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https://github.com/c64scene-ar/llvm-6502.git
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ea54ab9cd2
Cleanup code a lot git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2547 91177308-0d34-0410-b5e6-96231b3b80d8
295 lines
10 KiB
C++
295 lines
10 KiB
C++
//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
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//
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// This file implements "aggressive" dead code elimination. ADCE is DCe where
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// values are assumed to be dead until proven otherwise. This is similar to
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// SCCP, except applied to the liveness of values.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Type.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/Writer.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iPHINode.h"
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#include "llvm/Support/CFG.h"
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#include "Support/STLExtras.h"
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#include "Support/DepthFirstIterator.h"
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#include <algorithm>
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#include <iostream>
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using std::cerr;
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#define DEBUG_ADCE 1
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namespace {
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//===----------------------------------------------------------------------===//
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// ADCE Class
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//
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// This class does all of the work of Aggressive Dead Code Elimination.
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// It's public interface consists of a constructor and a doADCE() method.
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//
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class ADCE : public FunctionPass {
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Function *Func; // The function that we are working on
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std::vector<Instruction*> WorkList; // Instructions that just became live
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std::set<Instruction*> LiveSet; // The set of live instructions
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bool MadeChanges;
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//===--------------------------------------------------------------------===//
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// The public interface for this class
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//
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public:
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const char *getPassName() const { return "Aggressive Dead Code Elimination"; }
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// doADCE - Execute the Aggressive Dead Code Elimination Algorithm
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//
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virtual bool runOnFunction(Function *F) {
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Func = F; MadeChanges = false;
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doADCE(getAnalysis<DominanceFrontier>(DominanceFrontier::PostDomID));
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assert(WorkList.empty());
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LiveSet.clear();
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return MadeChanges;
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}
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// getAnalysisUsage - We require post dominance frontiers (aka Control
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// Dependence Graph)
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired(DominanceFrontier::PostDomID);
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}
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//===--------------------------------------------------------------------===//
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// The implementation of this class
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//
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private:
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// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
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// true if the function was modified.
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//
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void doADCE(DominanceFrontier &CDG);
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inline void markInstructionLive(Instruction *I) {
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if (LiveSet.count(I)) return;
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#ifdef DEBUG_ADCE
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cerr << "Insn Live: " << I;
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#endif
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LiveSet.insert(I);
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WorkList.push_back(I);
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}
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inline void markTerminatorLive(const BasicBlock *BB) {
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#ifdef DEBUG_ADCE
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cerr << "Terminat Live: " << BB->getTerminator();
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#endif
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markInstructionLive((Instruction*)BB->getTerminator());
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}
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// fixupCFG - Walk the CFG in depth first order, eliminating references to
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// dead blocks.
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//
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BasicBlock *fixupCFG(BasicBlock *Head, std::set<BasicBlock*> &VisitedBlocks,
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const std::set<BasicBlock*> &AliveBlocks);
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};
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} // End of anonymous namespace
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Pass *createAggressiveDCEPass() {
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return new ADCE();
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}
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// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
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// true if the function was modified.
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//
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void ADCE::doADCE(DominanceFrontier &CDG) {
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#ifdef DEBUG_ADCE
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cerr << "Function: " << Func;
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#endif
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// Iterate over all of the instructions in the function, eliminating trivially
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// dead instructions, and marking instructions live that are known to be
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// needed. Perform the walk in depth first order so that we avoid marking any
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// instructions live in basic blocks that are unreachable. These blocks will
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// be eliminated later, along with the instructions inside.
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//
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for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
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BBI != BBE; ++BBI) {
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BasicBlock *BB = *BBI;
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for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
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Instruction *I = *II;
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if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
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markInstructionLive(I);
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++II; // Increment the inst iterator if the inst wasn't deleted
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} else if (isInstructionTriviallyDead(I)) {
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// Remove the instruction from it's basic block...
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delete BB->getInstList().remove(II);
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MadeChanges = true;
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} else {
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++II; // Increment the inst iterator if the inst wasn't deleted
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}
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}
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}
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#ifdef DEBUG_ADCE
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cerr << "Processing work list\n";
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#endif
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// AliveBlocks - Set of basic blocks that we know have instructions that are
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// alive in them...
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//
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std::set<BasicBlock*> AliveBlocks;
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// Process the work list of instructions that just became live... if they
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// became live, then that means that all of their operands are neccesary as
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// well... make them live as well.
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//
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while (!WorkList.empty()) {
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Instruction *I = WorkList.back(); // Get an instruction that became live...
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WorkList.pop_back();
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BasicBlock *BB = I->getParent();
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if (AliveBlocks.count(BB) == 0) { // Basic block not alive yet...
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// Mark the basic block as being newly ALIVE... and mark all branches that
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// this block is control dependant on as being alive also...
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//
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AliveBlocks.insert(BB); // Block is now ALIVE!
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DominanceFrontier::const_iterator It = CDG.find(BB);
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if (It != CDG.end()) {
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// Get the blocks that this node is control dependant on...
