// * 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
+// * Proves conditional branches constant, and unconditionalizes them
// * Folds multiple identical constants in the constant pool together
//
// Notice that:
//
//===----------------------------------------------------------------------===//
-#include "llvm/Transforms/Scalar/ConstantProp.h"
+#include "llvm/Transforms/Scalar.h"
#include "llvm/ConstantHandling.h"
#include "llvm/Function.h"
+#include "llvm/BasicBlock.h"
#include "llvm/iPHINode.h"
#include "llvm/iMemory.h"
#include "llvm/iTerminators.h"
#include <iostream>
using std::cerr;
+#if 0 // Enable this to get SCCP debug output
+#define DEBUG_SCCP(X) X
+#else
+#define DEBUG_SCCP(X)
+#endif
+
// InstVal class - This class represents the different lattice values that an
// instruction may occupy. It is a simple class with value semantics.
//
bool runOnFunction(Function *F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- // FIXME: SCCP does not preserve the CFG because it folds terminators!
- //AU.preservesCFG();
+ AU.preservesCFG();
}
// the users of the instruction are updated later.
//
inline bool markConstant(Instruction *I, Constant *V) {
- //cerr << "markConstant: " << V << " = " << I;
+ DEBUG_SCCP(cerr << "markConstant: " << V << " = " << I);
+
if (ValueState[I].markConstant(V)) {
InstWorkList.push_back(I);
return true;
inline bool markOverdefined(Value *V) {
if (ValueState[V].markOverdefined()) {
if (Instruction *I = dyn_cast<Instruction>(V)) {
- //cerr << "markOverdefined: " << V;
+ DEBUG_SCCP(cerr << "markOverdefined: " << V);
InstWorkList.push_back(I); // Only instructions go on the work list
}
return true;
//
void markExecutable(BasicBlock *BB) {
if (BBExecutable.count(BB)) return;
- //cerr << "Marking BB Executable: " << BB;
+ DEBUG_SCCP(cerr << "Marking BB Executable: " << BB);
BBExecutable.insert(BB); // Basic block is executable!
BBWorkList.push_back(BB); // Add the block to the work list!
}
// Terminators
void visitReturnInst(ReturnInst *I) { /*does not have an effect*/ }
- void visitBranchInst(BranchInst *I);
- void visitSwitchInst(SwitchInst *I);
+ void visitTerminatorInst(TerminatorInst *TI);
void visitUnaryOperator(Instruction *I);
void visitCastInst(CastInst *I) { visitUnaryOperator(I); }
void visitShiftInst(ShiftInst *I) { visitBinaryOperator(I); }
// Instructions that cannot be folded away...
+ void visitStoreInst (Instruction *I) { /*returns void*/ }
void visitMemAccessInst (Instruction *I) { markOverdefined(I); }
void visitCallInst (Instruction *I) { markOverdefined(I); }
void visitInvokeInst (Instruction *I) { markOverdefined(I); }
void visitAllocationInst(Instruction *I) { markOverdefined(I); }
- void visitFreeInst (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...
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 *I, 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);
+ void OperandChangedState(User *U) {
+ // Only instructions use other variable values!
+ Instruction *I = cast<Instruction>(U);
+ if (!BBExecutable.count(I->getParent())) return;// Inst not executable yet!
+ visit(I);
+ }
};
} // end anonymous namespace
Instruction *I = InstWorkList.back();
InstWorkList.pop_back();
- //cerr << "\nPopped off I-WL: " << I;
+ DEBUG_SCCP(cerr << "\nPopped off I-WL: " << I);
// "I" got into the work list because it either made the transition from
BasicBlock *BB = BBWorkList.back();
BBWorkList.pop_back();
- //cerr << "\nPopped off BBWL: " << BB;
+ DEBUG_SCCP(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.
InstVal &IV = ValueState[Inst];
if (IV.isConstant()) {
Constant *Const = IV.getConstant();
- // cerr << "Constant: " << Inst << " is: " << Const;
+ DEBUG_SCCP(cerr << "Constant: " << Inst << " is: " << Const);
// Replaces all of the uses of a variable with uses of the constant.
Inst->replaceAllUsesWith(Const);
// Hey, we just changed something!
MadeChanges = true;
-
- // Do NOT advance the iterator, skipping the next instruction...
- continue;
-
- } else if (TerminatorInst *TI = dyn_cast<TerminatorInst>(Inst)) {
- MadeChanges |= ConstantFoldTerminator(BB, BI, TI);
+ } else {
+ ++BI;
}
-
- ++BI;
}
}
- // Reset state so that the next invokation will have empty data structures
+ // Reset state so that the next invocation will have empty data structures
BBExecutable.clear();
ValueState.clear();
- // 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;
}
+// 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) {
+ assert(Succs.size() == TI->getNumSuccessors() && "Succs vector wrong size!");
+ 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 {
+ 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(FT->getNumSuccessors());
+ 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
// If there are no executable operands, the PHI remains undefined.
//
for (i = 0; i < NumValues; ++i) {
- if (BBExecutable.count(PN->getIncomingBlock(i))) {
+ if (isEdgeFeasible(PN->getIncomingBlock(i), PN->getParent())) {
InstVal &IV = getValueState(PN->getIncomingValue(i));
if (IV.isUndefined()) continue; // Doesn't influence PHI node.
if (IV.isOverdefined()) { // PHI node becomes overdefined!
}
}
-void SCCP::visitBranchInst(BranchInst *BI) {
- 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.
- if (BCValue.getConstant() == ConstantBool::True)
- markExecutable(BI->getSuccessor(0));
- else
- markExecutable(BI->getSuccessor(1));
- }
-}
-
-void SCCP::visitSwitchInst(SwitchInst *SI) {
- InstVal &SCValue = getValueState(SI->getCondition());
- if (SCValue.isOverdefined()) { // Overdefined condition? All dests are exe
- for(unsigned i = 0, E = SI->getNumSuccessors(); i != E; ++i)
- markExecutable(SI->getSuccessor(i));
- } 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...
- markExecutable(SI->getSuccessor(i));
- return;
- }
- }
+void SCCP::visitTerminatorInst(TerminatorInst *TI) {
+ std::vector<bool> SuccFeasible(TI->getNumSuccessors());
+ getFeasibleSuccessors(TI, SuccFeasible);
- // Constant value not equal to any of the branches... must execute
- // default branch then...
- markExecutable(SI->getDefaultDest());
- }
+ // Mark all feasible successors executable...
+ for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
+ if (SuccFeasible[i])
+ markExecutable(TI->getSuccessor(i));
}
void SCCP::visitUnaryOperator(Instruction *I) {
if (V1State.isOverdefined() || V2State.isOverdefined()) {
markOverdefined(I);
} else if (V1State.isConstant() && V2State.isConstant()) {
- Constant *Result = ConstantFoldBinaryInstruction(I->getOpcode(),
- V1State.getConstant(),
- V2State.getConstant());
+ 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. :(
}
}
-
-// 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 = cast<Instruction>(U);
- if (!BBExecutable.count(I->getParent())) return; // Inst not executable yet!
-
- visit(I);
-}
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