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llvm-project/llvm/lib/Transforms/Scalar/SCCP.cpp

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//===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
//
// This file implements sparse conditional constant propagation 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 to be unconditional
//
// 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/Transforms/Scalar.h"
#include "llvm/ConstantHandling.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Support/InstVisitor.h"
#include "Support/STLExtras.h"
#include "Support/Statistic.h"
#include <algorithm>
#include <set>
// InstVal class - This class represents the different lattice values that an
// instruction may occupy. It is a simple class with value semantics.
//
namespace {
Statistic<> NumInstRemoved("sccp", "Number of instructions removed");
class InstVal {
enum {
undefined, // This instruction has no known value
constant, // This instruction has a constant value
overdefined // This instruction has an unknown value
} LatticeValue; // The current lattice position
Constant *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(Constant *V) {
if (LatticeValue != constant) {
LatticeValue = constant;
ConstantVal = V;
return true;
} else {
assert(ConstantVal == 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 Constant *getConstant() const { return ConstantVal; }
};
} // end anonymous namespace
//===----------------------------------------------------------------------===//
// SCCP Class
//
// This class does all of the work of Sparse Conditional Constant Propagation.
//
namespace {
class SCCP : public FunctionPass, public InstVisitor<SCCP> {
std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable
std::map<Value*, InstVal> ValueState; // The state each value is in...
std::vector<Instruction*> InstWorkList;// The instruction work list
std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list
public:
// runOnFunction - Run the Sparse Conditional Constant Propagation algorithm,
// and return true if the function was modified.
//
bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
}
//===--------------------------------------------------------------------===//
// The implementation of this class
//
private:
friend class InstVisitor<SCCP>; // Allow callbacks from visitor
// 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, Constant *V) {
if (ValueState[I].markConstant(V)) {
DEBUG(std::cerr << "markConstant: " << V << " = " << I);
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 = dyn_cast<Instruction>(V)) {
DEBUG(std::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... Argument'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) {
std::map<Value*, InstVal>::iterator I = ValueState.find(V);
if (I != ValueState.end()) return I->second; // Common case, in the map
if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant
ValueState[CPV].markConstant(CPV);
} else if (isa<Argument>(V)) { // Arguments are overdefined
ValueState[V].markOverdefined();
} else if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
// The address of a global is a constant...
ValueState[V].markConstant(ConstantPointerRef::get(GV));
}
// 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)) {
// BB is already executable, but we may have just made an edge feasible
// that wasn't before. Add the PHI nodes to the work list so that they
// can be rechecked.
for (BasicBlock::iterator I = BB->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I)
visitPHINode(*PN);
} else {
DEBUG(std::cerr << "Marking BB Executable: " << *BB);
BBExecutable.insert(BB); // Basic block is executable!
BBWorkList.push_back(BB); // Add the block to the work list!
}
}
// visit implementations - 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.
//
void visitPHINode(PHINode &I);
// Terminators
void visitReturnInst(ReturnInst &I) { /*does not have an effect*/ }
void visitTerminatorInst(TerminatorInst &TI);
void visitCastInst(CastInst &I);
void visitBinaryOperator(Instruction &I);
void visitShiftInst(ShiftInst &I) { visitBinaryOperator(I); }
// Instructions that cannot be folded away...
void visitStoreInst (Instruction &I) { /*returns void*/ }
void visitLoadInst (Instruction &I) { markOverdefined(&I); }
void visitGetElementPtrInst(GetElementPtrInst &I);
void visitCallInst (Instruction &I) { markOverdefined(&I); }
void visitInvokeInst (Instruction &I) { markOverdefined(&I); }
void visitAllocationInst(Instruction &I) { markOverdefined(&I); }
void visitVarArgInst (Instruction &I) { markOverdefined(&I); }
void visitFreeInst (Instruction &I) { /*returns void*/ }
void visitInstruction(Instruction &I) {
// If a new instruction is added to LLVM that we don't handle...
std::cerr << "SCCP: Don't know how to handle: " << I;
markOverdefined(&I); // Just in case
}
// getFeasibleSuccessors - Return a vector of booleans to indicate which
// successors are reachable from a given terminator instruction.
