// * removes basic blocks with no predecessors
// * merges a basic block into its predecessor if there is only one and the
// predecessor only has one successor.
+// * Eliminates PHI nodes for basic blocks with a single predecessor
+// * Eliminates a basic block that only contains an unconditional branch
//
// TODO: This should REALLY be recursive instead of iterative. Right now, we
// scan linearly through values, removing unused ones as we go. The problem is
#include "llvm/Method.h"
#include "llvm/BasicBlock.h"
#include "llvm/iTerminators.h"
+#include "llvm/iOther.h"
#include "llvm/Opt/AllOpts.h"
+#include "llvm/Assembly/Writer.h"
struct ConstPoolDCE {
enum { EndOffs = 0 };
}
};
+
template<class ValueSubclass, class ItemParentType, class DCEController>
static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
DCEController DCEControl) {
// Look for un"used" definitions...
if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
// Bye bye
+ //cerr << "Removing: " << *DI;
delete Vals.remove(DI);
Changed = true;
} else {
return Changed;
}
+// RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
+// a single predecessor. This means that the PHI node must only have a single
+// RHS value and can be eliminated.
+//
+// This routine is very simple because we know that PHI nodes must be the first
+// things in a basic block, if they are present.
+//
+static bool RemoveSingularPHIs(BasicBlock *BB) {
+ BasicBlock::pred_iterator PI(BB->pred_begin());
+ if (PI == BB->pred_end() || ++PI != BB->pred_end())
+ return false; // More than one predecessor...
+
+ Instruction *I = BB->getInstList().front();
+ if (I->getInstType() != Instruction::PHINode) return false; // No PHI nodes
+
+ cerr << "Killing PHIs from " << BB;
+ cerr << "Pred #0 = " << *BB->pred_begin();
+
+ cerr << "Method == " << BB->getParent();
+
+ do {
+ PHINode *PN = (PHINode*)I;
+ assert(PN->getOperand(1) == 0 && "PHI node should only have one value!");
+ Value *V = PN->getOperand(0);
+
+ PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
+ delete BB->getInstList().remove(BB->getInstList().begin());
+
+ I = BB->getInstList().front();
+ } while (I->getInstType() == Instruction::PHINode);
+
+ return true; // Yes, we nuked at least one phi node
+}
bool DoRemoveUnusedConstants(SymTabValue *S) {
bool Changed = false;
return Changed;
}
-
static void ReplaceUsesWithConstant(Instruction *I) {
// Get the method level constant pool
ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
I->replaceAllUsesWith(CPV);
}
+// RemovePredecessorFromBlock - This function is called when we are about
+// to remove a predecessor from a basic block. This function takes care of
+// removing the predecessor from the PHI nodes in BB so that after the pred
+// is removed, the number of PHI slots per bb is equal to the number of
+// predecessors.
+//
+static void RemovePredecessorFromBlock(BasicBlock *BB, BasicBlock *Pred) {
+ BasicBlock::pred_iterator PI(BB->pred_begin()), EI(BB->pred_end());
+ unsigned pred_idx = 0, max_idx;
+
+ cerr << "RPFB: " << Pred << "From Block: " << BB;
+
+ // Find out what index the predecessor is...
+ for (; *PI != BB; ++PI, ++pred_idx) {
+ assert(PI != EI && "Pred is not a predecessor of BB!");
+ }
+
+ // Loop over the rest of the predecssors until we run out, or until we find
+ // out that there are more than 2 predecessors.
+ for (max_idx = pred_idx; PI != EI && max_idx < 2; ++PI, ++max_idx) /*empty*/;
+
+ // If there are exactly two predecessors, then we want to nuke the PHI nodes
+ // altogether.
+ bool NukePHIs = max_idx == 1;
+
+ // Okay, now we know that we need to remove predecessor #pred_idx from all
+ // PHI nodes. Iterate over each PHI node fixing them up
+ BasicBlock::InstListType::iterator II(BB->getInstList().begin());
+ for (; (*II)->getInstType() == Instruction::PHINode; ++II) {
+ PHINode *PN = (PHINode*)*II;
+ PN->removeIncomingValue(pred_idx);
+
+ if (NukePHIs) { // Destroy the PHI altogether??