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const DominanceFrontier::DomSetType &CDB = It->second;
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for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
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bind_obj(this, &ADCE::markTerminatorLive));
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}
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// If this basic block is live, then the terminator must be as well!
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markTerminatorLive(BB);
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}
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// Loop over all of the operands of the live instruction, making sure that
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// they are known to be alive as well...
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//
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for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
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if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
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markInstructionLive(Operand);
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}
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#ifdef DEBUG_ADCE
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cerr << "Current Function: X = Live\n";
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for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
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for (BasicBlock::iterator BI = (*I)->begin(), BE = (*I)->end();
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BI != BE; ++BI) {
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if (LiveSet.count(*BI)) cerr << "X ";
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cerr << *BI;
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}
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#endif
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// After the worklist is processed, recursively walk the CFG in depth first
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// order, patching up references to dead blocks...
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//
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std::set<BasicBlock*> VisitedBlocks;
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BasicBlock *EntryBlock = fixupCFG(Func->front(), VisitedBlocks, AliveBlocks);
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if (EntryBlock && EntryBlock != Func->front()) {
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// We need to move the new entry block to be the first bb of the function
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Function::iterator EBI = find(Func->begin(), Func->end(), EntryBlock);
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std::swap(*EBI, *Func->begin()); // Exchange old location with start of fn
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while (PHINode *PN = dyn_cast<PHINode>(EntryBlock->front())) {
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assert(PN->getNumIncomingValues() == 1 &&
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"Can only have a single incoming value at this point...");
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// The incoming value must be outside of the scope of the function, a
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// global variable, constant or parameter maybe...
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//
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PN->replaceAllUsesWith(PN->getIncomingValue(0));
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// Nuke the phi node...
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delete EntryBlock->getInstList().remove(EntryBlock->begin());
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}
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}
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// Now go through and tell dead blocks to drop all of their references so they
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// can be safely deleted.
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//
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for (Function::iterator BI = Func->begin(), BE = Func->end(); BI != BE; ++BI){
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BasicBlock *BB = *BI;
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if (!AliveBlocks.count(BB)) {
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BB->dropAllReferences();
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}
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}
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// Now loop through all of the blocks and delete them. We can safely do this
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// now because we know that there are no references to dead blocks (because
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// they have dropped all of their references...
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//
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for (Function::iterator BI = Func->begin(); BI != Func->end();) {
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if (!AliveBlocks.count(*BI)) {
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delete Func->getBasicBlocks().remove(BI);
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MadeChanges = true;
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continue; // Don't increment iterator
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}
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++BI; // Increment iterator...
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}
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}
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// fixupCFG - Walk the CFG in depth first order, eliminating references to
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// dead blocks:
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// If the BB is alive (in AliveBlocks):
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// 1. Eliminate all dead instructions in the BB
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// 2. Recursively traverse all of the successors of the BB:
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// - If the returned successor is non-null, update our terminator to
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// reference the returned BB
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// 3. Return 0 (no update needed)
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//
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// If the BB is dead (not in AliveBlocks):
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// 1. Add the BB to the dead set
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// 2. Recursively traverse all of the successors of the block:
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// - Only one shall return a nonnull value (or else this block should have
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// been in the alive set).
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// 3. Return the nonnull child, or 0 if no non-null children.
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//
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BasicBlock *ADCE::fixupCFG(BasicBlock *BB, std::set<BasicBlock*> &VisitedBlocks,
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const std::set<BasicBlock*> &AliveBlocks) {
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if (VisitedBlocks.count(BB)) return 0; // Revisiting a node? No update.
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VisitedBlocks.insert(BB); // We have now visited this node!
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#ifdef DEBUG_ADCE
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cerr << "Fixing up BB: " << BB;
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#endif
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if (AliveBlocks.count(BB)) { // Is the block alive?
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// Yes it's alive: loop through and eliminate all dead instructions in block
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for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; )
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if (!LiveSet.count(*II)) { // Is this instruction alive?
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// Nope... remove the instruction from it's basic block...
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delete BB->getInstList().remove(II);
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MadeChanges = true;
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} else {
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++II;
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}
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// Recursively traverse successors of this basic block.
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI) {
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BasicBlock *Succ = *SI;
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BasicBlock *Repl = fixupCFG(Succ, VisitedBlocks, AliveBlocks);
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if (Repl && Repl != Succ) { // We have to replace the successor
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Succ->replaceAllUsesWith(Repl);
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MadeChanges = true;
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}
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}
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return BB;
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} else { // Otherwise the block is dead...
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BasicBlock *ReturnBB = 0; // Default to nothing live down here
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// Recursively traverse successors of this basic block.
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI) {
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BasicBlock *RetBB = fixupCFG(*SI, VisitedBlocks, AliveBlocks);
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if (RetBB) {
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assert(ReturnBB == 0 && "One one live child allowed!");
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ReturnBB = RetBB;
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}
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}
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return ReturnBB; // Return the result of traversal
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}
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}
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