#include "llvm/Analysis/Dominators.h"
#include "llvm/Support/STLExtras.h"
#include "llvm/Analysis/Writer.h"
+#include "llvm/CFG.h"
+#include "llvm/iTerminators.h"
#include <set>
#include <algorithm>
+//#define DEBUG_ADCE 1
+
//===----------------------------------------------------------------------===//
// ADCE Class
//
Method *M; // The method that we are working on...
vector<Instruction*> WorkList; // Instructions that just became live
set<Instruction*> LiveSet; // The set of live instructions
+ bool MadeChanges;
//===--------------------------------------------------------------------===//
// The public interface for this class
//
public:
// ADCE Ctor - Save the method to operate on...
- inline ADCE(Method *m) : M(m) {}
+ inline ADCE(Method *m) : M(m), MadeChanges(false) {}
// doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
// true if the method was modified.
private:
inline void markInstructionLive(Instruction *I) {
if (LiveSet.count(I)) return;
+#ifdef DEBUG_ADCE
cerr << "Insn Live: " << I;
+#endif
LiveSet.insert(I);
WorkList.push_back(I);
}
inline void markTerminatorLive(const BasicBlock *BB) {
- cerr << "Marking Term Live\n";
- markInstructionLive((Instruction*)BB->back());
+#ifdef DEBUG_ADCE
+ cerr << "Terminat Live: " << BB->getTerminator();
+#endif
+ markInstructionLive((Instruction*)BB->getTerminator());
}
+
+ // fixupCFG - Walk the CFG in depth first order, eliminating references to
+ // dead blocks.
+ //
+ BasicBlock *fixupCFG(BasicBlock *Head, set<BasicBlock*> &VisitedBlocks,
+ const set<BasicBlock*> &AliveBlocks);
};
// true if the method was modified.
//
bool ADCE::doADCE() {
+ // Compute the control dependence graph... Note that this has a side effect
+ // on the CFG: a new return bb is added and all returns are merged here.
+ //
+ cfg::DominanceFrontier CDG(cfg::DominatorSet(M, true));
+
+#ifdef DEBUG_ADCE
+ cerr << "Method: " << M;
+#endif
+
// Iterate over all of the instructions in the method, eliminating trivially
// dead instructions, and marking instructions live that are known to be
- // needed.
+ // needed. Perform the walk in depth first order so that we avoid marking any
+ // instructions live in basic blocks that are unreachable. These blocks will
+ // be eliminated later, along with the instructions inside.
//
- for (Method::inst_iterator II = M->inst_begin(); II != M->inst_end(); ) {
- Instruction *I = *II;
- switch (I->getInstType()) {
- case Instruction::Ret:
- case Instruction::Call:
- case Instruction::Store:
- markInstructionLive(I);
- break;
- default:
- // Check to see if anything is trivially dead
- if (I->use_size() == 0 && I->getType() != Type::VoidTy) {
- // Remove the instruction from it's basic block...
- BasicBlock *BB = I->getParent();
- delete BB->getInstList().remove(II.getInstructionIterator());
-
- // Make sure to sync up the iterator again...
- II.resyncInstructionIterator();
- continue; // Don't increment the iterator past the current slot
+ for (cfg::df_iterator BBI = cfg::df_begin(M), BBE = cfg::df_end(M);
+ BBI != BBE; ++BBI) {
+ BasicBlock *BB = *BBI;
+ for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
+ Instruction *I = *II;
+
+ if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
+ markInstructionLive(I);
+ } else {
+ // Check to see if anything is trivially dead
+ if (I->use_size() == 0 && I->getType() != Type::VoidTy) {
+ // Remove the instruction from it's basic block...
+ delete BB->getInstList().remove(II);
+ MadeChanges = true;
+ continue; // Don't increment the iterator past the current slot
+ }
}
- }
- ++II; // Increment the iterator
+ ++II; // Increment the inst iterator if the inst wasn't deleted
+ }
}
- // Compute the control dependence graph...
- cfg::DominanceFrontier CDG(cfg::DominatorSet(M, true));
-
+#ifdef DEBUG_ADCE
cerr << "Processing work list\n";
+#endif
// AliveBlocks - Set of basic blocks that we know have instructions that are
// alive in them...
for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
bind_obj(this, &ADCE::markTerminatorLive));
}
+
+ // If this basic block is live, then the terminator must be as well!
+ markTerminatorLive(BB);
}
+ // Loop over all of the operands of the live instruction, making sure that
+ // they are known to be alive as well...
+ //
for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op) {
- Instruction *Operand = I->getOperand(op)->castInstruction();
- if (Operand) markInstructionLive(Operand);
+ if (Instruction *Operand = I->getOperand(op)->castInstruction())
+ markInstructionLive(Operand);
+ }
+ }
+
+#ifdef DEBUG_ADCE
+ cerr << "Current Method: X = Live\n";
+ for (Method::inst_iterator IL = M->inst_begin(); IL != M->inst_end(); ++IL) {
+ if (LiveSet.count(*IL)) cerr << "X ";
+ cerr << *IL;
+ }
+#endif
+
+ // After the worklist is processed, recursively walk the CFG in depth first
+ // order, patching up references to dead blocks...
