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https://github.com/c64scene-ar/llvm-6502.git
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2700 91177308-0d34-0410-b5e6-96231b3b80d8
101 lines
2.9 KiB
C++
101 lines
2.9 KiB
C++
//===- SimplifyCFG.cpp - CFG Simplification Pass --------------------------===//
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//
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// This file implements dead code elimination and basic block merging.
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//
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// Specifically, this:
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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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//
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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/Module.h"
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#include "llvm/GlobalVariable.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Pass.h"
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#include "Support/StatisticReporter.h"
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#include <set>
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static Statistic<> NumSimpl("cfgsimplify\t- Number of blocks simplified");
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namespace {
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struct CFGSimplifyPass : public FunctionPass {
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const char *getPassName() const { return "Simplify CFG"; }
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virtual bool runOnFunction(Function *F);
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};
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}
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Pass *createCFGSimplificationPass() {
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return new CFGSimplifyPass();
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}
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static bool MarkAliveBlocks(BasicBlock *BB, std::set<BasicBlock*> &Reachable) {
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if (Reachable.count(BB)) return false;
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Reachable.insert(BB);
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bool Changed = ConstantFoldTerminator(BB);
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
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MarkAliveBlocks(*SI, Reachable);
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return Changed;
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}
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// It is possible that we may require multiple passes over the code to fully
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// simplify the CFG.
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//
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bool CFGSimplifyPass::runOnFunction(Function *F) {
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std::set<BasicBlock*> Reachable;
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bool Changed = MarkAliveBlocks(F->front(), Reachable);
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// If there are unreachable blocks in the CFG...
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if (Reachable.size() != F->size()) {
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assert(Reachable.size() < F->size());
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NumSimpl += F->size()-Reachable.size();
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// Loop over all of the basic blocks that are not reachable, dropping all of
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// their internal references...
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for (Function::iterator I = F->begin()+1, E = F->end(); I != E; ++I)
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if (!Reachable.count(*I)) {
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BasicBlock *BB = *I;
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI!=SE; ++SI)
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if (Reachable.count(*SI))
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(*SI)->removePredecessor(BB);
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BB->dropAllReferences();
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}
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for (Function::iterator I = F->begin()+1; I != F->end();)
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if (!Reachable.count(*I))
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delete F->getBasicBlocks().remove(I);
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else
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++I;
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Changed = true;
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}
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bool LocalChange = true;
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while (LocalChange) {
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LocalChange = false;
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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 (Function::iterator BBIt = F->begin()+1; BBIt != F->end(); ) {
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if (SimplifyCFG(BBIt)) {
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LocalChange = true;
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++NumSimpl;
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} else {
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++BBIt;
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}
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}
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Changed |= LocalChange;
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}
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return Changed;
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}
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