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Make ADCE more robust, it still has problems, but it's getting closer 

llvm-svn: 521
This commit is contained in:
Chris Lattner 2001-09-09 22:26:47 +00:00
parent c49f5f35db
commit acfd27d502
1 changed files with 186 additions and 42 deletions
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228
llvm/lib/Transforms/Scalar/ADCE.cpp
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@ -12,9 +12,13 @@
#include "llvm/Analysis/Dominators.h"
#include "llvm/Support/STLExtras.h"
#include "llvm/Analysis/Writer.h"
#include "llvm/CFG.h"
#include "llvm/iTerminators.h"
#include <set>
#include <algorithm>
//#define DEBUG_ADCE 1
//===----------------------------------------------------------------------===//
// ADCE Class
//
@ -25,13 +29,14 @@ class ADCE {
Method *M; // The method that we are working on...
vector<Instruction*> WorkList; // Instructions that just became live
set<Instruction*> LiveSet; // The set of live instructions
bool MadeChanges;
//===--------------------------------------------------------------------===//
// The public interface for this class
//
public:
// ADCE Ctor - Save the method to operate on...
inline ADCE(Method *m) : M(m) {}
inline ADCE(Method *m) : M(m), MadeChanges(false) {}
// doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
// true if the method was modified.
@ -43,15 +48,25 @@ public:
private:
inline void markInstructionLive(Instruction *I) {
if (LiveSet.count(I)) return;
#ifdef DEBUG_ADCE
cerr << "Insn Live: " << I;
#endif
LiveSet.insert(I);
WorkList.push_back(I);
}
inline void markTerminatorLive(const BasicBlock *BB) {
cerr << "Marking Term Live\n";
markInstructionLive((Instruction*)BB->back());
#ifdef DEBUG_ADCE
cerr << "Terminat Live: " << BB->getTerminator();
#endif
markInstructionLive((Instruction*)BB->getTerminator());
}
// fixupCFG - Walk the CFG in depth first order, eliminating references to
// dead blocks.
//
BasicBlock *fixupCFG(BasicBlock *Head, set<BasicBlock*> &VisitedBlocks,
const set<BasicBlock*> &AliveBlocks);
};
@ -60,38 +75,46 @@ private:
// true if the method was modified.
//
bool ADCE::doADCE() {
// Iterate over all of the instructions in the method, eliminating trivially
// dead instructions, and marking instructions live that are known to be
// needed.
// Compute the control dependence graph... Note that this has a side effect
// on the CFG: a new return bb is added and all returns are merged here.
//
for (Method::inst_iterator II = M->inst_begin(); II != M->inst_end(); ) {
Instruction *I = *II;
switch (I->getInstType()) {
case Instruction::Ret:
case Instruction::Call:
case Instruction::Store:
markInstructionLive(I);
break;
default:
// Check to see if anything is trivially dead
if (I->use_size() == 0 && I->getType() != Type::VoidTy) {
// Remove the instruction from it's basic block...
BasicBlock *BB = I->getParent();
delete BB->getInstList().remove(II.getInstructionIterator());
// Make sure to sync up the iterator again...
II.resyncInstructionIterator();
continue; // Don't increment the iterator past the current slot
}
}
++II; // Increment the iterator
}
// Compute the control dependence graph...
cfg::DominanceFrontier CDG(cfg::DominatorSet(M, true));
#ifdef DEBUG_ADCE
cerr << "Method: " << M;
#endif
// Iterate over all of the instructions in the method, eliminating trivially
// dead instructions, and marking instructions live that are known to be
// needed. Perform the walk in depth first order so that we avoid marking any
// instructions live in basic blocks that are unreachable. These blocks will
// be eliminated later, along with the instructions inside.
//
for (cfg::df_iterator BBI = cfg::df_begin(M), BBE = cfg::df_end(M);
BBI != BBE; ++BBI) {
BasicBlock *BB = *BBI;
for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
Instruction *I = *II;
if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
markInstructionLive(I);
} else {
// Check to see if anything is trivially dead
if (I->use_size() == 0 && I->getType() != Type::VoidTy) {
// Remove the instruction from it's basic block...
delete BB->getInstList().remove(II);
MadeChanges = true;
continue; // Don't increment the iterator past the current slot
}
}
++II; // Increment the inst iterator if the inst wasn't deleted
}
}
#ifdef DEBUG_ADCE
cerr << "Processing work list\n";
#endif
// AliveBlocks - Set of basic blocks that we know have instructions that are
// alive in them...
@ -119,33 +142,154 @@ bool ADCE::doADCE() {
for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
bind_obj(this, &ADCE::markTerminatorLive));
}
// If this basic block is live, then the terminator must be as well!
markTerminatorLive(BB);
}
// Loop over all of the operands of the live instruction, making sure that
// they are known to be alive as well...
//
for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op) {
Instruction *Operand = I->getOperand(op)->castInstruction();
if (Operand) markInstructionLive(Operand);
if (Instruction *Operand = I->getOperand(op)->castInstruction())
markInstructionLive(Operand);
}
}
// After the worklist is processed, loop through the instructions again,
// removing any that are not live... by the definition of the LiveSet.