+ //
+ set<BasicBlock*> VisitedBlocks;
+ BasicBlock *EntryBlock = fixupCFG(M->front(), VisitedBlocks, AliveBlocks);
+ if (EntryBlock && EntryBlock != M->front()) {
+ if (EntryBlock->front()->isPHINode()) {
+ // Cannot make the first block be a block with a PHI node in it! Instead,
+ // strip the first basic block of the method to contain no instructions,
+ // then add a simple branch to the "real" entry node...
+ //
+ BasicBlock *E = M->front();
+ if (!E->front()->isTerminator() || // Check for an actual change...
+ ((TerminatorInst*)E->front())->getNumSuccessors() != 1 ||
+ ((TerminatorInst*)E->front())->getSuccessor(0) != EntryBlock) {
+ E->getInstList().delete_all(); // Delete all instructions in block
+ E->getInstList().push_back(new BranchInst(EntryBlock));
+ MadeChanges = true;
+ }
+ AliveBlocks.insert(E);
+ } else {
+ // We need to move the new entry block to be the first bb of the method.
+ Method::iterator EBI = find(M->begin(), M->end(), EntryBlock);
+ swap(*EBI, *M->begin()); // Exchange old location with start of method
+ MadeChanges = true;
}
}
- // After the worklist is processed, loop through the instructions again,
- // removing any that are not live... by the definition of the LiveSet.
+ // Now go through and tell dead blocks to drop all of their references so they
+ // can be safely deleted.
//
- for (Method::inst_iterator II = M->inst_begin(); II != M->inst_end(); ) {
- Instruction *I = *II;
- if (!LiveSet.count(I)) {
- cerr << "Instruction Dead: " << I;
+ for (Method::iterator BI = M->begin(), BE = M->end(); BI != BE; ++BI) {
+ BasicBlock *BB = *BI;
+ if (!AliveBlocks.count(BB)) {
+ BB->dropAllReferences();
}
+ }
- ++II; // Increment the iterator
+ // Now loop through all of the blocks and delete them. We can safely do this
+ // now because we know that there are no references to dead blocks (because
+ // they have dropped all of their references...
+ //
+ for (Method::iterator BI = M->begin(); BI != M->end();) {
+ if (!AliveBlocks.count(*BI)) {
+ delete M->getBasicBlocks().remove(BI);
+ MadeChanges = true;
+ continue; // Don't increment iterator
+ }
+ ++BI; // Increment iterator...
}
- return false;
+ return MadeChanges;
+}
+
+
+// fixupCFG - Walk the CFG in depth first order, eliminating references to
+// dead blocks:
+// If the BB is alive (in AliveBlocks):
+// 1. Eliminate all dead instructions in the BB
+// 2. Recursively traverse all of the successors of the BB:
+// - If the returned successor is non-null, update our terminator to
+// reference the returned BB
+// 3. Return 0 (no update needed)
+//
+// If the BB is dead (not in AliveBlocks):
+// 1. Add the BB to the dead set
+// 2. Recursively traverse all of the successors of the block:
+// - Only one shall return a nonnull value (or else this block should have
+// been in the alive set).
+// 3. Return the nonnull child, or 0 if no non-null children.
+//
+BasicBlock *ADCE::fixupCFG(BasicBlock *BB, set<BasicBlock*> &VisitedBlocks,
+ const set<BasicBlock*> &AliveBlocks) {
+ if (VisitedBlocks.count(BB)) return 0; // Revisiting a node? No update.
+ VisitedBlocks.insert(BB); // We have now visited this node!
+
+#ifdef DEBUG_ADCE
+ cerr << "Fixing up BB: " << BB;
+#endif
+
+ if (AliveBlocks.count(BB)) { // Is the block alive?
+ // Yes it's alive: loop through and eliminate all dead instructions in block
+ for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; ) {
+ Instruction *I = *II;
+ if (!LiveSet.count(I)) { // Is this instruction alive?
+ // Nope... remove the instruction from it's basic block...
+ delete BB->getInstList().remove(II);
+ MadeChanges = true;
+ continue; // Don't increment II
+ }
+ ++II;
+ }
+
+ // Recursively traverse successors of this basic block.
+ cfg::succ_iterator SI = cfg::succ_begin(BB), SE = cfg::succ_end(BB);
+ for (; SI != SE; ++SI) {
+ BasicBlock *Succ = *SI;
+ BasicBlock *Repl = fixupCFG(Succ, VisitedBlocks, AliveBlocks);
+ if (Repl && Repl != Succ) { // We have to replace the successor
+ Succ->replaceAllUsesWith(Repl);
+ MadeChanges = true;
+ }
+ }
+ return BB;
+ } else { // Otherwise the block is dead...
+ BasicBlock *ReturnBB = 0; // Default to nothing live down here
+
+ // Recursively traverse successors of this basic block.
+ cfg::succ_iterator SI = cfg::succ_begin(BB), SE = cfg::succ_end(BB);
+ for (; SI != SE; ++SI) {
+ BasicBlock *RetBB = fixupCFG(*SI, VisitedBlocks, AliveBlocks);
+ if (RetBB) {
+ assert(ReturnBB == 0 && "One one live child allowed!");
+ ReturnBB = RetBB;
+ }
+ }
+ return ReturnBB; // Return the result of traversal
+ }
}
+
// DoADCE - Execute the Agressive Dead Code Elimination Algorithm
//
bool opt::DoADCE(Method *M) {
+ if (M->isExternal()) return false;
ADCE DCE(M);
return DCE.doADCE();
}
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