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llvm-mirror/lib/Transforms/Scalar/SCCP.cpp
Chris Lattner e87a1360b3 Two minor improvements: 
1. Speedup getValueState by having it not consider Arguments.  It's better
    to just add them before we start SCCP'ing.
 2. SCCP can delete the contents of dead blocks.  No really, it's ok!  This
    reduces the size of the IR for subsequent passes, even though
    simplifycfg would do the same job.  In practice, simplifycfg does not
    run until much later than sccp in gccas

llvm-svn: 17820
2004-11-15 05:45:33 +00:00

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//===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "sccp"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Type.h"
#include "llvm/Support/InstVisitor.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/hash_map"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
#include <set>
using namespace llvm;
// LatticeVal 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 LatticeVal {
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 LatticeVal() : 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 {
assert(isConstant() && "Cannot get the constant of a non-constant!");
return ConstantVal;
}
};
} // end anonymous namespace
//===----------------------------------------------------------------------===//
//
/// SCCPSolver - This class is a general purpose solver for Sparse Conditional
/// Constant Propagation.
///
class SCCPSolver : public InstVisitor<SCCPSolver> {
std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable
hash_map<Value*, LatticeVal> ValueState; // The state each value is in...
// The reason for two worklists is that overdefined is the lowest state
// on the lattice, and moving things to overdefined as fast as possible
// makes SCCP converge much faster.
// By having a separate worklist, we accomplish this because everything
// possibly overdefined will become overdefined at the soonest possible
// point.
std::vector<Instruction*> OverdefinedInstWorkList;// The overdefined
// instruction work list
std::vector<Instruction*> InstWorkList;// The instruction work list
std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list
/// UsersOfOverdefinedPHIs - Keep track of any users of PHI nodes that are not
/// overdefined, despite the fact that the PHI node is overdefined.
std::multimap<PHINode*, Instruction*> UsersOfOverdefinedPHIs;
/// KnownFeasibleEdges - Entries in this set are edges which have already had
/// PHI nodes retriggered.
typedef std::pair<BasicBlock*,BasicBlock*> Edge;
std::set<Edge> KnownFeasibleEdges;
public:
/// MarkBlockExecutable - This method can be used by clients to mark all of
/// the blocks that are known to be intrinsically live in the processed unit.
void MarkBlockExecutable(BasicBlock *BB) {
DEBUG(std::cerr << "Marking Block Executable: " << BB->getName() << "\n");
BBExecutable.insert(BB); // Basic block is executable!
BBWorkList.push_back(BB); // Add the block to the work list!
}
/// Solve - Solve for constants and executable blocks.
///
void Solve();
/// getExecutableBlocks - Once we have solved for constants, return the set of
/// blocks that is known to be executable.
std::set<BasicBlock*> &getExecutableBlocks() {
return BBExecutable;
}
/// getValueMapping - Once we have solved for constants, return the mapping of
/// LLVM values to LatticeVals.
hash_map<Value*, LatticeVal> &getValueMapping() {
return ValueState;
}
private:
// markConstant - 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 void markConstant(LatticeVal &IV, Instruction *I, Constant *C) {
if (IV.markConstant(C)) {
DEBUG(std::cerr << "markConstant: " << *C << ": " << *I);
InstWorkList.push_back(I);
}
}
inline void markConstant(Instruction *I, Constant *C) {
markConstant(ValueState[I], I, C);
}
// markOverdefined - Make a value be marked as "overdefined". If the
// value is not already overdefined, add it to the overdefined instruction
// work list so that the users of the instruction are updated later.
inline void markOverdefined(LatticeVal &IV, Instruction *I) {
if (IV.markOverdefined()) {
DEBUG(std::cerr << "markOverdefined: " << *I);
// Only instructions go on the work list
OverdefinedInstWorkList.push_back(I);
}
}
inline void markOverdefined(Instruction *I) {
markOverdefined(ValueState[I], I);
}
// getValueState - Return the LatticeVal object that corresponds to the value.
// This function is necessary 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 LatticeVal &getValueState(Value *V) {
hash_map<Value*, LatticeVal>::iterator I = ValueState.find(V);
if (I != ValueState.end()) return I->second; // Common case, in the map
if (Constant *CPV = dyn_cast<Constant>(V)) {
if (isa<UndefValue>(V)) {
// Nothing to do, remain undefined.
