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* Factored RemovePredecessorFromBlock into BasicBlock::removePredecessor

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* Avoid messing around with this case:
  br label %A
%A:  br label %A
* Enable optimizations that are correct now.

llvm-svn: 102
This commit is contained in:
Chris Lattner 2001-06-29 05:24:28 +00:00
parent f5b28afa04
commit e005dc792d
1 changed files with 29 additions and 67 deletions
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96
lib/Transforms/Scalar/DCE.cpp
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@ -10,10 +10,10 @@
// * 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
// that this may cause other earlier values to become unused. To make sure that
// we get them all, we iterate until things stop changing. Instead, when
// TODO: This should REALLY be worklist driven instead of iterative. Right now,
// we scan linearly through values, removing unused ones as we go. The problem
// is that this may cause other earlier values to become unused. To make sure
// that we get them all, we iterate until things stop changing. Instead, when
// removing a value, recheck all of its operands to see if they are now unused.
// Piece of cake, and more efficient as well.
//
@ -30,6 +30,8 @@
#include "llvm/Opt/AllOpts.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/CFG.h"
#include "llvm/Tools/STLExtras.h"
#include <algorithm>
using namespace cfg;
@ -131,44 +133,6 @@ 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) {
pred_iterator PI(pred_begin(BB)), EI(pred_end(BB));
unsigned max_idx;
//cerr << "RPFB: " << Pred << "From Block: " << 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 = 0; PI != EI && max_idx < 3; ++PI, ++max_idx) /*empty*/;
// If there are exactly two predecessors, then we want to nuke the PHI nodes
// altogether.
bool NukePHIs = max_idx == 2;
assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
// 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::iterator II(BB->begin());
for (; (*II)->isPHINode(); ++II) {
PHINode *PN = (PHINode*)*II;
PN->removeIncomingValue(BB);
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
@ -218,16 +182,15 @@ static bool DoDCEPass(Method *M) {
BasicBlock *BB = *BBIt;
assert(BB->getTerminator() && "Degenerate basic block encountered!");
#if 0 // This is know to basically work?
// Remove basic blocks that have no predecessors... which are unreachable.
if (pred_begin(BB) == pred_end(BB) &&
!BB->hasConstantPoolReferences() && 0) {
cerr << "Removing BB: \n" << BB;
//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_each(succ_begin(BB), succ_end(BB),
bind_2nd(RemovePredecessorFromBlock, BB));
bind_obj(BB, &BasicBlock::removePredecessor));
while (!BB->empty()) {
Instruction *I = BB->front();
@ -244,9 +207,7 @@ static bool DoDCEPass(Method *M) {
Changed = true;
continue;
}
#endif
#if 0 // This has problems
// Check to see if this block has no instructions and only a single
// successor. If so, replace block references with successor.
succ_iterator SI(succ_begin(BB));
@ -254,30 +215,31 @@ static bool DoDCEPass(Method *M) {
Instruction *I = BB->front();
if (I->isTerminator()) { // Terminator is the only instruction!
BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
cerr << "Killing Trivial BB: \n" << BB;
//cerr << "Killing Trivial BB: \n" << BB;
if (Succ->front()->isPHINode()) {
// If our successor has PHI nodes, then we need to update them to
// include entries for BB's predecessors, not for BB itself.
//
PropogatePredecessorsForPHIs(BB, Succ);
if (Succ != BB) { // Arg, don't hurt infinite loops!
if (Succ->front()->isPHINode()) {
// If our successor has PHI nodes, then we need to update them to
// include entries for BB's predecessors, not for BB itself.
//
PropogatePredecessorsForPHIs(BB, Succ);
}
BB->replaceAllUsesWith(Succ);
BB = M->getBasicBlocks().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;
}
BB->replaceAllUsesWith(Succ);
BB = M->getBasicBlocks().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.