forked from OSchip/llvm-project
276 lines
11 KiB
C++
276 lines
11 KiB
C++
//===-- GCSE.cpp - SSA based Global Common Subexpr Elimination ------------===//
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//
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// This pass is designed to be a very quick global transformation that
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// eliminates global common subexpressions from a function. It does this by
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// examining the SSA value graph of the function, instead of doing slow, dense,
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// bit-vector computations.
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//
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// This pass works best if it is proceeded with a simple constant propogation
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// pass and an instruction combination pass because this pass does not do any
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// value numbering (in order to be speedy).
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//
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// This pass does not attempt to CSE load instructions, because it does not use
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// pointer analysis to determine when it is safe.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/InstrTypes.h"
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#include "llvm/iMemory.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Support/InstVisitor.h"
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#include "llvm/Support/InstIterator.h"
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#include <algorithm>
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namespace {
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class GCSE : public FunctionPass, public InstVisitor<GCSE, bool> {
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set<Instruction*> WorkList;
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DominatorSet *DomSetInfo;
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ImmediateDominators *ImmDominator;
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public:
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const char *getPassName() const {
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return "Global Common Subexpression Elimination";
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}
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virtual bool runOnFunction(Function *F);
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// Visitation methods, these are invoked depending on the type of
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// instruction being checked. They should return true if a common
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// subexpression was folded.
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//
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bool visitUnaryOperator(Instruction *I);
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bool visitBinaryOperator(Instruction *I);
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bool visitGetElementPtrInst(GetElementPtrInst *I);
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bool visitCastInst(CastInst *I){return visitUnaryOperator((Instruction*)I);}
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bool visitShiftInst(ShiftInst *I) {
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return visitBinaryOperator((Instruction*)I);
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}
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bool visitInstruction(Instruction *) { return false; }
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private:
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void ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI);
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void CommonSubExpressionFound(Instruction *I, Instruction *Other);
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// This transformation requires dominator and immediate dominator info
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.preservesCFG();
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AU.addRequired(DominatorSet::ID);
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AU.addRequired(ImmediateDominators::ID);
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}
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};
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}
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// createGCSEPass - The public interface to this file...
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Pass *createGCSEPass() { return new GCSE(); }
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// GCSE::runOnFunction - This is the main transformation entry point for a
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// function.
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//
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bool GCSE::runOnFunction(Function *F) {
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bool Changed = false;
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DomSetInfo = &getAnalysis<DominatorSet>();
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ImmDominator = &getAnalysis<ImmediateDominators>();
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// Step #1: Add all instructions in the function to the worklist for
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// processing. All of the instructions are considered to be our
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// subexpressions to eliminate if possible.
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//
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WorkList.insert(inst_begin(F), inst_end(F));
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// Step #2: WorkList processing. Iterate through all of the instructions,
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// checking to see if there are any additionally defined subexpressions in the
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// program. If so, eliminate them!
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//
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while (!WorkList.empty()) {
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Instruction *I = *WorkList.begin(); // Get an instruction from the worklist
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WorkList.erase(WorkList.begin());
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// Visit the instruction, dispatching to the correct visit function based on
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// the instruction type. This does the checking.
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//
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Changed |= visit(I);
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}
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// When the worklist is empty, return whether or not we changed anything...
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return Changed;
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}
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// ReplaceInstWithInst - Destroy the instruction pointed to by SI, making all
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// uses of the instruction use First now instead.
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//
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void GCSE::ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI) {
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Instruction *Second = *SI;
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//cerr << "DEL " << (void*)Second << Second;
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// Add the first instruction back to the worklist
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WorkList.insert(First);
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// Add all uses of the second instruction to the worklist
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for (Value::use_iterator UI = Second->use_begin(), UE = Second->use_end();
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UI != UE; ++UI)
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WorkList.insert(cast<Instruction>(*UI));
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// Make all users of 'Second' now use 'First'
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Second->replaceAllUsesWith(First);
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// Erase the second instruction from the program
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delete Second->getParent()->getInstList().remove(SI);
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}
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// CommonSubExpressionFound - The two instruction I & Other have been found to
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// be common subexpressions. This function is responsible for eliminating one
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// of them, and for fixing the worklist to be correct.
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//
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void GCSE::CommonSubExpressionFound(Instruction *I, Instruction *Other) {
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// I has already been removed from the worklist, Other needs to be.
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assert(I != Other && WorkList.count(I) == 0 && "I shouldn't be on worklist!");
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WorkList.erase(Other); // Other may not actually be on the worklist anymore...
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// Handle the easy case, where both instructions are in the same basic block
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BasicBlock *BB1 = I->getParent(), *BB2 = Other->getParent();
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if (BB1 == BB2) {
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// Eliminate the second occuring instruction. Add all uses of the second
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// instruction to the worklist.
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//
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// Scan the basic block looking for the "first" instruction
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BasicBlock::iterator BI = BB1->begin();
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while (*BI != I && *BI != Other) {
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++BI;
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assert(BI != BB1->end() && "Instructions not found in parent BB!");
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}
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// Keep track of which instructions occurred first & second
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Instruction *First = *BI;
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Instruction *Second = I != First ? I : Other; // Get iterator to second inst
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BI = find(BI, BB1->end(), Second);
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assert(BI != BB1->end() && "Second instruction not found in parent block!");
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// Destroy Second, using First instead.
