forked from OSchip/llvm-project
* Remove all cfg simplification stuff for a new cfg simplify pass (todo)
* Convert to worklist instead of iterative algorithm llvm-svn: 2510
This commit is contained in:
Chris Lattner
parent
a8292f31c8
commit
87e8806f05
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@ -1,38 +1,24 @@
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//===- DCE.cpp - Code to perform dead code elimination --------------------===//
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//
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// This file implements dead code elimination and basic block merging.
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// This file implements dead inst elimination and dead code elimination.
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//
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// Specifically, this:
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// * removes definitions with no uses
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// * removes basic blocks with no predecessors
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// * merges a basic block into its predecessor if there is only one and the
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// predecessor only has one successor.
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// * Eliminates PHI nodes for basic blocks with a single predecessor
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// * Eliminates a basic block that only contains an unconditional branch
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// * Eliminates function prototypes that are not referenced
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//
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// TODO: This should REALLY be worklist driven instead of iterative. Right now,
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// we scan linearly through values, removing unused ones as we go. The problem
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// is that this may cause other earlier values to become unused. To make sure
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// that we get them all, we iterate until things stop changing. Instead, when
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// removing a value, recheck all of its operands to see if they are now unused.
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// Piece of cake, and more efficient as well.
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//
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// Note, this is not trivial, because we have to worry about invalidating
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// iterators. :(
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// Dead Inst Elimination performs a single pass over the function removing
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// instructions that are obviously dead. Dead Code Elimination is similar, but
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// it rechecks instructions that were used by removed instructions to see if
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// they are newly dead.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/DCE.h"
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#include "llvm/Module.h"
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#include "llvm/GlobalVariable.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iPHINode.h"
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#include "llvm/Constant.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Pass.h"
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#include "Support/STLExtras.h"
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#include <algorithm>
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#include "llvm/InstrTypes.h"
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#include "llvm/Function.h"
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#include "llvm/Support/InstIterator.h"
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#include <set>
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static inline bool isInstDead(Instruction *I) {
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return I->use_empty() && !I->hasSideEffects() && !isa<TerminatorInst>(I);
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}
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// dceInstruction - Inspect the instruction at *BBI and figure out if it's
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// [trivially] dead. If so, remove the instruction and update the iterator
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@ -42,339 +28,108 @@
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bool dceInstruction(BasicBlock::InstListType &BBIL,
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BasicBlock::iterator &BBI) {
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// Look for un"used" definitions...
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if ((*BBI)->use_empty() && !(*BBI)->hasSideEffects() &&
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!isa<TerminatorInst>(*BBI)) {
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if (isInstDead(*BBI)) {
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delete BBIL.remove(BBI); // Bye bye
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return true;
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}
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return false;
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}
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static inline bool RemoveUnusedDefs(BasicBlock::InstListType &Vals) {
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bool Changed = false;
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for (BasicBlock::InstListType::iterator DI = Vals.begin();
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DI != Vals.end(); )
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if (dceInstruction(Vals, DI))
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Changed = true;
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else
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++DI;
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return Changed;
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//===----------------------------------------------------------------------===//
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// DeadInstElimination pass implementation
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//
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namespace {
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struct DeadInstElimination : public BasicBlockPass {
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const char *getPassName() const { return "Dead Instruction Elimination"; }
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virtual bool runOnBasicBlock(BasicBlock *BB) {
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BasicBlock::InstListType &Vals = BB->getInstList();
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bool Changed = false;
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for (BasicBlock::iterator DI = Vals.begin(); DI != Vals.end(); )
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if (dceInstruction(Vals, DI))
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Changed = true;
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else
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++DI;
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return Changed;
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}
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.preservesCFG();
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}
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};
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}
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struct DeadInstElimination : public BasicBlockPass {
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const char *getPassName() const { return "Dead Instruction Elimination"; }
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virtual bool runOnBasicBlock(BasicBlock *BB) {
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return RemoveUnusedDefs(BB->getInstList());
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}
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};
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Pass *createDeadInstEliminationPass() {
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return new DeadInstElimination();
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}
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// RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
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// a single predecessor. This means that the PHI node must only have a single
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// RHS value and can be eliminated.
