Significant rework. DCE is still not done (see #ifdef'd out parts)
but at least the stuff that is checked in, now works. llvm-svn: 10
This commit is contained in:
Chris Lattner
parent
a2f01878b7
commit
d821c2af37
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@ -7,6 +7,8 @@
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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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// TODO: This should REALLY be recursive instead of iterative. Right now, we
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// scan linearly through values, removing unused ones as we go. The problem is
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@ -21,7 +23,9 @@
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#include "llvm/Method.h"
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#include "llvm/BasicBlock.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iOther.h"
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#include "llvm/Opt/AllOpts.h"
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#include "llvm/Assembly/Writer.h"
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struct ConstPoolDCE {
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enum { EndOffs = 0 };
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@ -35,6 +39,7 @@ struct BasicBlockDCE {
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}
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};
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template<class ValueSubclass, class ItemParentType, class DCEController>
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static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
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DCEController DCEControl) {
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@ -46,6 +51,7 @@ static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
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// Look for un"used" definitions...
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if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
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// Bye bye
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//cerr << "Removing: " << *DI;
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delete Vals.remove(DI);
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Changed = true;
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} else {
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@ -55,6 +61,39 @@ static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
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return Changed;
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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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// 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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BasicBlock::pred_iterator PI(BB->pred_begin());
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if (PI == BB->pred_end() || ++PI != BB->pred_end())
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return false; // More than one predecessor...
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Instruction *I = BB->getInstList().front();
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if (I->getInstType() != Instruction::PHINode) return false; // No PHI nodes
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cerr << "Killing PHIs from " << BB;
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cerr << "Pred #0 = " << *BB->pred_begin();
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cerr << "Method == " << BB->getParent();
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do {
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PHINode *PN = (PHINode*)I;
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assert(PN->getOperand(1) == 0 && "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->getInstList().begin());
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I = BB->getInstList().front();
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} while (I->getInstType() == Instruction::PHINode);
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return true; // Yes, we nuked at least one phi node
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}
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bool DoRemoveUnusedConstants(SymTabValue *S) {
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bool Changed = false;
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@ -64,7 +103,6 @@ bool DoRemoveUnusedConstants(SymTabValue *S) {
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return Changed;
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}
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static void ReplaceUsesWithConstant(Instruction *I) {
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// Get the method level constant pool
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ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
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@ -87,25 +125,97 @@ static void ReplaceUsesWithConstant(Instruction *I) {
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I->replaceAllUsesWith(CPV);
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}
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// RemovePredecessorFromBlock - This function is called when we are about
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// to remove a predecessor from a basic block. This function takes care of
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// removing the predecessor from the PHI nodes in BB so that after the pred
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// is removed, the number of PHI slots per bb is equal to the number of
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// predecessors.
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//
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static void RemovePredecessorFromBlock(BasicBlock *BB, BasicBlock *Pred) {
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BasicBlock::pred_iterator PI(BB->pred_begin()), EI(BB->pred_end());
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unsigned pred_idx = 0, max_idx;
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cerr << "RPFB: " << Pred << "From Block: " << BB;
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// Find out what index the predecessor is...
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for (; *PI != BB; ++PI, ++pred_idx) {
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assert(PI != EI && "Pred is not a predecessor of BB!");
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}
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// Loop over the rest of the predecssors until we run out, or until we find
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// out that there are more than 2 predecessors.
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for (max_idx = pred_idx; PI != EI && max_idx < 2; ++PI, ++max_idx) /*empty*/;
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// If there are exactly two predecessors, then we want to nuke the PHI nodes
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// altogether.
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bool NukePHIs = max_idx == 1;
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// Okay, now we know that we need to remove predecessor #pred_idx from all
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// PHI nodes. Iterate over each PHI node fixing them up
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BasicBlock::InstListType::iterator II(BB->getInstList().begin());
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for (; (*II)->getInstType() == Instruction::PHINode; ++II) {
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PHINode *PN = (PHINode*)*II;
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PN->removeIncomingValue(pred_idx);
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if (NukePHIs) { // Destroy the PHI altogether??
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assert(PN->getOperand(1) == 0 && "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(II);
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}
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}
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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.
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//
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// Assumption: BB is the single predecessor of Succ.
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//
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static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
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assert(BB && Succ && *Succ->pred_begin() == BB && "BB is only pred of Succ" &&
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++Succ->pred_begin() == Succ->pred_end());
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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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BasicBlock::pred_iterator PI(BB->pred_begin());
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for (; PI != BB->pred_end(); ++PI) {
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// TODO:
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}
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}
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static bool DoDCEPass(Method *M) {
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Method::BasicBlocksType::iterator BBIt;
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Method::BasicBlocksType &BBs = M->getBasicBlocks();
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Method::BasicBlocksType::iterator BBIt, BBEnd = BBs.end();
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if (BBs.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 = BBs.begin(); BBIt != BBs.end(); BBIt++)
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for (BBIt = BBs.begin(); BBIt != BBEnd; BBIt++) {
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Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
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Changed |= RemoveSingularPHIs(*BBIt);
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}
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// Scan through and remove basic blocks that have no predecessors (except,
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// of course, the first one. :) (so skip first block)
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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 = BBs.begin(), ++BBIt; BBIt != BBs.end(); BBIt++) {
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for (BBIt = BBs.begin(), ++BBIt; BBIt != BBs.end(); ++BBIt) {
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BasicBlock *BB = *BBIt;
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assert(BB->getTerminator() &&
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"Degenerate basic block encountered!"); // Empty bb???
