1 //===-- Local.cpp - Functions to perform local transformations ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This family of functions perform various local transformations to the
13 //===----------------------------------------------------------------------===//
15 #include "llvm/Transforms/Utils/Local.h"
16 #include "llvm/Constants.h"
17 #include "llvm/DerivedTypes.h"
18 #include "llvm/Instructions.h"
19 #include "llvm/Intrinsics.h"
20 #include "llvm/Analysis/ConstantFolding.h"
21 #include "llvm/Support/GetElementPtrTypeIterator.h"
22 #include "llvm/Support/MathExtras.h"
27 //===----------------------------------------------------------------------===//
28 // Local constant propagation...
31 /// doConstantPropagation - If an instruction references constants, try to fold
34 bool llvm::doConstantPropagation(BasicBlock::iterator &II) {
35 if (Constant *C = ConstantFoldInstruction(II)) {
36 // Replaces all of the uses of a variable with uses of the constant.
37 II->replaceAllUsesWith(C);
39 // Remove the instruction from the basic block...
40 II = II->getParent()->getInstList().erase(II);
47 /// ConstantFoldInstruction - Attempt to constant fold the specified
48 /// instruction. If successful, the constant result is returned, if not, null
49 /// is returned. Note that this function can only fail when attempting to fold
50 /// instructions like loads and stores, which have no constant expression form.
52 Constant *llvm::ConstantFoldInstruction(Instruction *I) {
53 if (PHINode *PN = dyn_cast<PHINode>(I)) {
54 if (PN->getNumIncomingValues() == 0)
55 return Constant::getNullValue(PN->getType());
57 Constant *Result = dyn_cast<Constant>(PN->getIncomingValue(0));
58 if (Result == 0) return 0;
60 // Handle PHI nodes specially here...
61 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
62 if (PN->getIncomingValue(i) != Result && PN->getIncomingValue(i) != PN)
63 return 0; // Not all the same incoming constants...
65 // If we reach here, all incoming values are the same constant.
67 } else if (CallInst *CI = dyn_cast<CallInst>(I)) {
68 if (Function *F = CI->getCalledFunction())
69 if (canConstantFoldCallTo(F)) {
70 std::vector<Constant*> Args;
71 for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
72 if (Constant *Op = dyn_cast<Constant>(CI->getOperand(i)))
76 return ConstantFoldCall(F, Args);
81 Constant *Op0 = 0, *Op1 = 0;
82 switch (I->getNumOperands()) {
85 Op1 = dyn_cast<Constant>(I->getOperand(1));
86 if (Op1 == 0) return 0; // Not a constant?, can't fold
88 Op0 = dyn_cast<Constant>(I->getOperand(0));
89 if (Op0 == 0) return 0; // Not a constant?, can't fold
94 if (isa<BinaryOperator>(I) || isa<ShiftInst>(I))
95 return ConstantExpr::get(I->getOpcode(), Op0, Op1);
97 switch (I->getOpcode()) {
99 case Instruction::Cast:
100 return ConstantExpr::getCast(Op0, I->getType());
101 case Instruction::Select:
102 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(2)))
103 return ConstantExpr::getSelect(Op0, Op1, Op2);
105 case Instruction::ExtractElement:
106 return ConstantExpr::getExtractElement(Op0, Op1);
107 case Instruction::GetElementPtr:
108 std::vector<Constant*> IdxList;
109 IdxList.reserve(I->getNumOperands()-1);
110 if (Op1) IdxList.push_back(Op1);
111 for (unsigned i = 2, e = I->getNumOperands(); i != e; ++i)
112 if (Constant *C = dyn_cast<Constant>(I->getOperand(i)))
113 IdxList.push_back(C);
115 return 0; // Non-constant operand
116 return ConstantExpr::getGetElementPtr(Op0, IdxList);
120 // ConstantFoldTerminator - If a terminator instruction is predicated on a
121 // constant value, convert it into an unconditional branch to the constant
124 bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
125 TerminatorInst *T = BB->getTerminator();
127 // Branch - See if we are conditional jumping on constant
128 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
129 if (BI->isUnconditional()) return false; // Can't optimize uncond branch
130 BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
131 BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
133 if (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
134 // Are we branching on constant?
135 // YES. Change to unconditional branch...
136 BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
137 BasicBlock *OldDest = Cond->getValue() ? Dest2 : Dest1;
139 //cerr << "Function: " << T->getParent()->getParent()
140 // << "\nRemoving branch from " << T->getParent()
141 // << "\n\nTo: " << OldDest << endl;
143 // Let the basic block know that we are letting go of it. Based on this,
144 // it will adjust it's PHI nodes.