+ assert(PN->getOperand(1) == 0 && "PHI node should only have one value!");
+ Value *V = PN->getOperand(0);
+
+ PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
+ delete BB->getInstList().remove(II);
+ }
+ }
+}
+
+// PropogatePredecessors - This gets "Succ" ready to have the predecessors from
+// "BB". This is a little tricky because "Succ" has PHI nodes, which need to
+// have extra slots added to them to hold the merge edges from BB's
+// predecessors.
+//
+// Assumption: BB is the single predecessor of Succ.
+//
+static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
+ assert(BB && Succ && *Succ->pred_begin() == BB && "BB is only pred of Succ" &&
+ ++Succ->pred_begin() == Succ->pred_end());
+
+ // If there is more than one predecessor, and there are PHI nodes in
+ // the successor, then we need to add incoming edges for the PHI nodes
+ BasicBlock::pred_iterator PI(BB->pred_begin());
+ for (; PI != BB->pred_end(); ++PI) {
+ // TODO:
+ }
+}
+
static bool DoDCEPass(Method *M) {
- Method::BasicBlocksType::iterator BBIt;
Method::BasicBlocksType &BBs = M->getBasicBlocks();
+ Method::BasicBlocksType::iterator BBIt, BBEnd = BBs.end();
+ if (BBs.begin() == BBEnd) return false; // Nothing to do
bool Changed = false;
// Loop through now and remove instructions that have no uses...
- for (BBIt = BBs.begin(); BBIt != BBs.end(); BBIt++)
+ for (BBIt = BBs.begin(); BBIt != BBEnd; BBIt++) {
Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
+ Changed |= RemoveSingularPHIs(*BBIt);
+ }
- // Scan through and remove basic blocks that have no predecessors (except,
- // of course, the first one. :) (so skip first block)
+ // Loop over all of the basic blocks (except the first one) and remove them
+ // if they are unneeded...
//
- for (BBIt = BBs.begin(), ++BBIt; BBIt != BBs.end(); BBIt++) {
+ for (BBIt = BBs.begin(), ++BBIt; BBIt != BBs.end(); ++BBIt) {
BasicBlock *BB = *BBIt;
- assert(BB->getTerminator() &&
- "Degenerate basic block encountered!"); // Empty bb???
+ assert(BB->getTerminator() && "Degenerate basic block encountered!");
+#if 0
+ // Remove basic blocks that have no predecessors... which are unreachable.
if (BB->pred_begin() == BB->pred_end() &&
- !BB->hasConstantPoolReferences()) {
+ !BB->hasConstantPoolReferences() && 0) {
+ cerr << "Removing BB: \n" << BB;
+
+ // Loop through all of our successors and make sure they know that one
+ // of their predecessors is going away.
+ for (BasicBlock::succ_iterator SI = BB->succ_begin(), EI = BB->succ_end();
+ SI != EI; ++SI)
+ RemovePredecessorFromBlock(*SI, BB);
while (!BB->getInstList().empty()) {
Instruction *I = BB->getInstList().front();
if (!I->use_empty()) ReplaceUsesWithConstant(I);
// Remove the instruction from the basic block
- BasicBlock::InstListType::iterator f = BB->getInstList().begin();
- delete BB->getInstList().remove(f);
+ delete BB->getInstList().remove(BB->getInstList().begin());
}
-
delete BBs.remove(BBIt);
- ++BBIt; // remove puts use on the previous block, we want the next one
+ --BBIt; // remove puts use on the next block, we want the previous one
Changed = true;
+ continue;
+ }
+
+ // Check to see if this block has no instructions and only a single
+ // successor. If so, replace block references with successor.
+ BasicBlock::succ_iterator SI(BB->succ_begin());
+ if (SI != BB->succ_end() && ++SI == BB->succ_end()) { // One succ?
+ Instruction *I = BB->getInstList().front();
+ if (I->isTerminator()) { // Terminator is the only instruction!
