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Teach jump threading to clean up after itself, DCE and constfolding the 

new instructions it simplifies.  Because we're threading jumps on edges
with constants coming in from PHI's, we inherently are exposing a lot more
constants to the new block.  Folding them and deleting dead conditions
allows the cost model in jump threading to be more accurate as it iterates.

llvm-svn: 60327
This commit is contained in:
Chris Lattner 2008-12-01 04:48:07 +00:00
parent 856684d360
commit 88a1f0213d
1 changed files with 24 additions and 1 deletions
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25
llvm/lib/Transforms/Scalar/JumpThreading.cpp
View File

@ -17,8 +17,10 @@
#include "llvm/Pass.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
@ -51,10 +53,15 @@ namespace {
/// revectored to the false side of the second if.
///
class VISIBILITY_HIDDEN JumpThreading : public FunctionPass {
TargetData *TD;
public:
static char ID; // Pass identification
JumpThreading() : FunctionPass(&ID) {}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<TargetData>();
}
bool runOnFunction(Function &F);
bool ProcessBlock(BasicBlock *BB);
void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
@ -79,6 +86,7 @@ FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
///
bool JumpThreading::runOnFunction(Function &F) {
DOUT << "Jump threading on function '" << F.getNameStart() << "'\n";
TD = &getAnalysis<TargetData>();
bool AnotherIteration = true, EverChanged = false;
while (AnotherIteration) {
@ -679,7 +687,7 @@ void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
PN->addIncoming(IV, NewBB);
}
// Finally, NewBB is good to go. Update the terminator of PredBB to jump to
// Ok, NewBB is good to go. Update the terminator of PredBB to jump to
// NewBB instead of BB. This eliminates predecessors from BB, which requires
// us to simplify any PHI nodes in BB.
TerminatorInst *PredTerm = PredBB->getTerminator();
@ -688,4 +696,19 @@ void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
BB->removePredecessor(PredBB);
PredTerm->setSuccessor(i, NewBB);
}
// At this point, the IR is fully up to date and consistent. Do a quick scan
// over the new instructions and zap any that are constants or dead. This
// frequently happens because of phi translation.
BI = NewBB->begin();
for (BasicBlock::iterator E = NewBB->end(); BI != E; ) {
Instruction *Inst = BI++;
if (Constant *C = ConstantFoldInstruction(Inst, TD)) {
Inst->replaceAllUsesWith(C);
Inst->eraseFromParent();
continue;
}
RecursivelyDeleteTriviallyDeadInstructions(Inst);
}
}