//
// This pass is designed to be a very quick global transformation that
// eliminates global common subexpressions from a function. It does this by
-// examining the SSA value graph of the function, instead of doing slow, dense,
-// bit-vector computations.
-//
-// This pass works best if it is proceeded with a simple constant propogation
-// pass and an instruction combination pass because this pass does not do any
-// value numbering (in order to be speedy).
-//
-// This pass does not attempt to CSE load instructions, because it does not use
-// pointer analysis to determine when it is safe.
+// using an existing value numbering implementation to identify the common
+// subexpressions, eliminating them when possible.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
-#include "llvm/InstrTypes.h"
#include "llvm/iMemory.h"
+#include "llvm/Type.h"
#include "llvm/Analysis/Dominators.h"
-#include "llvm/Support/InstVisitor.h"
+#include "llvm/Analysis/ValueNumbering.h"
#include "llvm/Support/InstIterator.h"
-#include "llvm/Support/CFG.h"
-#include "Support/StatisticReporter.h"
+#include "Support/Statistic.h"
#include <algorithm>
-using std::set;
-using std::map;
-
-
-static Statistic<> NumInstRemoved("gcse\t\t- Number of instructions removed");
-static Statistic<> NumLoadRemoved("gcse\t\t- Number of loads removed");
namespace {
- class GCSE : public FunctionPass, public InstVisitor<GCSE, bool> {
- set<Instruction*> WorkList;
+ Statistic<> NumInstRemoved("gcse", "Number of instructions removed");
+ Statistic<> NumLoadRemoved("gcse", "Number of loads removed");
+ Statistic<> NumNonInsts ("gcse", "Number of instructions removed due "
+ "to non-instruction values");
+
+ class GCSE : public FunctionPass {
+ std::set<Instruction*> WorkList;
DominatorSet *DomSetInfo;
+#if 0
ImmediateDominators *ImmDominator;
-
- // BBContainsStore - Contains a value that indicates whether a basic block
- // has a store or call instruction in it. This map is demand populated, so
- // not having an entry means that the basic block has not been scanned yet.
- //
- map<BasicBlock*, bool> BBContainsStore;
+#endif
+ ValueNumbering *VN;
public:
virtual bool runOnFunction(Function &F);
- // Visitation methods, these are invoked depending on the type of
- // instruction being checked. They should return true if a common
- // subexpression was folded.
- //
- bool visitUnaryOperator(Instruction &I);
- bool visitBinaryOperator(Instruction &I);
- bool visitGetElementPtrInst(GetElementPtrInst &I);
- bool visitCastInst(CastInst &I){return visitUnaryOperator((Instruction&)I);}
- bool visitShiftInst(ShiftInst &I) {
- return visitBinaryOperator((Instruction&)I);
- }
- bool visitLoadInst(LoadInst &LI);
- bool visitInstruction(Instruction &) { return false; }
-
private:
+ bool EliminateRedundancies(Instruction *I,std::vector<Value*> &EqualValues);
+ Instruction *EliminateCSE(Instruction *I, Instruction *Other);
void ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI);
- void CommonSubExpressionFound(Instruction *I, Instruction *Other);
-
- // TryToRemoveALoad - Try to remove one of L1 or L2. The problem with
- // removing loads is that intervening stores might make otherwise identical
- // load's yield different values. To ensure that this is not the case, we
- // check that there are no intervening stores or calls between the
- // instructions.
- //
- bool TryToRemoveALoad(LoadInst *L1, LoadInst *L2);
-
- // CheckForInvalidatingInst - Return true if BB or any of the predecessors
- // of BB (until DestBB) contain a store (or other invalidating) instruction.
- //
- bool CheckForInvalidatingInst(BasicBlock *BB, BasicBlock *DestBB,
- set<BasicBlock*> &VisitedSet);
// This transformation requires dominator and immediate dominator info
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- AU.preservesCFG();
- AU.addRequired(DominatorSet::ID);
- AU.addRequired(ImmediateDominators::ID);
+ AU.setPreservesCFG();
+ AU.addRequired<DominatorSet>();
+ AU.addRequired<ImmediateDominators>();
+ AU.addRequired<ValueNumbering>();
}
};
- RegisterPass<GCSE> X("gcse", "Global Common Subexpression Elimination");
+ RegisterOpt<GCSE> X("gcse", "Global Common Subexpression Elimination");
}
// createGCSEPass - The public interface to this file...
bool GCSE::runOnFunction(Function &F) {
bool Changed = false;
+ // Get pointers to the analysis results that we will be using...