#ifdef DEBUG_ADCE
cerr << "Current Method: X = Live\n";
for (Method::inst_iterator IL = M->inst_begin(); IL != M->inst_end(); ++IL) {
if (LiveSet.count(*IL)) cerr << "X ";
cerr << *IL;
}
#endif
// After the worklist is processed, recursively walk the CFG in depth first
// order, patching up references to dead blocks...
//
for (Method::inst_iterator II = M->inst_begin(); II != M->inst_end(); ) {
Instruction *I = *II;
if (!LiveSet.count(I)) {
cerr << "Instruction Dead: " << I;
set<BasicBlock*> VisitedBlocks;
BasicBlock *EntryBlock = fixupCFG(M->front(), VisitedBlocks, AliveBlocks);
if (EntryBlock && EntryBlock != M->front()) {
if (EntryBlock->front()->isPHINode()) {
// Cannot make the first block be a block with a PHI node in it! Instead,
// strip the first basic block of the method to contain no instructions,
// then add a simple branch to the "real" entry node...
//
BasicBlock *E = M->front();
if (!E->front()->isTerminator() || // Check for an actual change...
((TerminatorInst*)E->front())->getNumSuccessors() != 1 ||
((TerminatorInst*)E->front())->getSuccessor(0) != EntryBlock) {
E->getInstList().delete_all(); // Delete all instructions in block
E->getInstList().push_back(new BranchInst(EntryBlock));
MadeChanges = true;
}
AliveBlocks.insert(E);
} else {
// We need to move the new entry block to be the first bb of the method.
Method::iterator EBI = find(M->begin(), M->end(), EntryBlock);
swap(*EBI, *M->begin()); // Exchange old location with start of method
MadeChanges = true;
}
++II; // Increment the iterator
}
return false;
// Now go through and tell dead blocks to drop all of their references so they
// can be safely deleted.
//
for (Method::iterator BI = M->begin(), BE = M->end(); BI != BE; ++BI) {
BasicBlock *BB = *BI;
if (!AliveBlocks.count(BB)) {
BB->dropAllReferences();
}
}
// Now loop through all of the blocks and delete them. We can safely do this
// now because we know that there are no references to dead blocks (because
// they have dropped all of their references...
//
for (Method::iterator BI = M->begin(); BI != M->end();) {
if (!AliveBlocks.count(*BI)) {
delete M->getBasicBlocks().remove(BI);
MadeChanges = true;
continue; // Don't increment iterator
}
++BI; // Increment iterator...
}
return MadeChanges;
}
// fixupCFG - Walk the CFG in depth first order, eliminating references to
// dead blocks:
// If the BB is alive (in AliveBlocks):
// 1. Eliminate all dead instructions in the BB
// 2. Recursively traverse all of the successors of the BB:
// - If the returned successor is non-null, update our terminator to
// reference the returned BB
// 3. Return 0 (no update needed)
//
// If the BB is dead (not in AliveBlocks):
// 1. Add the BB to the dead set
// 2. Recursively traverse all of the successors of the block:
// - Only one shall return a nonnull value (or else this block should have
// been in the alive set).
// 3. Return the nonnull child, or 0 if no non-null children.
//
BasicBlock *ADCE::fixupCFG(BasicBlock *BB, set<BasicBlock*> &VisitedBlocks,
const set<BasicBlock*> &AliveBlocks) {
if (VisitedBlocks.count(BB)) return 0; // Revisiting a node? No update.
VisitedBlocks.insert(BB); // We have now visited this node!
#ifdef DEBUG_ADCE
cerr << "Fixing up BB: " << BB;
#endif
if (AliveBlocks.count(BB)) { // Is the block alive?
// Yes it's alive: loop through and eliminate all dead instructions in block
for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; ) {
Instruction *I = *II;
if (!LiveSet.count(I)) { // Is this instruction alive?
// Nope... remove the instruction from it's basic block...
delete BB->getInstList().remove(II);
MadeChanges = true;
continue; // Don't increment II
}
++II;
}
// Recursively traverse successors of this basic block.
cfg::succ_iterator SI = cfg::succ_begin(BB), SE = cfg::succ_end(BB);
for (; SI != SE; ++SI) {
BasicBlock *Succ = *SI;
BasicBlock *Repl = fixupCFG(Succ, VisitedBlocks, AliveBlocks);
if (Repl && Repl != Succ) { // We have to replace the successor
Succ->replaceAllUsesWith(Repl);
MadeChanges = true;
}
}
return BB;
} else { // Otherwise the block is dead...
BasicBlock *ReturnBB = 0; // Default to nothing live down here
// Recursively traverse successors of this basic block.
cfg::succ_iterator SI = cfg::succ_begin(BB), SE = cfg::succ_end(BB);
for (; SI != SE; ++SI) {
BasicBlock *RetBB = fixupCFG(*SI, VisitedBlocks, AliveBlocks);
if (RetBB) {
assert(ReturnBB == 0 && "One one live child allowed!");
ReturnBB = RetBB;
}
}
return ReturnBB; // Return the result of traversal
}
}
// DoADCE - Execute the Agressive Dead Code Elimination Algorithm
//
bool opt::DoADCE(Method *M) {
if (M->isExternal()) return false;
ADCE DCE(M);
return DCE.doADCE();
}