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Add a new Sparse Conditional Constant Propogation pass

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llvm-svn: 95
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Chris Lattner 2001-06-27 23:38:11 +00:00
parent 9e4c6fe810
commit 0a37aa1884
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lib/Transforms/Scalar/SCCP.cpp Normal file
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//===- SCCP.cpp - Sparse Conditional Constant Propogation -----------------===//
//
// This file implements sparse conditional constant propogation and merging:
//
// Specifically, this:
// * Assumes values are constant unless proven otherwise
// * Assumes BasicBlocks are dead unless proven otherwise
// * Proves values to be constant, and replaces them with constants
// . Proves conditional branches constant, and unconditionalizes them
// * Folds multiple identical constants in the constant pool together
//
// Notice that:
// * This pass has a habit of making definitions be dead. It is a good idea
// to to run a DCE pass sometime after running this pass.
//
//===----------------------------------------------------------------------===//
#include "llvm/Opt/AllOpts.h"
#include "llvm/Method.h"
#include "llvm/BasicBlock.h"
#include "llvm/ConstPoolVals.h"
#include "llvm/ConstantPool.h"
#include "llvm/Opt/ConstantHandling.h"
#include "llvm/InstrTypes.h"
#include "llvm/iOther.h"
#include "llvm/iTerminators.h"
//#include "llvm/Assembly/Writer.h"
#include <algorithm>
#include <map>
#include <set>
// InstVal class - This class represents the different lattice values that an
// instruction may occupy. It is a simple class with value semantics. The
// potential constant value that is pointed to is owned by the constant pool
// for the method being optimized.
//
class InstVal {
enum {
Undefined, // This instruction has no known value
Constant, // This instruction has a constant value
// Range, // This instruction is known to fall within a range
Overdefined // This instruction has an unknown value
} LatticeValue; // The current lattice position
ConstPoolVal *ConstantVal; // If Constant value, the current value
public:
inline InstVal() : LatticeValue(Undefined), ConstantVal(0) {}
// markOverdefined - Return true if this is a new status to be in...
inline bool markOverdefined() {
if (LatticeValue != Overdefined) {
LatticeValue = Overdefined;
return true;
}
return false;
}
// markConstant - Return true if this is a new status for us...
inline bool markConstant(ConstPoolVal *V) {
if (LatticeValue != Constant) {
LatticeValue = Constant;
ConstantVal = V;
return true;
} else {
assert(ConstantVal->equals(V) && "Marking constant with different value");
}
return false;
}
inline bool isUndefined() const { return LatticeValue == Undefined; }
inline bool isConstant() const { return LatticeValue == Constant; }
inline bool isOverdefined() const { return LatticeValue == Overdefined; }
inline ConstPoolVal *getConstant() const { return ConstantVal; }
};
//===----------------------------------------------------------------------===//
// SCCP Class
//
// This class does all of the work of Sparse Conditional Constant Propogation.
// It's public interface consists of a constructor and a doSCCP() method.
//
class SCCP {
Method *M; // The method that we are working on...
set<BasicBlock*> BBExecutable; // The basic blocks that are executable
map<Value*, InstVal> ValueState; // The state each value is in...
vector<Instruction*> InstWorkList; // The instruction work list
vector<BasicBlock*> BBWorkList; // The BasicBlock work list
//===--------------------------------------------------------------------===//
// The public interface for this class
//
public:
// SCCP Ctor - Save the method to operate on...
inline SCCP(Method *m) : M(m) {}
// doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and
// return true if the method was modified.
bool doSCCP();
//===--------------------------------------------------------------------===//
// The implementation of this class
//
private:
// markValueOverdefined - Make a value be marked as "constant". If the value
// is not already a constant, add it to the instruction work list so that
// the users of the instruction are updated later.
//
inline bool markConstant(Instruction *I, ConstPoolVal *V) {
//cerr << "markConstant: " << V << " = " << I;
if (ValueState[I].markConstant(V)) {
InstWorkList.push_back(I);
return true;
}
return false;
}
// markValueOverdefined - Make a value be marked as "overdefined". If the
// value is not already overdefined, add it to the instruction work list so
// that the users of the instruction are updated later.
