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Significant rework. DCE is still not done (see #ifdef'd out parts)

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but at least the stuff that is checked in, now works.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2001-06-07 16:59:26 +00:00
parent b9a4578df5
commit f155e13092
1 changed files with 186 additions and 51 deletions
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221
lib/Transforms/Scalar/DCE.cpp
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@ -7,6 +7,8 @@
// * 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
@ -21,7 +23,9 @@
#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 };
@ -35,6 +39,7 @@ struct BasicBlockDCE {
}
};
template<class ValueSubclass, class ItemParentType, class DCEController>
static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
DCEController DCEControl) {
@ -46,6 +51,7 @@ static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
// Look for un"used" definitions...
if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
// Bye bye
//cerr << "Removing: " << *DI;
delete Vals.remove(DI);
Changed = true;
} else {
@ -55,6 +61,39 @@ static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
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;
@ -64,7 +103,6 @@ bool DoRemoveUnusedConstants(SymTabValue *S) {
return Changed;
}
static void ReplaceUsesWithConstant(Instruction *I) {
// Get the method level constant pool
ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
@ -87,25 +125,97 @@ static void ReplaceUsesWithConstant(Instruction *I) {
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();
@ -115,59 +225,84 @@ static bool DoDCEPass(Method *M) {
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;
}
// 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;
// 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!
// Is there exactly one predecessor to this block?
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
// 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!
assert(Term != 0 && "malformed basic block without terminator!");
// Is it an unconditional branch?
if (Term->getInstType() != Instruction::Br ||
!((BranchInst*)Term)->isUnconditional())
continue; // Nope, maybe next time...
// 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;
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...
// 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); // Remove terminator
delete Pred->getInstList().remove(--DI);
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...
// 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...
BBs.remove(Pred);
// 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;
// Always inherit predecessors name if it exists...
if (Pred->hasName()) BB->setName(Pred->getName());
// Inherit predecessors name if it exists...
if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
// So long you waste of a basic block you...
delete Pred;
// You ARE the weakest link... goodbye
delete BB;
}
}
}