DomSetInfo = &getAnalysis<DominatorSet>();
+#if 0
ImmDominator = &getAnalysis<ImmediateDominators>();
+#endif
+ VN = &getAnalysis<ValueNumbering>();
// Step #1: Add all instructions in the function to the worklist for
// processing. All of the instructions are considered to be our
Instruction &I = **WorkList.begin(); // Get an instruction from the worklist
WorkList.erase(WorkList.begin());
- // Visit the instruction, dispatching to the correct visit function based on
- // the instruction type. This does the checking.
+ // If this instruction computes a value, try to fold together common
+ // instructions that compute it.
//
- Changed |= visit(I);
+ if (I.getType() != Type::VoidTy) {
+ std::vector<Value*> EqualValues;
+ VN->getEqualNumberNodes(&I, EqualValues);
+
+ if (!EqualValues.empty())
+ Changed |= EliminateRedundancies(&I, EqualValues);
+ }
}
- // Clear out data structure so that next function starts fresh
- BBContainsStore.clear();
-
// When the worklist is empty, return whether or not we changed anything...
return Changed;
}
+bool GCSE::EliminateRedundancies(Instruction *I,
+ std::vector<Value*> &EqualValues) {
+ // If the EqualValues set contains any non-instruction values, then we know
+ // that all of the instructions can be replaced with the non-instruction value
+ // because it is guaranteed to dominate all of the instructions in the
+ // function. We only have to do hard work if all we have are instructions.
+ //
+ for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
+ if (!isa<Instruction>(EqualValues[i])) {
+ // Found a non-instruction. Replace all instructions with the
+ // non-instruction.
+ //
+ Value *Replacement = EqualValues[i];
+
+ // Make sure we get I as well...
+ EqualValues[i] = I;
+
+ // Replace all instructions with the Replacement value.
+ for (i = 0; i != e; ++i)
+ if (Instruction *I = dyn_cast<Instruction>(EqualValues[i])) {
+ // Change all users of I to use Replacement.
+ I->replaceAllUsesWith(Replacement);
+
+ if (isa<LoadInst>(I))
+ ++NumLoadRemoved; // Keep track of loads eliminated
+ ++NumInstRemoved; // Keep track of number of instructions eliminated
+ ++NumNonInsts; // Keep track of number of insts repl with values
+
+ // Erase the instruction from the program.
+ I->getParent()->getInstList().erase(I);
+ }
+
+ return true;
+ }
+
+ // Remove duplicate entries from EqualValues...
+ std::sort(EqualValues.begin(), EqualValues.end());
+ EqualValues.erase(std::unique(EqualValues.begin(), EqualValues.end()),
+ EqualValues.end());
+
+ // From this point on, EqualValues is logically a vector of instructions.
+ //
+ bool Changed = false;
+ EqualValues.push_back(I); // Make sure I is included...
+ while (EqualValues.size() > 1) {
+ // FIXME, this could be done better than simple iteration!
+ Instruction *Test = cast<Instruction>(EqualValues.back());
+ EqualValues.pop_back();
+
+ for (unsigned i = 0, e = EqualValues.size(); i != e; ++i)
+ if (Instruction *Ret = EliminateCSE(Test,
+ cast<Instruction>(EqualValues[i]))) {
+ if (Ret == Test) // Eliminated EqualValues[i]
+ EqualValues[i] = Test; // Make sure that we reprocess I at some point
+ Changed = true;
+ break;
+ }
+ }
+ return Changed;
+}
+
// ReplaceInstWithInst - Destroy the instruction pointed to by SI, making all
// uses of the instruction use First now instead.