//
inline bool markOverdefined(Value *V) {
if (ValueState[V].markOverdefined()) {
if (Instruction *I = V->castInstruction()) {
//cerr << "markOverdefined: " << V;
InstWorkList.push_back(I); // Only instructions go on the work list
}
return true;
}
return false;
}
// getValueState - Return the InstVal object that corresponds to the value.
// This function is neccesary because not all values should start out in the
// underdefined state... MethodArgument's should be overdefined, and constants
// should be marked as constants. If a value is not known to be an
// Instruction object, then use this accessor to get its value from the map.
//
inline InstVal &getValueState(Value *V) {
map<Value*, InstVal>::iterator I = ValueState.find(V);
if (I != ValueState.end()) return I->second; // Common case, in the map
if (ConstPoolVal *CPV = V->castConstant()) { // Constants are constant
ValueState[CPV].markConstant(CPV);
} else if (V->isMethodArgument()) { // MethodArgs are overdefined
ValueState[V].markOverdefined();
}
// All others are underdefined by default...
return ValueState[V];
}
// markExecutable - Mark a basic block as executable, adding it to the BB
// work list if it is not already executable...
//
void markExecutable(BasicBlock *BB) {
if (BBExecutable.count(BB)) return;
//cerr << "Marking BB Executable: " << BB;
BBExecutable.insert(BB); // Basic block is executable!
BBWorkList.push_back(BB); // Add the block to the work list!
}
void OperandChangedState(User *U);
void UpdateInstruction(Instruction *I);
};
//===----------------------------------------------------------------------===//
// SCCP Class Implementation
// doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and
// return true if the method was modified.
//
bool SCCP::doSCCP() {
// Mark the first block of the method as being executable...
markExecutable(M->front());
// Process the work lists until their are empty!
while (!BBWorkList.empty() || !InstWorkList.empty()) {
// Process the instruction work list...
while (!InstWorkList.empty()) {
Instruction *I = InstWorkList.back();
InstWorkList.pop_back();
//cerr << "\nPopped off I-WL: " << I;
// "I" got into the work list because it either made the transition from
// bottom to constant, or to Overdefined.
//
// Update all of the users of this instruction's value...
//
for_each(I->use_begin(), I->use_end(),
bind_obj(this, &SCCP::OperandChangedState));
}
// Process the basic block work list...
while (!BBWorkList.empty()) {
BasicBlock *BB = BBWorkList.back();
BBWorkList.pop_back();
//cerr << "\nPopped off BBWL: " << BB;
// If this block only has a single successor, mark it as executable as
// well... if not, terminate the do loop.
//
if (BB->getTerminator()->getNumSuccessors() == 1)
markExecutable(BB->getTerminator()->getSuccessor(0));
// Loop over all of the instructions and notify them that they are newly
// executable...
for_each(BB->begin(), BB->end(),
bind_obj(this, &SCCP::UpdateInstruction));
}
}
#if 0
for (Method::iterator BBI = M->begin(), BBEnd = M->end(); BBI != BBEnd; ++BBI)
if (!BBExecutable.count(*BBI))
cerr << "BasicBlock Dead:" << *BBI;
#endif
// Iterate over all of the instructions in a method, replacing them with
// constants if we have found them to be of constant values.
//
bool MadeChanges = false;
for (Method::inst_iterator II = M->inst_begin(); II != M->inst_end(); ) {
Instruction *Inst = *II;
InstVal &IV = ValueState[Inst];
if (IV.isConstant()) {
ConstPoolVal *Const = IV.getConstant();
// cerr << "Constant: " << Inst << " is: " << Const;
// Replaces all of the uses of a variable with uses of the constant.
Inst->replaceAllUsesWith(Const);
// Remove the operator from the list of definitions...
Inst->getParent()->getInstList().remove(II.getInstructionIterator());
// The new constant inherits the old name of the operator...
if (Inst->hasName() && !Const->hasName())
Const->setName(Inst->getName());
// Delete the operator now...
delete Inst;
// Incrementing the iterator in an unchecked manner could mess up the
// internals of 'II'. To make sure everything is happy, tell it we might
// have broken it.