} else {
ValueState[CPV].markConstant(CPV); // Constants are constant
}
}
// All others are underdefined by default...
return ValueState[V];
}
// markEdgeExecutable - Mark a basic block as executable, adding it to the BB
// work list if it is not already executable...
//
void markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
return; // This edge is already known to be executable!
if (BBExecutable.count(Dest)) {
DEBUG(std::cerr << "Marking Edge Executable: " << Source->getName()
<< " -> " << Dest->getName() << "\n");
// The destination is already executable, but we just made an edge
// feasible that wasn't before. Revisit the PHI nodes in the block
// because they have potentially new operands.
for (BasicBlock::iterator I = Dest->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
visitPHINode(*PN);
}
} else {
MarkBlockExecutable(Dest);
}
}
// 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);
}
private:
friend class InstVisitor<SCCPSolver>;
// 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 visitSelectInst(SelectInst &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 (LoadInst &I);
void visitGetElementPtrInst(GetElementPtrInst &I);
void visitCallInst (CallInst &I);
void visitInvokeInst (TerminatorInst &I) {
if (I.getType() != Type::VoidTy) markOverdefined(&I);
visitTerminatorInst(I);
}
void visitUnwindInst (TerminatorInst &I) { /*returns void*/ }
void visitUnreachableInst(TerminatorInst &I) { /*returns void*/ }
void visitAllocationInst(Instruction &I) { markOverdefined(&I); }
void visitVANextInst (Instruction &I) { markOverdefined(&I); }
void visitVAArgInst (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 SCCPSolver::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 {
LatticeVal &BCValue = getValueState(BI->getCondition());
if (BCValue.isOverdefined() ||
(BCValue.isConstant() && !isa<ConstantBool>(BCValue.getConstant()))) {
// Overdefined condition variables, and branches on unfoldable constant
// conditions, 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)) {
LatticeVal &SCValue = getValueState(SI->getCondition());
if (SCValue.isOverdefined() || // Overdefined condition?
(SCValue.isConstant() && !isa<ConstantInt>(SCValue.getConstant()))) {
// 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 SCCPSolver::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 *TI = From->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (BI->isUnconditional())
return true;
else {
LatticeVal &BCValue = getValueState(BI->getCondition());
if (BCValue.isOverdefined()) {
// Overdefined condition variables mean the branch could go either way.
return true;
} else if (BCValue.isConstant()) {
// Not branching on an evaluatable constant?
if (!isa<ConstantBool>(BCValue.getConstant())) return true;
// Constant condition variables mean the branch can only go a single way
return BI->getSuccessor(BCValue.getConstant() ==
ConstantBool::False) == To;
}
return false;
}
} else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
// Invoke instructions successors are always executable.
return true;
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
LatticeVal &SCValue = getValueState(SI->getCondition());
if (SCValue.isOverdefined()) { // Overdefined condition?
// All destinations are executable!
return true;
} else if (SCValue.isConstant()) {
Constant *CPV = SCValue.getConstant();
if (!isa<ConstantInt>(CPV))
return true; // not a foldable constant?
// 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 taken branch...
return SI->getSuccessor(i) == To;
// Constant value not equal to any of the branches... must execute
// default branch then...
return SI->getDefaultDest() == To;
}
return false;
} else {
std::cerr << "Unknown terminator instruction: " << *TI;
abort();
}
}
// 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 SCCPSolver::visitPHINode(PHINode &PN) {
LatticeVal &PNIV = getValueState(&PN);
if (PNIV.isOverdefined()) {
// There may be instructions using this PHI node that are not overdefined
// themselves. If so, make sure that they know that the PHI node operand
// changed.
std::multimap<PHINode*, Instruction*>::iterator I, E;
tie(I, E) = UsersOfOverdefinedPHIs.equal_range(&PN);
if (I != E) {
std::vector<Instruction*> Users;
Users.reserve(std::distance(I, E));
for (; I != E; ++I) Users.push_back(I->second);
while (!Users.empty()) {
visit(Users.back());
Users.pop_back();
}
}
return; // Quick exit
}
// Super-extra-high-degree PHI nodes are unlikely to ever be marked constant,
// and slow us down a lot. Just mark them overdefined.
if (PN.getNumIncomingValues() > 64) {
markOverdefined(PNIV, &PN);
return;
}
// 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) {
LatticeVal &IV = getValueState(PN.getIncomingValue(i));
if (IV.isUndefined()) continue; // Doesn't influence PHI node.
