mirror of
https://github.com/c64scene-ar/llvm-6502.git
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@7492 91177308-0d34-0410-b5e6-96231b3b80d8
536 lines
20 KiB
C++
536 lines
20 KiB
C++
//===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
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//
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// This file implements sparse conditional constant propagation and merging:
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//
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// Specifically, this:
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// * Assumes values are constant unless proven otherwise
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// * Assumes BasicBlocks are dead unless proven otherwise
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// * Proves values to be constant, and replaces them with constants
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// * Proves conditional branches to be unconditional
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//
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// Notice that:
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// * This pass has a habit of making definitions be dead. It is a good idea
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// to to run a DCE pass sometime after running this pass.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/ConstantHandling.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/InstVisitor.h"
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#include "Support/Debug.h"
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#include "Support/Statistic.h"
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#include "Support/STLExtras.h"
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#include <algorithm>
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#include <set>
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// InstVal class - This class represents the different lattice values that an
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// instruction may occupy. It is a simple class with value semantics.
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//
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namespace {
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Statistic<> NumInstRemoved("sccp", "Number of instructions removed");
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class InstVal {
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enum {
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undefined, // This instruction has no known value
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constant, // This instruction has a constant value
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overdefined // This instruction has an unknown value
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} LatticeValue; // The current lattice position
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Constant *ConstantVal; // If Constant value, the current value
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public:
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inline InstVal() : LatticeValue(undefined), ConstantVal(0) {}
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// markOverdefined - Return true if this is a new status to be in...
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inline bool markOverdefined() {
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if (LatticeValue != overdefined) {
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LatticeValue = overdefined;
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return true;
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}
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return false;
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}
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// markConstant - Return true if this is a new status for us...
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inline bool markConstant(Constant *V) {
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if (LatticeValue != constant) {
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LatticeValue = constant;
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ConstantVal = V;
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return true;
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} else {
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assert(ConstantVal == V && "Marking constant with different value");
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}
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return false;
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}
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inline bool isUndefined() const { return LatticeValue == undefined; }
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inline bool isConstant() const { return LatticeValue == constant; }
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inline bool isOverdefined() const { return LatticeValue == overdefined; }
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inline Constant *getConstant() const { return ConstantVal; }
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};
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} // end anonymous namespace
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//===----------------------------------------------------------------------===//
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// SCCP Class
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//
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// This class does all of the work of Sparse Conditional Constant Propagation.
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//
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namespace {
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class SCCP : public FunctionPass, public InstVisitor<SCCP> {
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std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable
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std::map<Value*, InstVal> ValueState; // The state each value is in...
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std::vector<Instruction*> InstWorkList;// The instruction work list
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std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list
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public:
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// runOnFunction - Run the Sparse Conditional Constant Propagation algorithm,
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// and return true if the function was modified.
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//
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bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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}
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//===--------------------------------------------------------------------===//
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// The implementation of this class
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//
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private:
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friend class InstVisitor<SCCP>; // Allow callbacks from visitor
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// markValueOverdefined - Make a value be marked as "constant". If the value
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// is not already a constant, add it to the instruction work list so that
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// the users of the instruction are updated later.
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//
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inline bool markConstant(Instruction *I, Constant *V) {
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if (ValueState[I].markConstant(V)) {
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DEBUG(std::cerr << "markConstant: " << V << " = " << I);
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InstWorkList.push_back(I);
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return true;
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}
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return false;
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}
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// markValueOverdefined - Make a value be marked as "overdefined". If the
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// value is not already overdefined, add it to the instruction work list so
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// that the users of the instruction are updated later.
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//
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inline bool markOverdefined(Value *V) {
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if (ValueState[V].markOverdefined()) {
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if (Instruction *I = dyn_cast<Instruction>(V)) {
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DEBUG(std::cerr << "markOverdefined: " << V);
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InstWorkList.push_back(I); // Only instructions go on the work list
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}
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return true;
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}
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return false;
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}
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// getValueState - Return the InstVal object that corresponds to the value.