II.resyncInstructionIterator();
// Hey, we just changed something!
MadeChanges = true;
} else {
++II;
}
}
// Merge identical constants last: this is important because we may have just
// introduced constants that already exist, and we don't want to pollute later
// stages with extraneous constants.
//
return MadeChanges | DoConstantPoolMerging(M->getConstantPool());
}
// UpdateInstruction - Something changed in this instruction... Either an
// operand made a transition, or the instruction is newly executable. Change
// the value type of I to reflect these changes if appropriate. This method
// makes sure to do the following actions:
//
// 1. If a phi node merges two constants in, and has conflicting value coming
// from different branches, or if the PHI node merges in an overdefined
// value, then the PHI node becomes overdefined.
// 2. If a phi node merges only constants in, and they all agree on value, the
// PHI node becomes a constant value equal to that.
// 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
// 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
// 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
// 6. If a conditional branch has a value that is constant, make the selected
// destination executable
// 7. If a conditional branch has a value that is overdefined, make all
// successors executable.
//
void SCCP::UpdateInstruction(Instruction *I) {
InstVal &IValue = ValueState[I];
if (IValue.isOverdefined())
return; // If already overdefined, we aren't going to effect anything
switch (I->getInstType()) {
//===-----------------------------------------------------------------===//
// Handle PHI nodes...
//
case Instruction::PHINode: {
PHINode *PN = (PHINode*)I;
unsigned NumValues = PN->getNumIncomingValues(), i;
InstVal *OperandIV = 0;
// Look at all of the executable operands of the PHI node. If any of them
// are overdefined, the PHI becomes overdefined as well. If they are all
// constant, and they agree with each other, the PHI becomes the identical
// constant. If they are constant and don't agree, the PHI is overdefined.
// If there are no executable operands, the PHI remains undefined.
//
for (i = 0; i < NumValues; ++i) {
if (BBExecutable.count(PN->getIncomingBlock(i))) {
InstVal &IV = getValueState(PN->getIncomingValue(i));
if (IV.isUndefined()) continue; // Doesn't influence PHI node.
if (IV.isOverdefined()) { // PHI node becomes overdefined!
markOverdefined(PN);
return;
}
if (OperandIV == 0) { // Grab the first value...
OperandIV = &IV;
} else { // Another value is being merged in!
// There is already a reachable operand. If we conflict with it,
// then the PHI node becomes overdefined. If we agree with it, we
// can continue on.
// Check to see if there are two different constants merging...
if (!IV.getConstant()->equals(OperandIV->getConstant())) {
// Yes there is. This means the PHI node is not constant.
// You must be overdefined poor PHI.
//
markOverdefined(I); // The PHI node now becomes overdefined
return; // I'm done analyzing you
}
}
}
}
// If we exited the loop, this means that the PHI node only has constant
// arguments that agree with each other(and OperandIV is a pointer to one
// of their InstVal's) or OperandIV is null because there are no defined
// incoming arguments. If this is the case, the PHI remains undefined.
//
if (OperandIV) {
assert(OperandIV->isConstant() && "Should only be here for constants!");
markConstant(I, OperandIV->getConstant()); // Aquire operand value
}
return;
}
//===-----------------------------------------------------------------===//
// Handle instructions that unconditionally provide overdefined values...
//
case Instruction::Malloc:
case Instruction::Free:
case Instruction::Alloca:
case Instruction::Load:
case Instruction::Store:
// TODO: getfield/putfield?
case Instruction::Call:
markOverdefined(I); // Memory and call's are all overdefined
return;
//===-----------------------------------------------------------------===//
// Handle Terminator instructions...
//
case Instruction::Ret: return; // Method return doesn't affect anything
case Instruction::Br: { // Handle conditional branches...
BranchInst *BI = (BranchInst*)I;
if (BI->isUnconditional())
return; // Unconditional branches are already handled!