if (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) {
if (IV.isOverdefined()) { // PHI node becomes overdefined!
markOverdefined(PNIV, &PN);
return;
}
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(PNIV, &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(PNIV, &PN, OperandVal); // Acquire operand value
}
void SCCPSolver::visitTerminatorInst(TerminatorInst &TI) {
std::vector<bool> SuccFeasible;
getFeasibleSuccessors(TI, SuccFeasible);
BasicBlock *BB = TI.getParent();
// Mark all feasible successors executable...
for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
if (SuccFeasible[i])
markEdgeExecutable(BB, TI.getSuccessor(i));
}
void SCCPSolver::visitCastInst(CastInst &I) {
Value *V = I.getOperand(0);
LatticeVal &VState = getValueState(V);
if (VState.isOverdefined()) // Inherit overdefinedness of operand
markOverdefined(&I);
else if (VState.isConstant()) // Propagate constant value
markConstant(&I, ConstantExpr::getCast(VState.getConstant(), I.getType()));
}
void SCCPSolver::visitSelectInst(SelectInst &I) {
LatticeVal &CondValue = getValueState(I.getCondition());
if (CondValue.isOverdefined())
markOverdefined(&I);
else if (CondValue.isConstant()) {
if (CondValue.getConstant() == ConstantBool::True) {
LatticeVal &Val = getValueState(I.getTrueValue());
if (Val.isOverdefined())
markOverdefined(&I);
else if (Val.isConstant())
markConstant(&I, Val.getConstant());
} else if (CondValue.getConstant() == ConstantBool::False) {
LatticeVal &Val = getValueState(I.getFalseValue());
if (Val.isOverdefined())
markOverdefined(&I);
else if (Val.isConstant())
markConstant(&I, Val.getConstant());
} else
markOverdefined(&I);
}
}
// Handle BinaryOperators and Shift Instructions...
void SCCPSolver::visitBinaryOperator(Instruction &I) {
LatticeVal &IV = ValueState[&I];
if (IV.isOverdefined()) return;
LatticeVal &V1State = getValueState(I.getOperand(0));
LatticeVal &V2State = getValueState(I.getOperand(1));
if (V1State.isOverdefined() || V2State.isOverdefined()) {
// If both operands are PHI nodes, it is possible that this instruction has
// a constant value, despite the fact that the PHI node doesn't. Check for
// this condition now.
if (PHINode *PN1 = dyn_cast<PHINode>(I.getOperand(0)))
if (PHINode *PN2 = dyn_cast<PHINode>(I.getOperand(1)))
if (PN1->getParent() == PN2->getParent()) {
// Since the two PHI nodes are in the same basic block, they must have
// entries for the same predecessors. Walk the predecessor list, and
// if all of the incoming values are constants, and the result of
// evaluating this expression with all incoming value pairs is the
// same, then this expression is a constant even though the PHI node
// is not a constant!
LatticeVal Result;
for (unsigned i = 0, e = PN1->getNumIncomingValues(); i != e; ++i) {
LatticeVal &In1 = getValueState(PN1->getIncomingValue(i));
BasicBlock *InBlock = PN1->getIncomingBlock(i);
LatticeVal &In2 =
getValueState(PN2->getIncomingValueForBlock(InBlock));
if (In1.isOverdefined() || In2.isOverdefined()) {
Result.markOverdefined();
break; // Cannot fold this operation over the PHI nodes!
} else if (In1.isConstant() && In2.isConstant()) {
Constant *V = ConstantExpr::get(I.getOpcode(), In1.getConstant(),
In2.getConstant());
if (Result.isUndefined())
Result.markConstant(V);
else if (Result.isConstant() && Result.getConstant() != V) {
Result.markOverdefined();
break;
}
}
}
// If we found a constant value here, then we know the instruction is
// constant despite the fact that the PHI nodes are overdefined.
if (Result.isConstant()) {
markConstant(IV, &I, Result.getConstant());
// Remember that this instruction is virtually using the PHI node
// operands.