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// This function is neccesary because not all values should start out in the
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// underdefined state... Argument's should be overdefined, and
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// constants should be marked as constants. If a value is not known to be an
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// Instruction object, then use this accessor to get its value from the map.
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//
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inline InstVal &getValueState(Value *V) {
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std::map<Value*, InstVal>::iterator I = ValueState.find(V);
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if (I != ValueState.end()) return I->second; // Common case, in the map
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if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant
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ValueState[CPV].markConstant(CPV);
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} else if (isa<Argument>(V)) { // Arguments are overdefined
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ValueState[V].markOverdefined();
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} else if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
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// The address of a global is a constant...
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ValueState[V].markConstant(ConstantPointerRef::get(GV));
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}
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// All others are underdefined by default...
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return ValueState[V];
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}
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// markExecutable - Mark a basic block as executable, adding it to the BB
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// work list if it is not already executable...
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//
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void markExecutable(BasicBlock *BB) {
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if (BBExecutable.count(BB)) {
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// BB is already executable, but we may have just made an edge feasible
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// that wasn't before. Add the PHI nodes to the work list so that they
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// can be rechecked.
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for (BasicBlock::iterator I = BB->begin();
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PHINode *PN = dyn_cast<PHINode>(I); ++I)
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visitPHINode(*PN);
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} else {
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DEBUG(std::cerr << "Marking BB Executable: " << *BB);
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BBExecutable.insert(BB); // Basic block is executable!
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BBWorkList.push_back(BB); // Add the block to the work list!
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}
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}
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// visit implementations - Something changed in this instruction... Either an
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// operand made a transition, or the instruction is newly executable. Change
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// the value type of I to reflect these changes if appropriate.
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//
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void visitPHINode(PHINode &I);
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// Terminators
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void visitReturnInst(ReturnInst &I) { /*does not have an effect*/ }
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void visitTerminatorInst(TerminatorInst &TI);
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void visitCastInst(CastInst &I);
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void visitBinaryOperator(Instruction &I);
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void visitShiftInst(ShiftInst &I) { visitBinaryOperator(I); }
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// Instructions that cannot be folded away...
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void visitStoreInst (Instruction &I) { /*returns void*/ }
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void visitLoadInst (Instruction &I) { markOverdefined(&I); }
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void visitGetElementPtrInst(GetElementPtrInst &I);
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void visitCallInst (Instruction &I) { markOverdefined(&I); }
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void visitInvokeInst (Instruction &I) { markOverdefined(&I); }
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void visitAllocationInst(Instruction &I) { markOverdefined(&I); }
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void visitVarArgInst (Instruction &I) { markOverdefined(&I); }
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void visitFreeInst (Instruction &I) { /*returns void*/ }
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void visitInstruction(Instruction &I) {
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// If a new instruction is added to LLVM that we don't handle...
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std::cerr << "SCCP: Don't know how to handle: " << I;
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markOverdefined(&I); // Just in case
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}
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// getFeasibleSuccessors - Return a vector of booleans to indicate which
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// successors are reachable from a given terminator instruction.
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//
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void getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs);
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// isEdgeFeasible - Return true if the control flow edge from the 'From' basic
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// block to the 'To' basic block is currently feasible...
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//
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bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
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// OperandChangedState - This method is invoked on all of the users of an
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// instruction that was just changed state somehow.... Based on this
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// information, we need to update the specified user of this instruction.
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//
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void OperandChangedState(User *U) {
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// Only instructions use other variable values!
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Instruction &I = cast<Instruction>(*U);
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if (BBExecutable.count(I.getParent())) // Inst is executable?
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visit(I);
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}
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};
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RegisterOpt<SCCP> X("sccp", "Sparse Conditional Constant Propagation");
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} // end anonymous namespace
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// createSCCPPass - This is the public interface to this file...
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//
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Pass *createSCCPPass() {
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return new SCCP();
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}
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//===----------------------------------------------------------------------===//
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// SCCP Class Implementation
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// runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
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// and return true if the function was modified.