+
+ if (Succ->getInstList().front()->getInstType() == Instruction::PHINode){
+ // Add entries to the PHI nodes so that the PHI nodes have the right
+ // number of entries...
+ PropogatePredecessorsForPHIs(BB, Succ);
+ }
+
+ BasicBlock *Succ = *BB->succ_begin(); // There is exactly one successor
+ BB->replaceAllUsesWith(Succ);
+ cerr << "Killing Trivial BB: \n" << BB;
+
+ BB = BBs.remove(BBIt);
+ --BBIt; // remove puts use on the next block, we want the previous one
+
+ if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
+ Succ->setName(BB->getName());
+ delete BB; // Delete basic block
+
+ cerr << "Method after removal: \n" << M;
+ Changed = true;
+ continue;
+ }
}
- }
+#endif
- // Loop through an merge basic blocks into their predecessor if there is only
- // one, and if there is only one successor of the predecessor.
- //
- for (BBIt = BBs.begin(); BBIt != BBs.end(); BBIt++) {
- BasicBlock *BB = *BBIt;
-
- // Is there exactly one predecessor to this block?
+ // Merge basic blocks into their predecessor if there is only one pred,
+ // and if there is only one successor of the predecessor.
BasicBlock::pred_iterator PI(BB->pred_begin());
- if (PI != BB->pred_end() && ++PI == BB->pred_end() &&
- !BB->hasConstantPoolReferences()) {
+ if (PI != BB->pred_end() && *PI != BB && // Not empty? Not same BB?
+ ++PI == BB->pred_end() && !BB->hasConstantPoolReferences()) {
BasicBlock *Pred = *BB->pred_begin();
TerminatorInst *Term = Pred->getTerminator();
- if (Term == 0) continue; // Err... malformed basic block!
-
- // Is it an unconditional branch?
- if (Term->getInstType() != Instruction::Br ||
- !((BranchInst*)Term)->isUnconditional())
- continue; // Nope, maybe next time...
-
- Changed = true;
-
- // Make all branches to the predecessor now point to the successor...
- Pred->replaceAllUsesWith(BB);
-
- // Move all definitions in the predecessor to the successor...
- BasicBlock::InstListType::iterator DI = Pred->getInstList().end();
- assert(Pred->getTerminator() &&
- "Degenerate basic block encountered!"); // Empty bb???
- delete Pred->getInstList().remove(--DI); // Remove terminator
-
- while (Pred->getInstList().begin() != (DI = Pred->getInstList().end())) {
- Instruction *Def = Pred->getInstList().remove(--DI); // Remove from end
- BB->getInstList().push_front(Def); // Add to front...
+ assert(Term != 0 && "malformed basic block without terminator!");
+
+ // Does the predecessor block only have a single successor?
+ BasicBlock::succ_iterator SI(Pred->succ_begin());
+ if (++SI == Pred->succ_end()) {
+ //cerr << "Merging: " << BB << "into: " << Pred;
+
+ // Delete the unconditianal branch from the predecessor...
+ BasicBlock::InstListType::iterator DI = Pred->getInstList().end();
+ assert(Pred->getTerminator() &&
+ "Degenerate basic block encountered!"); // Empty bb???
+ delete Pred->getInstList().remove(--DI);
+
+ // Move all definitions in the succecessor to the predecessor...
+ while (!BB->getInstList().empty()) {
+ DI = BB->getInstList().begin();
+ Instruction *Def = BB->getInstList().remove(DI); // Remove from front
+ Pred->getInstList().push_back(Def); // Add to end...
+ }
+
+ // Remove basic block from the method... and advance iterator to the
+ // next valid block...
+ BB = BBs.remove(BBIt);
+ --BBIt; // remove puts us on the NEXT bb. We want the prev BB
+ Changed = true;
+
+ // Inherit predecessors name if it exists...
+ if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
+
+ // You ARE the weakest link... goodbye
+ delete BB;
}
-
- // Remove basic block from the method...
- BBs.remove(Pred);
-
- // Always inherit predecessors name if it exists...
- if (Pred->hasName()) BB->setName(Pred->getName());
-
- // So long you waste of a basic block you...
- delete Pred;
}
}
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