UsersOfOverdefinedPHIs.insert(std::make_pair(PN1, &I));
UsersOfOverdefinedPHIs.insert(std::make_pair(PN2, &I));
return;
} else if (Result.isUndefined()) {
return;
}
// Okay, this really is overdefined now. Since we might have
// speculatively thought that this was not overdefined before, and
// added ourselves to the UsersOfOverdefinedPHIs list for the PHIs,
// make sure to clean out any entries that we put there, for
// efficiency.
std::multimap<PHINode*, Instruction*>::iterator It, E;
tie(It, E) = UsersOfOverdefinedPHIs.equal_range(PN1);
while (It != E) {
if (It->second == &I) {
UsersOfOverdefinedPHIs.erase(It++);
} else
++It;
}
tie(It, E) = UsersOfOverdefinedPHIs.equal_range(PN2);
while (It != E) {
if (It->second == &I) {
UsersOfOverdefinedPHIs.erase(It++);
} else
++It;
}
}
markOverdefined(IV, &I);
} else if (V1State.isConstant() && V2State.isConstant()) {
markConstant(IV, &I, ConstantExpr::get(I.getOpcode(), V1State.getConstant(),
V2State.getConstant()));
}
}
// Handle getelementptr instructions... if all operands are constants then we
// can turn this into a getelementptr ConstantExpr.
//
void SCCPSolver::visitGetElementPtrInst(GetElementPtrInst &I) {
LatticeVal &IV = ValueState[&I];
if (IV.isOverdefined()) return;
std::vector<Constant*> Operands;
Operands.reserve(I.getNumOperands());
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
LatticeVal &State = getValueState(I.getOperand(i));
if (State.isUndefined())
return; // Operands are not resolved yet...
else if (State.isOverdefined()) {
markOverdefined(IV, &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(IV, &I, ConstantExpr::getGetElementPtr(Ptr, Operands));
}
/// GetGEPGlobalInitializer - Given a constant and a getelementptr constantexpr,
/// return the constant value being addressed by the constant expression, or
/// null if something is funny.
///
static Constant *GetGEPGlobalInitializer(Constant *C, ConstantExpr *CE) {
if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
return 0; // Do not allow stepping over the value!
// Loop over all of the operands, tracking down which value we are
// addressing...
for (unsigned i = 2, e = CE->getNumOperands(); i != e; ++i)
if (ConstantUInt *CU = dyn_cast<ConstantUInt>(CE->getOperand(i))) {
ConstantStruct *CS = dyn_cast<ConstantStruct>(C);
if (CS == 0) return 0;
if (CU->getValue() >= CS->getNumOperands()) return 0;
C = CS->getOperand(CU->getValue());
} else if (ConstantSInt *CS = dyn_cast<ConstantSInt>(CE->getOperand(i))) {
ConstantArray *CA = dyn_cast<ConstantArray>(C);
if (CA == 0) return 0;
if ((uint64_t)CS->getValue() >= CA->getNumOperands()) return 0;
C = CA->getOperand(CS->getValue());
} else
return 0;
return C;
}
// Handle load instructions. If the operand is a constant pointer to a constant
// global, we can replace the load with the loaded constant value!
void SCCPSolver::visitLoadInst(LoadInst &I) {
LatticeVal &IV = ValueState[&I];
if (IV.isOverdefined()) return;
LatticeVal &PtrVal = getValueState(I.getOperand(0));
if (PtrVal.isUndefined()) return; // The pointer is not resolved yet!
if (PtrVal.isConstant() && !I.isVolatile()) {
Value *Ptr = PtrVal.getConstant();
if (isa<ConstantPointerNull>(Ptr)) {
// load null -> null
markConstant(IV, &I, Constant::getNullValue(I.getType()));
return;
}
// Transform load (constant global) into the value loaded.
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr))
if (GV->isConstant() && !GV->isExternal()) {
markConstant(IV, &I, GV->getInitializer());
return;
}
// Transform load (constantexpr_GEP global, 0, ...) into the value loaded.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
if (CE->getOpcode() == Instruction::GetElementPtr)
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getOperand(0)))
if (GV->isConstant() && !GV->isExternal())
if (Constant *V =
GetGEPGlobalInitializer(GV->getInitializer(), CE)) {
markConstant(IV, &I, V);
return;
}
}
// Otherwise we cannot say for certain what value this load will produce.