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//
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bool SCCP::runOnFunction(Function &F) {
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// Mark the first block of the function as being executable...
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markExecutable(&F.front());
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// Process the work lists until their are empty!
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while (!BBWorkList.empty() || !InstWorkList.empty()) {
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// Process the instruction work list...
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while (!InstWorkList.empty()) {
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Instruction *I = InstWorkList.back();
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InstWorkList.pop_back();
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DEBUG(std::cerr << "\nPopped off I-WL: " << I);
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// "I" got into the work list because it either made the transition from
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// bottom to constant, or to Overdefined.
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//
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// Update all of the users of this instruction's value...
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//
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for_each(I->use_begin(), I->use_end(),
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bind_obj(this, &SCCP::OperandChangedState));
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}
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// Process the basic block work list...
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while (!BBWorkList.empty()) {
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BasicBlock *BB = BBWorkList.back();
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BBWorkList.pop_back();
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DEBUG(std::cerr << "\nPopped off BBWL: " << BB);
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// Notify all instructions in this basic block that they are newly
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// executable.
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visit(BB);
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}
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}
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if (DebugFlag) {
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
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if (!BBExecutable.count(I))
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std::cerr << "BasicBlock Dead:" << *I;
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}
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// Iterate over all of the instructions in a function, replacing them with
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// constants if we have found them to be of constant values.
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//
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bool MadeChanges = false;
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for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB)
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for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
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Instruction &Inst = *BI;
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InstVal &IV = ValueState[&Inst];
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if (IV.isConstant()) {
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Constant *Const = IV.getConstant();
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DEBUG(std::cerr << "Constant: " << Const << " = " << Inst);
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// Replaces all of the uses of a variable with uses of the constant.
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Inst.replaceAllUsesWith(Const);
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// Remove the operator from the list of definitions... and delete it.
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BI = BB->getInstList().erase(BI);
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// Hey, we just changed something!
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MadeChanges = true;
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++NumInstRemoved;
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} else {
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++BI;
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}
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}
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// Reset state so that the next invocation will have empty data structures
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BBExecutable.clear();
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ValueState.clear();
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std::vector<Instruction*>().swap(InstWorkList);
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std::vector<BasicBlock*>().swap(BBWorkList);
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return MadeChanges;
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}
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// getFeasibleSuccessors - Return a vector of booleans to indicate which
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// successors are reachable from a given terminator instruction.
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//
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void SCCP::getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs) {
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Succs.resize(TI.getNumSuccessors());
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if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
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if (BI->isUnconditional()) {
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Succs[0] = true;
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} else {
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InstVal &BCValue = getValueState(BI->getCondition());
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if (BCValue.isOverdefined()) {
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// Overdefined condition variables mean the branch could go either way.
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Succs[0] = Succs[1] = true;
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} else if (BCValue.isConstant()) {
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// Constant condition variables mean the branch can only go a single way
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Succs[BCValue.getConstant() == ConstantBool::False] = true;
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}
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}
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} else if (InvokeInst *II = dyn_cast<InvokeInst>(&TI)) {
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// Invoke instructions successors are always executable.
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Succs[0] = Succs[1] = true;
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} else if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
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InstVal &SCValue = getValueState(SI->getCondition());
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if (SCValue.isOverdefined()) { // Overdefined condition?
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// All destinations are executable!
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Succs.assign(TI.getNumSuccessors(), true);
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} else if (SCValue.isConstant()) {
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Constant *CPV = SCValue.getConstant();
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// Make sure to skip the "default value" which isn't a value
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for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
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if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
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Succs[i] = true;
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return;
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}
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}
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// Constant value not equal to any of the branches... must execute
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// default branch then...
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Succs[0] = true;
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}
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} else {
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std::cerr << "SCCP: Don't know how to handle: " << TI;
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Succs.assign(TI.getNumSuccessors(), true);
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}
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}
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// isEdgeFeasible - Return true if the control flow edge from the 'From' basic
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// block to the 'To' basic block is currently feasible...
