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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@2701 91177308-0d34-0410-b5e6-96231b3b80d8
199 lines
7.7 KiB
C++
199 lines
7.7 KiB
C++
//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
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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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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Constant.h"
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#include "llvm/iPHINode.h"
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#include "llvm/Support/CFG.h"
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#include <algorithm>
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#include <functional>
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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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// Loop over all of the PHI nodes in the successor BB
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for (BasicBlock::iterator I = Succ->begin();
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PHINode *PN = dyn_cast<PHINode>(*I); ++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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}
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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())
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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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// 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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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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//cerr << "Killing Trivial BB: \n" << BB;
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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: " << OnlyPred;
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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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