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ReplaceInstWithInst(First, BI);
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// Otherwise, the two instructions are in different basic blocks. If one
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// dominates the other instruction, we can simply use it
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//
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} else if (DomSetInfo->dominates(BB1, BB2)) { // I dom Other?
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BasicBlock::iterator BI = find(BB2->begin(), BB2->end(), Other);
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assert(BI != BB2->end() && "Other not in parent basic block!");
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ReplaceInstWithInst(I, BI);
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} else if (DomSetInfo->dominates(BB2, BB1)) { // Other dom I?
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BasicBlock::iterator BI = find(BB1->begin(), BB1->end(), I);
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assert(BI != BB1->end() && "I not in parent basic block!");
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ReplaceInstWithInst(Other, BI);
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} else {
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// Handle the most general case now. In this case, neither I dom Other nor
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// Other dom I. Because we are in SSA form, we are guaranteed that the
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// operands of the two instructions both dominate the uses, so we _know_
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// that there must exist a block that dominates both instructions (if the
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// operands of the instructions are globals or constants, worst case we
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// would get the entry node of the function). Search for this block now.
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//
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// Search up the immediate dominator chain of BB1 for the shared dominator
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BasicBlock *SharedDom = (*ImmDominator)[BB1];
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while (!DomSetInfo->dominates(SharedDom, BB2))
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SharedDom = (*ImmDominator)[SharedDom];
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// At this point, shared dom must dominate BOTH BB1 and BB2...
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assert(SharedDom && DomSetInfo->dominates(SharedDom, BB1) &&
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DomSetInfo->dominates(SharedDom, BB2) && "Dominators broken!");
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// Rip 'I' out of BB1, and move it to the end of SharedDom.
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BB1->getInstList().remove(I);
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SharedDom->getInstList().insert(SharedDom->end()-1, I);
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// Eliminate 'Other' now.
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BasicBlock::iterator BI = find(BB2->begin(), BB2->end(), Other);
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assert(BI != BB2->end() && "I not in parent basic block!");
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ReplaceInstWithInst(I, BI);
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}
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}
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//===----------------------------------------------------------------------===//
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//
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// Visitation methods, these are invoked depending on the type of instruction
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// being checked. They should return true if a common subexpression was folded.
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//
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//===----------------------------------------------------------------------===//
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bool GCSE::visitUnaryOperator(Instruction *I) {
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Value *Op = I->getOperand(0);
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Function *F = I->getParent()->getParent();
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for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
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UI != UE; ++UI)
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if (Instruction *Other = dyn_cast<Instruction>(*UI))
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// Check to see if this new binary operator is not I, but same operand...
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if (Other != I && Other->getOpcode() == I->getOpcode() &&
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Other->getOperand(0) == Op && // Is the operand the same?
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// Is it embeded in the same function? (This could be false if LHS
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// is a constant or global!)
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Other->getParent()->getParent() == F &&
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// Check that the types are the same, since this code handles casts...
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Other->getType() == I->getType()) {
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// These instructions are identical. Handle the situation.
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CommonSubExpressionFound(I, Other);
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return true; // One instruction eliminated!
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}
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return false;
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}
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bool GCSE::visitBinaryOperator(Instruction *I) {
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Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
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Function *F = I->getParent()->getParent();
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for (Value::use_iterator UI = LHS->use_begin(), UE = LHS->use_end();
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UI != UE; ++UI)
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if (Instruction *Other = dyn_cast<Instruction>(*UI))
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// Check to see if this new binary operator is not I, but same operand...
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if (Other != I && Other->getOpcode() == I->getOpcode() &&
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// Are the LHS and RHS the same?
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Other->getOperand(0) == LHS && Other->getOperand(1) == RHS &&
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// Is it embeded in the same function? (This could be false if LHS
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// is a constant or global!)
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Other->getParent()->getParent() == F) {
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// These instructions are identical. Handle the situation.
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CommonSubExpressionFound(I, Other);
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return true; // One instruction eliminated!
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}
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return false;
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}
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bool GCSE::visitGetElementPtrInst(GetElementPtrInst *I) {
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Value *Op = I->getOperand(0);
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Function *F = I->getParent()->getParent();
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for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
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UI != UE; ++UI)
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if (GetElementPtrInst *Other = dyn_cast<GetElementPtrInst>(*UI))
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// Check to see if this new binary operator is not I, but same operand...
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if (Other != I && Other->getParent()->getParent() == F &&
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Other->getType() == I->getType()) {
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// Check to see that all operators past the 0th are the same...
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unsigned i = 1, e = I->getNumOperands();
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for (; i != e; ++i)
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if (I->getOperand(i) != Other->getOperand(i)) break;
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if (i == e) {
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// These instructions are identical. Handle the situation.
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CommonSubExpressionFound(I, Other);
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return true; // One instruction eliminated!
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}
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}
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return false;
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}
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