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//
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bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
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assert(BBExecutable.count(To) && "Dest should always be alive!");
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// Make sure the source basic block is executable!!
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if (!BBExecutable.count(From)) return false;
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// Check to make sure this edge itself is actually feasible now...
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TerminatorInst *FT = From->getTerminator();
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std::vector<bool> SuccFeasible;
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getFeasibleSuccessors(*FT, SuccFeasible);
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// Check all edges from From to To. If any are feasible, return true.
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for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
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if (FT->getSuccessor(i) == To && SuccFeasible[i])
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return true;
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// Otherwise, none of the edges are actually feasible at this time...
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return false;
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}
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// visit Implementations - Something changed in this instruction... Either an
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// operand made a transition, or the instruction is newly executable. Change
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// the value type of I to reflect these changes if appropriate. This method
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// makes sure to do the following actions:
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//
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// 1. If a phi node merges two constants in, and has conflicting value coming
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// from different branches, or if the PHI node merges in an overdefined
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// value, then the PHI node becomes overdefined.
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// 2. If a phi node merges only constants in, and they all agree on value, the
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// PHI node becomes a constant value equal to that.
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// 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
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// 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
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// 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
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// 6. If a conditional branch has a value that is constant, make the selected
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// destination executable
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// 7. If a conditional branch has a value that is overdefined, make all
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// successors executable.
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//
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void SCCP::visitPHINode(PHINode &PN) {
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if (getValueState(&PN).isOverdefined()) return; // Quick exit
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// Look at all of the executable operands of the PHI node. If any of them
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// are overdefined, the PHI becomes overdefined as well. If they are all
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// constant, and they agree with each other, the PHI becomes the identical
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// constant. If they are constant and don't agree, the PHI is overdefined.
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// If there are no executable operands, the PHI remains undefined.
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//
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Constant *OperandVal = 0;
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for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
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InstVal &IV = getValueState(PN.getIncomingValue(i));
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if (IV.isUndefined()) continue; // Doesn't influence PHI node.
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if (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) {
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if (IV.isOverdefined()) { // PHI node becomes overdefined!
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markOverdefined(&PN);
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return;
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}
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if (OperandVal == 0) { // Grab the first value...
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OperandVal = IV.getConstant();
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} else { // Another value is being merged in!
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// There is already a reachable operand. If we conflict with it,
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// then the PHI node becomes overdefined. If we agree with it, we
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// can continue on.
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// Check to see if there are two different constants merging...
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if (IV.getConstant() != OperandVal) {
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// Yes there is. This means the PHI node is not constant.
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// You must be overdefined poor PHI.
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//
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markOverdefined(&PN); // The PHI node now becomes overdefined
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return; // I'm done analyzing you
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}
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}
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}
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}
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// If we exited the loop, this means that the PHI node only has constant
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// arguments that agree with each other(and OperandVal is the constant) or
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// OperandVal is null because there are no defined incoming arguments. If
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// this is the case, the PHI remains undefined.
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//
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if (OperandVal)
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markConstant(&PN, OperandVal); // Aquire operand value
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}
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void SCCP::visitTerminatorInst(TerminatorInst &TI) {
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std::vector<bool> SuccFeasible;
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getFeasibleSuccessors(TI, SuccFeasible);
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// Mark all feasible successors executable...
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for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
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if (SuccFeasible[i]) {
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BasicBlock *Succ = TI.getSuccessor(i);
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markExecutable(Succ);
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}
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}
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void SCCP::visitCastInst(CastInst &I) {
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Value *V = I.getOperand(0);
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InstVal &VState = getValueState(V);
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if (VState.isOverdefined()) { // Inherit overdefinedness of operand
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markOverdefined(&I);
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} else if (VState.isConstant()) { // Propagate constant value
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Constant *Result =
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ConstantFoldCastInstruction(VState.getConstant(), I.getType());
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if (Result) {
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// This instruction constant folds!
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markConstant(&I, Result);
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} 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));
|
|
}
|