cpp | 194 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
1 files changed, 190 insertions(+), 4 deletions(-)
Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
diff -u llvm/lib/Transforms/Utils/SimplifyCFG.cpp:1.63 llvm/lib/Transforms/Utils/SimplifyCFG.cpp:1.64
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp:1.63 Fri Jan 28 18:38:26 2005
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp Thu Feb 24 00:17:52 2005
@@ -420,6 +420,175 @@
}
+// EliminateBlockCases - Given an vector of bb/value pairs, remove any entries
+// in the list that match the specified block.
+static void EliminateBlockCases(BasicBlock *BB,
+ std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) {
+ for (unsigned i = 0, e = Cases.size(); i != e; ++i)
+ if (Cases[i].second == BB) {
+ Cases.erase(Cases.begin()+i);
+ --i; --e;
+ }
+}
+
+// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as
+// well.
+static bool
+ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1,
+ std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) {
+ std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2;
+
+ // Make V1 be smaller than V2.
+ if (V1->size() > V2->size())
+ std::swap(V1, V2);
+
+ if (V1->size() == 0) return false;
+ if (V1->size() == 1) {
+ // Just scan V2.
+ ConstantInt *TheVal = (*V1)[0].first;
+ for (unsigned i = 0, e = V2->size(); i != e; ++i)
+ if (TheVal == (*V2)[i].first)
+ return true;
+ }
+
+ // Otherwise, just sort both lists and compare element by element.
+ std::sort(V1->begin(), V1->end());
+ std::sort(V2->begin(), V2->end());
+ unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
+ while (i1 != e1 && i2 != e2) {
+ if ((*V1)[i1].first == (*V2)[i2].first)
+ return true;
+ if ((*V1)[i1].first < (*V2)[i2].first)
+ ++i1;
+ else
+ ++i2;
+ }
+ return false;
+}
+
+// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a
+// terminator instruction and its block is known to only have a single
+// predecessor block, check to see if that predecessor is also a value
+// comparison with the same value, and if that comparison determines the outcome
+// of this comparison. If so, simplify TI. This does a very limited form of
+// jump threading.
+static bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
+ BasicBlock *Pred) {
+ Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
+ if (!PredVal) return false; // Not a value comparison in predecessor.
+
+ Value *ThisVal = isValueEqualityComparison(TI);
+ assert(ThisVal && "This isn't a value comparison!!");
+ if (ThisVal != PredVal) return false; // Different predicates.
+
+ // Find out information about when control will move from Pred to TI's block.
+ std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
+ BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(),
+ PredCases);
+ EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
+
+ // Find information about how control leaves this block.
+ std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases;
+ BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
+ EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
+
+ // If TI's block is the default block from Pred's comparison, potentially
+ // simplify TI based on this knowledge.
+ if (PredDef == TI->getParent()) {
+ // If we are here, we know that the value is none of those cases listed in
+ // PredCases. If there are any cases in ThisCases that are in PredCases, we
+ // can simplify TI.
+ if (ValuesOverlap(PredCases, ThisCases)) {
+ if (BranchInst *BTI = dyn_cast<BranchInst>(TI)) {
+ // Okay, one of the successors of this condbr is dead. Convert it to a
+ // uncond br.
+ assert(ThisCases.size() == 1 && "Branch can only have one case!");
+ Value *Cond = BTI->getCondition();
+ // Insert the new branch.
+ Instruction *NI = new BranchInst(ThisDef, TI);
+
+ // Remove PHI node entries for the dead edge.
+ ThisCases[0].second->removePredecessor(TI->getParent());
+
+ DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator()
+ << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
+
+ TI->eraseFromParent(); // Nuke the old one.
+ // If condition is now dead, nuke it.
+ if (Instruction *CondI = dyn_cast<Instruction>(Cond))
+ ErasePossiblyDeadInstructionTree(CondI);
+ return true;
+
+ } else {
+ SwitchInst *SI = cast<SwitchInst>(TI);
+ // Okay, TI has cases that are statically dead, prune them away.
+ std::set<Constant*> DeadCases;
+ for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
+ DeadCases.insert(PredCases[i].first);
+
+ DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator()
+ << "Through successor TI: " << *TI);
+
+ for (unsigned i = SI->getNumCases()-1; i != 0; --i)
+ if (DeadCases.count(SI->getCaseValue(i))) {
+ SI->getSuccessor(i)->removePredecessor(TI->getParent());
+ SI->removeCase(i);
+ }
+
+ DEBUG(std::cerr << "Leaving: " << *TI << "\n");
+ return true;
+ }
+ }
+
+ } else {
+ // Otherwise, TI's block must correspond to some matched value. Find out
+ // which value (or set of values) this is.
+ ConstantInt *TIV = 0;
+ BasicBlock *TIBB = TI->getParent();
+ for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
+ if (PredCases[i].second == TIBB)
+ if (TIV == 0)
+ TIV = PredCases[i].first;
+ else
+ return false; // Cannot handle multiple values coming to this block.
+ assert(TIV && "No edge from pred to succ?");
+
+ // Okay, we found the one constant that our value can be if we get into TI's
+ // BB. Find out which successor will unconditionally be branched to.
+ BasicBlock *TheRealDest = 0;
+ for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
+ if (ThisCases[i].first == TIV) {
+ TheRealDest = ThisCases[i].second;
+ break;
+ }
+
+ // If not handled by any explicit cases, it is handled by the default case.
+ if (TheRealDest == 0) TheRealDest = ThisDef;
+
+ // Remove PHI node entries for dead edges.
+ BasicBlock *CheckEdge = TheRealDest;
+ for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI)
+ if (*SI != CheckEdge)
+ (*SI)->removePredecessor(TIBB);
+ else
+ CheckEdge = 0;
+
+ // Insert the new branch.
+ Instruction *NI = new BranchInst(TheRealDest, TI);
+
+ DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator()
+ << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
+ Instruction *Cond = 0;
+ if (BranchInst *BI = dyn_cast<BranchInst>(TI))
+ Cond = dyn_cast<Instruction>(BI->getCondition());
+ TI->eraseFromParent(); // Nuke the old one.
+
+ if (Cond) ErasePossiblyDeadInstructionTree(Cond);
+ return true;
+ }
+ return false;
+}
+
// FoldValueComparisonIntoPredecessors - The specified terminator is a value
// equality comparison instruction (either a switch or a branch on "X == c").
// See if any of the predecessors of the terminator block are value comparisons
@@ -908,13 +1077,30 @@
return true;
}
- } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->begin())) {
- if (isValueEqualityComparison(SI))
- if (FoldValueComparisonIntoPredecessors(SI))
- return SimplifyCFG(BB) || 1;
+ } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
+ if (isValueEqualityComparison(SI)) {
+ // If we only have one predecessor, and if it is a branch on this value,
+ // see if that predecessor totally determines the outcome of this switch.
+ if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
+ if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred))
+ return SimplifyCFG(BB) || 1;
+
+ // If the block only contains the switch, see if we can fold the block
+ // away into any preds.
+ if (SI == &BB->front())
+ if (FoldValueComparisonIntoPredecessors(SI))
+ return SimplifyCFG(BB) || 1;
+ }
} else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
if (BI->isConditional()) {
if (Value *CompVal = isValueEqualityComparison(BI)) {
+ // If we only have one predecessor, and if it is a branch on this value,
+ // see if that predecessor totally determines the outcome of this
+ // switch.
+ if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
+ if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred))
+ return SimplifyCFG(BB) || 1;
+
// This block must be empty, except for the setcond inst, if it exists.
BasicBlock::iterator I = BB->begin();
if (&*I == BI ||
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