Second.getParent()->getInstList().erase(SI);
}
-// CommonSubExpressionFound - The two instruction I & Other have been found to
-// be common subexpressions. This function is responsible for eliminating one
-// of them, and for fixing the worklist to be correct.
+// EliminateCSE - The two instruction I & Other have been found to be common
+// subexpressions. This function is responsible for eliminating one of them,
+// and for fixing the worklist to be correct. The instruction that is preserved
+// is returned from the function if the other is eliminated, otherwise null is
+// returned.
//
-void GCSE::CommonSubExpressionFound(Instruction *I, Instruction *Other) {
+Instruction *GCSE::EliminateCSE(Instruction *I, Instruction *Other) {
assert(I != Other);
WorkList.erase(I);
WorkList.erase(Other); // Other may not actually be on the worklist anymore...
- ++NumInstRemoved; // Keep track of number of instructions eliminated
-
// Handle the easy case, where both instructions are in the same basic block
BasicBlock *BB1 = I->getParent(), *BB2 = Other->getParent();
+ Instruction *Ret = 0;
+
if (BB1 == BB2) {
// Eliminate the second occuring instruction. Add all uses of the second
// instruction to the worklist.
BI = Second;
// Destroy Second, using First instead.
- ReplaceInstWithInst(First, BI);
+ ReplaceInstWithInst(First, BI);
+ Ret = First;
// Otherwise, the two instructions are in different basic blocks. If one
// dominates the other instruction, we can simply use it
//
} else if (DomSetInfo->dominates(BB1, BB2)) { // I dom Other?
ReplaceInstWithInst(I, Other);
+ Ret = I;
} else if (DomSetInfo->dominates(BB2, BB1)) { // Other dom I?
ReplaceInstWithInst(Other, I);
+ Ret = Other;
} else {
+ // This code is disabled because it has several problems:
+ // One, the actual assumption is wrong, as shown by this code:
+ // int "test"(int %X, int %Y) {
+ // %Z = add int %X, %Y
+ // ret int %Z
+ // Unreachable:
+ // %Q = add int %X, %Y
+ // ret int %Q
+ // }
+ //
+ // Here there are no shared dominators. Additionally, this had the habit of
+ // moving computations where they were not always computed. For example, in
+ // a cast like this:
+ // if (c) {
+ // if (d) ...
+ // else ... X+Y ...
+ // } else {
+ // ... X+Y ...
+ // }
+ //
+ // In thiscase, the expression would be hoisted to outside the 'if' stmt,
+ // causing the expression to be evaluated, even for the if (d) path, which
+ // could cause problems, if, for example, it caused a divide by zero. In
+ // general the problem this case is trying to solve is better addressed with
+ // PRE than GCSE.
+ //
+ return 0;
+
+#if 0
// Handle the most general case now. In this case, neither I dom Other nor
// Other dom I. Because we are in SSA form, we are guaranteed that the
// operands of the two instructions both dominate the uses, so we _know_
// Eliminate 'Other' now.
ReplaceInstWithInst(I, Other);
+#endif
}
-}
-
-//===----------------------------------------------------------------------===//
-//
-// Visitation methods, these are invoked depending on the type of instruction
-// being checked. They should return true if a common subexpression was folded.
-//
-//===----------------------------------------------------------------------===//
-
-bool GCSE::visitUnaryOperator(Instruction &I) {
- Value *Op = I.getOperand(0);
- Function *F = I.getParent()->getParent();
-
- for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
- UI != UE; ++UI)
- if (Instruction *Other = dyn_cast<Instruction>(*UI))
- // Check to see if this new binary operator is not I, but same operand...
- if (Other != &I && Other->getOpcode() == I.getOpcode() &&
- Other->getOperand(0) == Op && // Is the operand the same?
- // Is it embeded in the same function? (This could be false if LHS
- // is a constant or global!)
- Other->getParent()->getParent() == F &&
-
- // Check that the types are the same, since this code handles casts...
- Other->getType() == I.getType()) {
-
- // These instructions are identical. Handle the situation.
- CommonSubExpressionFound(&I, Other);
- return true; // One instruction eliminated!
- }
-
- return false;
-}
-
-// isIdenticalBinaryInst - Return true if the two binary instructions are
-// identical.