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//===----------------------------------------------------------------------===//
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// DeadCodeElimination pass implementation
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//
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// This routine is very simple because we know that PHI nodes must be the first
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// things in a basic block, if they are present.
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//
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static bool RemoveSingularPHIs(BasicBlock *BB) {
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pred_iterator PI(pred_begin(BB));
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if (PI == pred_end(BB) || ++PI != pred_end(BB))
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return false; // More than one predecessor...
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Instruction *I = BB->front();
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if (!isa<PHINode>(I)) return false; // No PHI nodes
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//cerr << "Killing PHIs from " << BB;
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//cerr << "Pred #0 = " << *pred_begin(BB);
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//cerr << "Function == " << BB->getParent();
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do {
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PHINode *PN = cast<PHINode>(I);
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assert(PN->getNumOperands() == 2 && "PHI node should only have one value!");
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Value *V = PN->getOperand(0);
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PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
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delete BB->getInstList().remove(BB->begin());
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I = BB->front();
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} while (isa<PHINode>(I));
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return true; // Yes, we nuked at least one phi node
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}
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static void ReplaceUsesWithConstant(Instruction *I) {
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// Make all users of this instruction reference the constant instead
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I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
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}
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// PropogatePredecessors - This gets "Succ" ready to have the predecessors from
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// "BB". This is a little tricky because "Succ" has PHI nodes, which need to
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// have extra slots added to them to hold the merge edges from BB's
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// predecessors. This function returns true (failure) if the Succ BB already
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// has a predecessor that is a predecessor of BB.
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//
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// Assumption: Succ is the single successor for BB.
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//
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static bool PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
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assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
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assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
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// If there is more than one predecessor, and there are PHI nodes in
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// the successor, then we need to add incoming edges for the PHI nodes
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//
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const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
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// Check to see if one of the predecessors of BB is already a predecessor of
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// Succ. If so, we cannot do the transformation!
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//
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for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
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PI != PE; ++PI) {
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if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end())
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return true;
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}
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BasicBlock::iterator I = Succ->begin();
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do { // Loop over all of the PHI nodes in the successor BB
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PHINode *PN = cast<PHINode>(*I);
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Value *OldVal = PN->removeIncomingValue(BB);
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assert(OldVal && "No entry in PHI for Pred BB!");
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for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
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End = BBPreds.end(); PredI != End; ++PredI) {
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// Add an incoming value for each of the new incoming values...
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PN->addIncoming(OldVal, *PredI);
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}
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++I;
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} while (isa<PHINode>(*I));
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return false;
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}
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// SimplifyCFG - This function is used to do simplification of a CFG. For
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// example, it adjusts branches to branches to eliminate the extra hop, it
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// eliminates unreachable basic blocks, and does other "peephole" optimization
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// of the CFG. It returns true if a modification was made, and returns an
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// iterator that designates the first element remaining after the block that
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// was deleted.
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//
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// WARNING: The entry node of a function may not be simplified.
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//
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bool SimplifyCFG(Function::iterator &BBIt) {
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BasicBlock *BB = *BBIt;
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Function *M = BB->getParent();
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assert(BB && BB->getParent() && "Block not embedded in function!");
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assert(BB->getTerminator() && "Degenerate basic block encountered!");
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assert(BB->getParent()->front() != BB && "Can't Simplify entry block!");
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// Remove basic blocks that have no predecessors... which are unreachable.
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if (pred_begin(BB) == pred_end(BB) &&
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!BB->hasConstantReferences()) {
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//cerr << "Removing BB: \n" << BB;
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// Loop through all of our successors and make sure they know that one
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// of their predecessors is going away.
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for_each(succ_begin(BB), succ_end(BB),
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std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
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while (!BB->empty()) {
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Instruction *I = BB->back();
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// If this instruction is used, replace uses with an arbitrary
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// constant value. Because control flow can't get here, we don't care
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// what we replace the value with. Note that since this block is
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// unreachable, and all values contained within it must dominate their
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// uses, that all uses will eventually be removed.