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assert(BB->getTerminator() && "Degenerate basic block encountered!");
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#if 0
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// Remove basic blocks that have no predecessors... which are unreachable.
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if (BB->pred_begin() == BB->pred_end() &&
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!BB->hasConstantPoolReferences()) {
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!BB->hasConstantPoolReferences() && 0) {
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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 (BasicBlock::succ_iterator SI = BB->succ_begin(), EI = BB->succ_end();
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SI != EI; ++SI)
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RemovePredecessorFromBlock(*SI, BB);
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while (!BB->getInstList().empty()) {
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Instruction *I = BB->getInstList().front();
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@ -115,59 +225,84 @@ static bool DoDCEPass(Method *M) {
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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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BasicBlock::InstListType::iterator f = BB->getInstList().begin();
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delete BB->getInstList().remove(f);
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delete BB->getInstList().remove(BB->getInstList().begin());
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}
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delete BBs.remove(BBIt);
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++BBIt; // remove puts use on the previous block, we want the next one
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--BBIt; // remove puts use on the next block, we want the previous one
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Changed = true;
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continue;
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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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BasicBlock::succ_iterator SI(BB->succ_begin());
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if (SI != BB->succ_end() && ++SI == BB->succ_end()) { // One succ?
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Instruction *I = BB->getInstList().front();
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if (I->isTerminator()) { // Terminator is the only instruction!
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if (Succ->getInstList().front()->getInstType() == Instruction::PHINode){
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// Add entries to the PHI nodes so that the PHI nodes have the right
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// number of entries...
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PropogatePredecessorsForPHIs(BB, Succ);
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}
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BasicBlock *Succ = *BB->succ_begin(); // There is exactly one successor
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BB->replaceAllUsesWith(Succ);
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cerr << "Killing Trivial BB: \n" << BB;
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BB = BBs.remove(BBIt);
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--BBIt; // remove puts use on the next block, we want the previous one
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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 << "Method after removal: \n" << M;
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Changed = true;
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continue;
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}
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}
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}
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#endif
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// Loop through an merge basic blocks into their predecessor if there is only
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// one, and if there is only one successor of the predecessor.
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//
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for (BBIt = BBs.begin(); BBIt != BBs.end(); BBIt++) {
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BasicBlock *BB = *BBIt;
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// Is there exactly one predecessor to this block?
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// Merge basic blocks into their predecessor if there is only one pred,
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// and if there is only one successor of the predecessor.
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BasicBlock::pred_iterator PI(BB->pred_begin());
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if (PI != BB->pred_end() && ++PI == BB->pred_end() &&
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!BB->hasConstantPoolReferences()) {
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if (PI != BB->pred_end() && *PI != BB && // Not empty? Not same BB?
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++PI == BB->pred_end() && !BB->hasConstantPoolReferences()) {
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BasicBlock *Pred = *BB->pred_begin();
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TerminatorInst *Term = Pred->getTerminator();
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if (Term == 0) continue; // Err... malformed basic block!
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assert(Term != 0 && "malformed basic block without terminator!");
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// Is it an unconditional branch?
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if (Term->getInstType() != Instruction::Br ||
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!((BranchInst*)Term)->isUnconditional())
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continue; // Nope, maybe next time...
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// Does the predecessor block only have a single successor?
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BasicBlock::succ_iterator SI(Pred->succ_begin());
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if (++SI == Pred->succ_end()) {
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//cerr << "Merging: " << BB << "into: " << Pred;
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Changed = true;
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// Make all branches to the predecessor now point to the successor...
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Pred->replaceAllUsesWith(BB);
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// Move all definitions in the predecessor to the successor...
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BasicBlock::InstListType::iterator DI = Pred->getInstList().end();
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assert(Pred->getTerminator() &&
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"Degenerate basic block encountered!"); // Empty bb???
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delete Pred->getInstList().remove(--DI); // Remove terminator
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while (Pred->getInstList().begin() != (DI = Pred->getInstList().end())) {
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Instruction *Def = Pred->getInstList().remove(--DI); // Remove from end
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BB->getInstList().push_front(Def); // Add to front...
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// Delete the unconditianal branch from the predecessor...
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BasicBlock::InstListType::iterator DI = Pred->getInstList().end();
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assert(Pred->getTerminator() &&
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"Degenerate basic block encountered!"); // Empty bb???
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delete Pred->getInstList().remove(--DI);
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// Move all definitions in the succecessor to the predecessor...
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while (!BB->getInstList().empty()) {
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DI = BB->getInstList().begin();
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Instruction *Def = BB->getInstList().remove(DI); // Remove from front
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Pred->getInstList().push_back(Def); // Add to end...
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}
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// Remove basic block from the method... and advance iterator to the
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// next valid block...
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BB = BBs.remove(BBIt);
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--BBIt; // remove puts us on the NEXT bb. We want the prev BB
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Changed = true;
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// Inherit predecessors name if it exists...
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if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
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// You ARE the weakest link... goodbye
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delete BB;
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}
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// Remove basic block from the method...
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BBs.remove(Pred);
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// Always inherit predecessors name if it exists...
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if (Pred->hasName()) BB->setName(Pred->getName());
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// So long you waste of a basic block you...
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delete Pred;
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}
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}
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