-//
-static inline bool isIdenticalBinaryInst(const Instruction &I1,
- const Instruction *I2) {
- // Is it embeded in the same function? (This could be false if LHS
- // is a constant or global!)
- if (I1.getOpcode() != I2->getOpcode() ||
- I1.getParent()->getParent() != I2->getParent()->getParent())
- return false;
-
- // They are identical if both operands are the same!
- if (I1.getOperand(0) == I2->getOperand(0) &&
- I1.getOperand(1) == I2->getOperand(1))
- return true;
-
- // If the instruction is commutative and associative, the instruction can
- // match if the operands are swapped!
- //
- if ((I1.getOperand(0) == I2->getOperand(1) &&
- I1.getOperand(1) == I2->getOperand(0)) &&
- (I1.getOpcode() == Instruction::Add ||
- I1.getOpcode() == Instruction::Mul ||
- I1.getOpcode() == Instruction::And ||
- I1.getOpcode() == Instruction::Or ||
- I1.getOpcode() == Instruction::Xor))
- return true;
-
- return false;
-}
-
-bool GCSE::visitBinaryOperator(Instruction &I) {
- Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
- Function *F = I.getParent()->getParent();
-
- for (Value::use_iterator UI = LHS->use_begin(), UE = LHS->use_end();
- UI != UE; ++UI)
- if (Instruction *Other = dyn_cast<Instruction>(*UI))
- // Check to see if this new binary operator is not I, but same operand...
- if (Other != &I && isIdenticalBinaryInst(I, Other)) {
- // These instructions are identical. Handle the situation.
- CommonSubExpressionFound(&I, Other);
- return true; // One instruction eliminated!
- }
-
- return false;
-}
-
-// IdenticalComplexInst - Return true if the two instructions are the same, by
-// using a brute force comparison.
-//
-static bool IdenticalComplexInst(const Instruction *I1, const Instruction *I2) {
- assert(I1->getOpcode() == I2->getOpcode());
- // Equal if they are in the same function...
- return I1->getParent()->getParent() == I2->getParent()->getParent() &&
- // And return the same type...
- I1->getType() == I2->getType() &&
- // And have the same number of operands...
- I1->getNumOperands() == I2->getNumOperands() &&
- // And all of the operands are equal.
- std::equal(I1->op_begin(), I1->op_end(), I2->op_begin());
-}
-
-bool GCSE::visitGetElementPtrInst(GetElementPtrInst &I) {
- Value *Op = I.getOperand(0);
- Function *F = I.getParent()->getParent();
-
- for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
- UI != UE; ++UI)
- if (GetElementPtrInst *Other = dyn_cast<GetElementPtrInst>(*UI))
- // Check to see if this new getelementptr is not I, but same operand...
- if (Other != &I && IdenticalComplexInst(&I, Other)) {
- // These instructions are identical. Handle the situation.
- CommonSubExpressionFound(&I, Other);
- return true; // One instruction eliminated!
- }
-
- return false;
-}
-
-bool GCSE::visitLoadInst(LoadInst &LI) {
- Value *Op = LI.getOperand(0);
- Function *F = LI.getParent()->getParent();
-
- for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
- UI != UE; ++UI)
- if (LoadInst *Other = dyn_cast<LoadInst>(*UI))
- // Check to see if this new load is not LI, but has the same operands...
- if (Other != &LI && IdenticalComplexInst(&LI, Other) &&
- TryToRemoveALoad(&LI, Other))
- return true; // An instruction was eliminated!
-
- return false;
-}
-
-static inline bool isInvalidatingInst(const Instruction &I) {
- return I.getOpcode() == Instruction::Store ||
- I.getOpcode() == Instruction::Call ||
- I.getOpcode() == Instruction::Invoke;
-}
-
-// TryToRemoveALoad - Try to remove one of L1 or L2. The problem with removing
-// loads is that intervening stores might make otherwise identical load's yield
-// different values. To ensure that this is not the case, we check that there
-// are no intervening stores or calls between the instructions.