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if (!I->use_empty()) ReplaceUsesWithConstant(I);
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// Remove the instruction from the basic block
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delete BB->getInstList().pop_back();
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}
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delete M->getBasicBlocks().remove(BBIt);
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return true;
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}
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// Check to see if this block has no instructions and only a single
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// successor. If so, replace block references with successor.
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succ_iterator SI(succ_begin(BB));
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if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
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if (BB->front()->isTerminator()) { // Terminator is the only instruction!
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BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
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//cerr << "Killing Trivial BB: \n" << BB;
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if (Succ != BB) { // Arg, don't hurt infinite loops!
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// If our successor has PHI nodes, then we need to update them to
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// include entries for BB's predecessors, not for BB itself.
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// Be careful though, if this transformation fails (returns true) then
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// we cannot do this transformation!
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//
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if (!isa<PHINode>(Succ->front()) ||
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!PropogatePredecessorsForPHIs(BB, Succ)) {
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BB->replaceAllUsesWith(Succ);
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BB = M->getBasicBlocks().remove(BBIt);
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if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
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Succ->setName(BB->getName());
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delete BB; // Delete basic block
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//cerr << "Function after removal: \n" << M;
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return true;
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}
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}
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}
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}
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// Merge basic blocks into their predecessor if there is only one distinct
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// pred, and if there is only one distinct successor of the predecessor, and
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// if there are no PHI nodes.
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//
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if (!isa<PHINode>(BB->front()) && !BB->hasConstantReferences()) {
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pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
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BasicBlock *OnlyPred = *PI++;
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for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
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if (*PI != OnlyPred) {
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OnlyPred = 0; // There are multiple different predecessors...
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break;
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}
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BasicBlock *OnlySucc = 0;
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if (OnlyPred && OnlyPred != BB) { // Don't break self loops
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// Check to see if there is only one distinct successor...
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succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
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OnlySucc = BB;
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for (; SI != SE; ++SI)
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if (*SI != OnlySucc) {
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OnlySucc = 0; // There are multiple distinct successors!
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break;
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}
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}
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if (OnlySucc) {
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//cerr << "Merging: " << BB << "into: " << Pred;
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TerminatorInst *Term = OnlyPred->getTerminator();
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// Delete the unconditional branch from the predecessor...
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BasicBlock::iterator DI = OnlyPred->end();
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delete OnlyPred->getInstList().remove(--DI); // Destroy branch
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// Move all definitions in the succecessor to the predecessor...
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std::vector<Instruction*> Insts(BB->begin(), BB->end());
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BB->getInstList().remove(BB->begin(), BB->end());
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OnlyPred->getInstList().insert(OnlyPred->end(),
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Insts.begin(), Insts.end());
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// Remove basic block from the function... and advance iterator to the
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// next valid block...
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M->getBasicBlocks().remove(BBIt);
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// Make all PHI nodes that refered to BB now refer to Pred as their
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// source...
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BB->replaceAllUsesWith(OnlyPred);
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// Inherit predecessors name if it exists...
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if (BB->hasName() && !OnlyPred->hasName())
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OnlyPred->setName(BB->getName());
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delete BB; // You ARE the weakest link... goodbye
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return true;
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}
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}
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return false;
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}
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static bool DoDCEPass(Function *F) {
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Function::iterator BBIt, BBEnd = F->end();
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if (F->begin() == BBEnd) return false; // Nothing to do
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bool Changed = false;
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// Loop through now and remove instructions that have no uses...
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for (BBIt = F->begin(); BBIt != BBEnd; ++BBIt) {
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Changed |= RemoveUnusedDefs((*BBIt)->getInstList());
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Changed |= RemoveSingularPHIs(*BBIt);
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}
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// Loop over all of the basic blocks (except the first one) and remove them
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// if they are unneeded...
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//
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for (BBIt = F->begin(), ++BBIt; BBIt != F->end(); ) {
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if (SimplifyCFG(BBIt)) {
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Changed = true;
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} else {
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++BBIt;
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}
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}
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return Changed;
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}
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// Remove unused global values - This removes unused global values of no
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// possible value. This currently includes unused function prototypes and
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// unitialized global variables.