InstVal &BCValue = getValueState(BI->getCondition());
if (BCValue.isOverdefined()) {
// Overdefined condition variables mean the branch could go either way.
markExecutable(BI->getSuccessor(0));
markExecutable(BI->getSuccessor(1));
} else if (BCValue.isConstant()) {
// Constant condition variables mean the branch can only go a single way.
ConstPoolBool *CPB = (ConstPoolBool*)BCValue.getConstant();
if (CPB->getValue()) // If the branch condition is TRUE...
markExecutable(BI->getSuccessor(0));
else // Else if the br cond is FALSE...
markExecutable(BI->getSuccessor(1));
}
return;
}
case Instruction::Switch: {
SwitchInst *SI = (SwitchInst*)I;
InstVal &SCValue = getValueState(SI->getCondition());
if (SCValue.isOverdefined()) { // Overdefined condition? All dests are exe
for(unsigned i = 0; BasicBlock *Succ = SI->getSuccessor(i); ++i)
markExecutable(Succ);
} else if (SCValue.isConstant()) {
ConstPoolVal *CPV = SCValue.getConstant();
for (SwitchInst::dest_iterator I = SI->dest_begin(), E = SI->dest_end();
I != E; ++I) {
if (I->first->equals(CPV)) { // Found the right branch...
markExecutable(I->second);
return;
}
}
// Constant value not equal to any of the branches... must execute
// default branch then...
markExecutable(SI->getDefaultDest());
}
return;
}
default: break; // Handle math operators as groups.
} // end switch(I->getInstType())
//===-------------------------------------------------------------------===//
// Handle Unary instructions...
//
if (I->isUnaryOp()) {
Value *V = I->getOperand(0);
InstVal &VState = getValueState(V);
if (VState.isOverdefined()) { // Inherit overdefinedness of operand
markOverdefined(I);
} else if (VState.isConstant()) { // Propogate constant value
ConstPoolVal *Result =
ConstantFoldUnaryInstruction(I->getInstType(), VState.getConstant());
if (Result) {
// This instruction constant folds! The only problem is that the value
// returned is newly allocated. Make sure to stick it into the methods
// constant pool...
M->getConstantPool().insert(Result);
markConstant(I, Result);
} else {
markOverdefined(I); // Don't know how to fold this instruction. :(
}
}
return;
}
//===-----------------------------------------------------------------===//
// Handle Binary instructions...
//
if (I->isBinaryOp()) {
Value *V1 = I->getOperand(0);
Value *V2 = I->getOperand(1);
InstVal &V1State = getValueState(V1);
InstVal &V2State = getValueState(V2);
if (V1State.isOverdefined() || V2State.isOverdefined()) {
markOverdefined(I);
} else if (V1State.isConstant() && V2State.isConstant()) {
ConstPoolVal *Result =
ConstantFoldBinaryInstruction(I->getInstType(), V1State.getConstant(),
V2State.getConstant());
if (Result) {
// This instruction constant folds! The only problem is that the value
// returned is newly allocated. Make sure to stick it into the methods
// constant pool...
M->getConstantPool().insert(Result);
markConstant(I, Result);
} else {
markOverdefined(I); // Don't know how to fold this instruction. :(
}
}
return;
}
// Shouldn't get here... either the switch statement or one of the group
// handlers should have kicked in...
//
cerr << "SCCP: Don't know how to handle: " << I;
markOverdefined(I); // Just in case
}
// OperandChangedState - This method is invoked on all of the users of an
// instruction that was just changed state somehow.... Based on this
// information, we need to update the specified user of this instruction.
//
void SCCP::OperandChangedState(User *U) {
// Only instructions use other variable values!
Instruction *I = U->castInstructionAsserting();
if (!BBExecutable.count(I->getParent())) return; // Inst not executable yet!
UpdateInstruction(I);
}
// DoSparseConditionalConstantProp - Use Sparse Conditional Constant Propogation
// to prove whether a value is constant and whether blocks are used.
//
bool DoSparseConditionalConstantProp(Method *M) {
SCCP S(M);
return S.doSCCP();
}