-//
-bool GCSE::TryToRemoveALoad(LoadInst *L1, LoadInst *L2) {
- // Figure out which load dominates the other one. If neither dominates the
- // other we cannot eliminate one...
- //
- if (DomSetInfo->dominates(L2, L1))
- std::swap(L1, L2); // Make L1 dominate L2
- else if (!DomSetInfo->dominates(L1, L2))
- return false; // Neither instruction dominates the other one...
-
- BasicBlock *BB1 = L1->getParent(), *BB2 = L2->getParent();
-
- BasicBlock::iterator L1I = L1;
-
- // L1 now dominates L2. Check to see if the intervening instructions between
- // the two loads include a store or call...
- //
- if (BB1 == BB2) { // In same basic block?
- // In this degenerate case, no checking of global basic blocks has to occur
- // just check the instructions BETWEEN L1 & L2...
- //
- for (++L1I; &*L1I != L2; ++L1I)
- if (isInvalidatingInst(*L1I))
- return false; // Cannot eliminate load
-
- ++NumLoadRemoved;
- CommonSubExpressionFound(L1, L2);
- return true;
- } else {
- // Make sure that there are no store instructions between L1 and the end of
- // it's basic block...
- //
- for (++L1I; L1I != BB1->end(); ++L1I)
- if (isInvalidatingInst(*L1I)) {
- BBContainsStore[BB1] = true;
- return false; // Cannot eliminate load
- }
-
- // Make sure that there are no store instructions between the start of BB2
- // and the second load instruction...
- //
- for (BasicBlock::iterator II = BB2->begin(); &*II != L2; ++II)
- if (isInvalidatingInst(*II)) {
- BBContainsStore[BB2] = true;
- return false; // Cannot eliminate load
- }
-
- // Do a depth first traversal of the inverse CFG starting at L2's block,
- // looking for L1's block. The inverse CFG is made up of the predecessor
- // nodes of a block... so all of the edges in the graph are "backward".
- //
- set<BasicBlock*> VisitedSet;
- for (pred_iterator PI = pred_begin(BB2), PE = pred_end(BB2); PI != PE; ++PI)
- if (CheckForInvalidatingInst(*PI, BB1, VisitedSet))
- return false;
-
- ++NumLoadRemoved;
- CommonSubExpressionFound(L1, L2);
- return true;
- }
- return false;
-}
-
-// CheckForInvalidatingInst - Return true if BB or any of the predecessors of BB
-// (until DestBB) contain a store (or other invalidating) instruction.
-//
-bool GCSE::CheckForInvalidatingInst(BasicBlock *BB, BasicBlock *DestBB,
- set<BasicBlock*> &VisitedSet) {
- // Found the termination point!
- if (BB == DestBB || VisitedSet.count(BB)) return false;
- // Avoid infinite recursion!
- VisitedSet.insert(BB);
-
- // Have we already checked this block?
- map<BasicBlock*, bool>::iterator MI = BBContainsStore.find(BB);
-
- if (MI != BBContainsStore.end()) {
- // If this block is known to contain a store, exit the recursion early...
- if (MI->second) return true;
- // Otherwise continue checking predecessors...
- } else {
- // We don't know if this basic block contains an invalidating instruction.
- // Check now:
- bool HasStore = std::find_if(BB->begin(), BB->end(),
- isInvalidatingInst) != BB->end();
- if ((BBContainsStore[BB] = HasStore)) // Update map
- return true; // Exit recursion early...
- }
+ if (isa<LoadInst>(Ret))
+ ++NumLoadRemoved; // Keep track of loads eliminated
+ ++NumInstRemoved; // Keep track of number of instructions eliminated
- // Check all of our predecessor blocks...
- for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
- if (CheckForInvalidatingInst(*PI, DestBB, VisitedSet))
- return true;
+ // Add all users of Ret to the worklist...
+ for (Value::use_iterator I = Ret->use_begin(), E = Ret->use_end(); I != E;++I)
+ if (Instruction *Inst = dyn_cast<Instruction>(*I))
+ WorkList.insert(Inst);
- // None of our predecessor blocks contain a store, and we don't either!
- return false;
+ return Ret;
}
projects / oota-llvm.git / blobdiff