//
void getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs);
// isEdgeFeasible - Return true if the control flow edge from the 'From' basic
// block to the 'To' basic block is currently feasible...
//
bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
// 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 OperandChangedState(User *U) {
// Only instructions use other variable values!
Instruction &I = cast<Instruction>(*U);
if (BBExecutable.count(I.getParent())) // Inst is executable?
visit(I);
}
};
RegisterOpt<SCCP> X("sccp", "Sparse Conditional Constant Propagation");
} // end anonymous namespace
// createSCCPPass - This is the public interface to this file...
//
Pass *createSCCPPass() {
return new SCCP();
}
//===----------------------------------------------------------------------===//
// SCCP Class Implementation
// runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
// and return true if the function was modified.
//
bool SCCP::runOnFunction(Function &F) {
// Mark the first block of the function as being executable...
markExecutable(&F.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();
DEBUG(std::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();
DEBUG(std::cerr << "\nPopped off BBWL: " << BB);
// Notify all instructions in this basic block that they are newly
// executable.
visit(BB);
}
}
if (DebugFlag) {
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
if (!BBExecutable.count(I))
std::cerr << "BasicBlock Dead:" << *I;
}
// Iterate over all of the instructions in a function, replacing them with
// constants if we have found them to be of constant values.
//
bool MadeChanges = false;
for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB)
for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
Instruction &Inst = *BI;
InstVal &IV = ValueState[&Inst];
if (IV.isConstant()) {
Constant *Const = IV.getConstant();
DEBUG(std::cerr << "Constant: " << Const << " = " << Inst);
// Replaces all of the uses of a variable with uses of the constant.
Inst.replaceAllUsesWith(Const);
// Remove the operator from the list of definitions... and delete it.
BI = BB->getInstList().erase(BI);
// Hey, we just changed something!
MadeChanges = true;
++NumInstRemoved;
} else {
++BI;
}
}
// Reset state so that the next invocation will have empty data structures
BBExecutable.clear();
ValueState.clear();
std::vector<Instruction*>().swap(InstWorkList);
std::vector<BasicBlock*>().swap(BBWorkList);
return MadeChanges;
}
// getFeasibleSuccessors - Return a vector of booleans to indicate which
// successors are reachable from a given terminator instruction.
//
void SCCP::getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs) {
Succs.resize(TI.getNumSuccessors());
if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
if (BI->isUnconditional()) {
Succs[0] = true;
} else {
InstVal &BCValue = getValueState(BI->getCondition());
if (BCValue.isOverdefined()) {
// Overdefined condition variables mean the branch could go either way.
Succs[0] = Succs[1] = true;
} else if (BCValue.isConstant()) {
// Constant condition variables mean the branch can only go a single way
Succs[BCValue.getConstant() == ConstantBool::False] = true;
}
}
} else if (InvokeInst *II = dyn_cast<InvokeInst>(&TI)) {
// Invoke instructions successors are always executable.
Succs[0] = Succs[1] = true;
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
InstVal &SCValue = getValueState(SI->getCondition());
if (SCValue.isOverdefined()) { // Overdefined condition?
// All destinations are executable!
Succs.assign(TI.getNumSuccessors(), true);
} else if (SCValue.isConstant()) {
Constant *CPV = SCValue.getConstant();
// Make sure to skip the "default value" which isn't a value
for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
Succs[i] = true;
return;
}
}
// Constant value not equal to any of the branches... must execute
// default branch then...
Succs[0] = true;
}
} else {
std::cerr << "SCCP: Don't know how to handle: " << TI;
Succs.assign(TI.getNumSuccessors(), true);
}
}
// isEdgeFeasible - Return true if the control flow edge from the 'From' basic
// block to the 'To' basic block is currently feasible...