// Bail out.
markOverdefined(IV, &I);
}
void SCCPSolver::visitCallInst(CallInst &I) {
LatticeVal &IV = ValueState[&I];
if (IV.isOverdefined()) return;
Function *F = I.getCalledFunction();
if (F == 0 || !canConstantFoldCallTo(F)) {
markOverdefined(IV, &I);
return;
}
std::vector<Constant*> Operands;
Operands.reserve(I.getNumOperands()-1);
for (unsigned i = 1, e = I.getNumOperands(); i != e; ++i) {
LatticeVal &State = getValueState(I.getOperand(i));
if (State.isUndefined())
return; // Operands are not resolved yet...
else if (State.isOverdefined()) {
markOverdefined(IV, &I);
return;
}
assert(State.isConstant() && "Unknown state!");
Operands.push_back(State.getConstant());
}
if (Constant *C = ConstantFoldCall(F, Operands))
markConstant(IV, &I, C);
else
markOverdefined(IV, &I);
}
void SCCPSolver::Solve() {
// Process the work lists until they are empty!
while (!BBWorkList.empty() || !InstWorkList.empty() ||
!OverdefinedInstWorkList.empty()) {
// Process the instruction work list...
while (!OverdefinedInstWorkList.empty()) {
Instruction *I = OverdefinedInstWorkList.back();
OverdefinedInstWorkList.pop_back();
DEBUG(std::cerr << "\nPopped off OI-WL: " << I);
// "I" got into the work list because it either made the transition from
// bottom to constant
//
// Anything on this worklist that is overdefined need not be visited
// since all of its users will have already been marked as overdefined
// Update all of the users of this instruction's value...
//
for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
UI != E; ++UI)
OperandChangedState(*UI);
}
// 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
//
// Anything on this worklist that is overdefined need not be visited
// since all of its users will have already been marked as overdefined.
// Update all of the users of this instruction's value...
//
if (!getValueState(I).isOverdefined())
for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
UI != E; ++UI)
OperandChangedState(*UI);
}
// 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);
}
}
}
namespace {
//===----------------------------------------------------------------------===//
//
/// SCCP Class - This class does all of the work of Sparse Conditional Constant
/// Propagation.
///
class SCCP : public FunctionPass, public InstVisitor<SCCP> {
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();
}
};
RegisterOpt<SCCP> X("sccp", "Sparse Conditional Constant Propagation");
} // end anonymous namespace
// createSCCPPass - This is the public interface to this file...
FunctionPass *llvm::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) {
DEBUG(std::cerr << "SCCP on function '" << F.getName() << "'\n");
SCCPSolver Solver;
// Mark the first block of the function as being executable.
Solver.MarkBlockExecutable(F.begin());
// Mark all arguments to the function as being overdefined.
hash_map<Value*, LatticeVal> &Values = Solver.getValueMapping();
for (Function::aiterator AI = F.abegin(), E = F.aend(); AI != E; ++AI)
Values[AI].markOverdefined();
// Solve for constants.
Solver.Solve();
bool MadeChanges = false;
// If we decided that there are basic blocks that are dead in this function,
// delete their contents now. Note that we cannot actually delete the blocks,
// as we cannot modify the CFG of the function.
//
std::set<BasicBlock*> &ExecutableBBs = Solver.getExecutableBlocks();
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
if (!ExecutableBBs.count(BB)) {
DEBUG(std::cerr << " BasicBlock Dead:" << *BB);
// Delete the instructions backwards, as it has a reduced likelihood of
// having to update as many def-use and use-def chains.
std::vector<Instruction*> Insts;
for (BasicBlock::iterator I = BB->begin(), E = BB->getTerminator();
I != E; ++I)
Insts.push_back(I);
while (!Insts.empty()) {
Instruction *I = Insts.back();
Insts.pop_back();
if (!I->use_empty())
I->replaceAllUsesWith(UndefValue::get(I->getType()));
BB->getInstList().erase(I);
MadeChanges = true;
}
}
// Iterate over all of the instructions in a function, replacing them with
// constants if we have found them to be of constant values.
//
for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB)
for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) {
Instruction *Inst = BI++;
if (Inst->getType() != Type::VoidTy) {
LatticeVal &IV = Values[Inst];
if (IV.isConstant() || IV.isUndefined()) {
Constant *Const;
if (IV.isConstant()) {
Const = IV.getConstant();
DEBUG(std::cerr << " Constant: " << *Const << " = " << *Inst);
} else {
Const = UndefValue::get(Inst->getType());
DEBUG(std::cerr << " Undefined: " << *Inst);
}
// Replaces all of the uses of a variable with uses of the constant.
Inst->replaceAllUsesWith(Const);
// Delete the instruction.
BB->getInstList().erase(Inst);
// Hey, we just changed something!
MadeChanges = true;
++NumInstRemoved;
}
}
}
return MadeChanges;
}
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