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

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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.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60327 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2008-12-01 04:48:07 +00:00
parent 11240d0fdd
commit ef0c6744d5
1 changed files with 24 additions and 1 deletions
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25
lib/Transforms/Scalar/JumpThreading.cpp
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@ -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);
}
}