//
bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
assert(BBExecutable.count(To) && "Dest should always be alive!");
// Make sure the source basic block is executable!!
if (!BBExecutable.count(From)) return false;
// Check to make sure this edge itself is actually feasible now...
TerminatorInst *FT = From->getTerminator();
std::vector<bool> SuccFeasible;
getFeasibleSuccessors(*FT, SuccFeasible);
// Check all edges from From to To. If any are feasible, return true.
for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
if (FT->getSuccessor(i) == To && SuccFeasible[i])
return true;
// Otherwise, none of the edges are actually feasible at this time...
return false;
}
// visit Implementations - 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::visitPHINode(PHINode &PN) {
if (getValueState(&PN).isOverdefined()) return; // Quick exit
// 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.
//
Constant *OperandVal = 0;
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++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 (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) {
if (OperandVal == 0) { // Grab the first value...
OperandVal = IV.getConstant();
} 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() != OperandVal) {
// Yes there is. This means the PHI node is not constant.
// You must be overdefined poor PHI.
//
markOverdefined(&PN); // 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 OperandVal is the constant) or
// OperandVal is null because there are no defined incoming arguments. If
// this is the case, the PHI remains undefined.
//
if (OperandVal)
markConstant(&PN, OperandVal); // Aquire operand value
}
void SCCP::visitTerminatorInst(TerminatorInst &TI) {
std::vector<bool> SuccFeasible;
getFeasibleSuccessors(TI, SuccFeasible);
// Mark all feasible successors executable...
for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
if (SuccFeasible[i]) {
BasicBlock *Succ = TI.getSuccessor(i);
markExecutable(Succ);
}
}
void SCCP::visitCastInst(CastInst &I) {
Value *V = I.getOperand(0);
InstVal &VState = getValueState(V);
if (VState.isOverdefined()) { // Inherit overdefinedness of operand
markOverdefined(&I);
} else if (VState.isConstant()) { // Propagate constant value
Constant *Result =
ConstantFoldCastInstruction(VState.getConstant(), I.getType());
if (Result) {
// This instruction constant folds!
markConstant(&I, Result);
} else {
markOverdefined(&I); // Don't know how to fold this instruction. :(
}
}
}
// Handle BinaryOperators and Shift Instructions...
void SCCP::visitBinaryOperator(Instruction &I) {
InstVal &V1State = getValueState(I.getOperand(0));
InstVal &V2State = getValueState(I.getOperand(1));
if (V1State.isOverdefined() || V2State.isOverdefined()) {
markOverdefined(&I);
} else if (V1State.isConstant() && V2State.isConstant()) {
Constant *Result = 0;
if (isa<BinaryOperator>(I))
Result = ConstantFoldBinaryInstruction(I.getOpcode(),
V1State.getConstant(),
V2State.getConstant());
else if (isa<ShiftInst>(I))
Result = ConstantFoldShiftInstruction(I.getOpcode(),
V1State.getConstant(),
V2State.getConstant());
if (Result)
markConstant(&I, Result); // This instruction constant folds!
else
markOverdefined(&I); // Don't know how to fold this instruction. :(
}
}
// Handle getelementptr instructions... if all operands are constants then we
// can turn this into a getelementptr ConstantExpr.
//
void SCCP::visitGetElementPtrInst(GetElementPtrInst &I) {
std::vector<Constant*> Operands;
Operands.reserve(I.getNumOperands());
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
InstVal &State = getValueState(I.getOperand(i));
if (State.isUndefined())
return; // Operands are not resolved yet...
else if (State.isOverdefined()) {
markOverdefined(&I);
return;
}
assert(State.isConstant() && "Unknown state!");
Operands.push_back(State.getConstant());
}
Constant *Ptr = Operands[0];
Operands.erase(Operands.begin()); // Erase the pointer from idx list...
markConstant(&I, ConstantExpr::getGetElementPtr(Ptr, Operands));
}
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