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//
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static bool RemoveUnusedGlobalValues(Module *Mod) {
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bool Changed = false;
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for (Module::iterator MI = Mod->begin(); MI != Mod->end(); ) {
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Function *Meth = *MI;
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if (Meth->isExternal() && Meth->use_size() == 0) {
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// No references to prototype?
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//cerr << "Removing function proto: " << Meth->getName() << endl;
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delete Mod->getFunctionList().remove(MI); // Remove prototype
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// Remove moves iterator to point to the next one automatically
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Changed = true;
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} else {
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++MI; // Skip prototype in use.
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}
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}
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for (Module::giterator GI = Mod->gbegin(); GI != Mod->gend(); ) {
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GlobalVariable *GV = *GI;
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if (!GV->hasInitializer() && GV->use_size() == 0) {
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// No references to uninitialized global variable?
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//cerr << "Removing global var: " << GV->getName() << endl;
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delete Mod->getGlobalList().remove(GI);
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// Remove moves iterator to point to the next one automatically
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Changed = true;
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} else {
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++GI;
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}
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}
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return Changed;
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}
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namespace {
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struct DeadCodeElimination : public FunctionPass {
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struct DCE : public FunctionPass {
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const char *getPassName() const { return "Dead Code Elimination"; }
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// Pass Interface...
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virtual bool doInitialization(Module *M) {
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return RemoveUnusedGlobalValues(M);
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virtual bool runOnFunction(Function *F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.preservesCFG();
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}
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};
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}
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bool DCE::runOnFunction(Function *F) {
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// Start out with all of the instructions in the worklist...
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std::vector<Instruction*> WorkList(inst_begin(F), inst_end(F));
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std::set<Instruction*> DeadInsts;
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// Loop over the worklist finding instructions that are dead. If they are
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// dead make them drop all of their uses, making other instructions
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// potentially dead, and work until the worklist is empty.
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//
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while (!WorkList.empty()) {
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Instruction *I = WorkList.back();
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WorkList.pop_back();
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// It is possible that we may require multiple passes over the code to fully
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// eliminate dead code. Iterate until we are done.
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//
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virtual bool runOnFunction(Function *F) {
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bool Changed = false;
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while (DoDCEPass(F)) Changed = true;
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return Changed;
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if (isInstDead(I)) { // If the instruction is dead...
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// Loop over all of the values that the instruction uses, if there are
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// instructions being used, add them to the worklist, because they might
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// go dead after this one is removed.
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//
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for (User::use_iterator UI = I->use_begin(), UE = I->use_end();
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UI != UE; ++UI)
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if (Instruction *Used = dyn_cast<Instruction>(*UI))
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WorkList.push_back(Used);
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// Tell the instruction to let go of all of the values it uses...
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||||
I->dropAllReferences();
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// Keep track of this instruction, because we are going to delete it later
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DeadInsts.insert(I);
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}
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||||
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virtual bool doFinalization(Module *M) {
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return RemoveUnusedGlobalValues(M);
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}
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||||
};
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}
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// If we found no dead instructions, we haven't changed the function...
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if (DeadInsts.empty()) return false;
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// Otherwise, loop over the program, removing and deleting the instructions...
|
||||
for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
|
||||
BasicBlock::InstListType &BBIL = (*I)->getInstList();
|
||||
for (BasicBlock::iterator BI = BBIL.begin(); BI != BBIL.end(); )
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if (DeadInsts.count(*BI)) { // Is this instruction dead?
|
||||
delete BBIL.remove(BI); // Yup, remove and delete inst
|
||||
} else { // This instruction is not dead
|
||||
++BI; // Continue on to the next one...
|
||||
}
|
||||
}
|
||||
|
||||
return true;
|
||||
}
|
||||
|
||||
Pass *createDeadCodeEliminationPass() {
|
||||
return new DeadCodeElimination();
|
||||
return new DCE();
|
||||
}
|
||||
|
|
|
|||
Loading…
Reference in New Issue