145 assert(BI->getParent() && "Terminator not inserted in block!");
146 OldDest->removePredecessor(BI->getParent());
148 // Set the unconditional destination, and change the insn to be an
149 // unconditional branch.
150 BI->setUnconditionalDest(Destination);
152 } else if (Dest2 == Dest1) { // Conditional branch to same location?
153 // This branch matches something like this:
154 // br bool %cond, label %Dest, label %Dest
155 // and changes it into: br label %Dest
157 // Let the basic block know that we are letting go of one copy of it.
158 assert(BI->getParent() && "Terminator not inserted in block!");
159 Dest1->removePredecessor(BI->getParent());
161 // Change a conditional branch to unconditional.
162 BI->setUnconditionalDest(Dest1);
165 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
166 // If we are switching on a constant, we can convert the switch into a
167 // single branch instruction!
168 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
169 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest
170 BasicBlock *DefaultDest = TheOnlyDest;
171 assert(TheOnlyDest == SI->getDefaultDest() &&
172 "Default destination is not successor #0?");
174 // Figure out which case it goes to...
175 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
176 // Found case matching a constant operand?
177 if (SI->getSuccessorValue(i) == CI) {
178 TheOnlyDest = SI->getSuccessor(i);
182 // Check to see if this branch is going to the same place as the default
183 // dest. If so, eliminate it as an explicit compare.
184 if (SI->getSuccessor(i) == DefaultDest) {
185 // Remove this entry...
186 DefaultDest->removePredecessor(SI->getParent());
188 --i; --e; // Don't skip an entry...
192 // Otherwise, check to see if the switch only branches to one destination.
193 // We do this by reseting "TheOnlyDest" to null when we find two non-equal
195 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
198 if (CI && !TheOnlyDest) {
199 // Branching on a constant, but not any of the cases, go to the default
201 TheOnlyDest = SI->getDefaultDest();
204 // If we found a single destination that we can fold the switch into, do so
207 // Insert the new branch..
208 new BranchInst(TheOnlyDest, SI);
209 BasicBlock *BB = SI->getParent();
211 // Remove entries from PHI nodes which we no longer branch to...
212 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
213 // Found case matching a constant operand?
214 BasicBlock *Succ = SI->getSuccessor(i);
215 if (Succ == TheOnlyDest)
216 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest
218 Succ->removePredecessor(BB);
221 // Delete the old switch...
222 BB->getInstList().erase(SI);
224 } else if (SI->getNumSuccessors() == 2) {
225 // Otherwise, we can fold this switch into a conditional branch
226 // instruction if it has only one non-default destination.
227 Value *Cond = new SetCondInst(Instruction::SetEQ, SI->getCondition(),
228 SI->getSuccessorValue(1), "cond", SI);
229 // Insert the new branch...
230 new BranchInst(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
232 // Delete the old switch...
233 SI->getParent()->getInstList().erase(SI);
240 /// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a
241 /// getelementptr constantexpr, return the constant value being addressed by the
242 /// constant expression, or null if something is funny and we can't decide.
243 Constant *llvm::ConstantFoldLoadThroughGEPConstantExpr(Constant *C,
245 if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
246 return 0; // Do not allow stepping over the value!
248 // Loop over all of the operands, tracking down which value we are
250 gep_type_iterator I = gep_type_begin(CE), E = gep_type_end(CE);
251 for (++I; I != E; ++I)
252 if (const StructType *STy = dyn_cast<StructType>(*I)) {
253 ConstantUInt *CU = cast<ConstantUInt>(I.getOperand());
254 assert(CU->getValue() < STy->getNumElements() &&
255 "Struct index out of range!");
256 unsigned El = (unsigned)CU->getValue();
257 if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) {
258 C = CS->getOperand(El);
259 } else if (isa<ConstantAggregateZero>(C)) {
260 C = Constant::getNullValue(STy->getElementType(El));
261 } else if (isa<UndefValue>(C)) {
262 C = UndefValue::get(STy->getElementType(El));
266 } else if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) {
267 const ArrayType *ATy = cast<ArrayType>(*I);
268 if ((uint64_t)CI->getRawValue() >= ATy->getNumElements()) return 0;
269 if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
270 C = CA->getOperand((unsigned)CI->getRawValue());
271 else if (isa<ConstantAggregateZero>(C))
272 C = Constant::getNullValue(ATy->getElementType());
273 else if (isa<UndefValue>(C))
274 C = UndefValue::get(ATy->getElementType());
284 //===----------------------------------------------------------------------===//
285 // Local dead code elimination...
288 bool llvm::isInstructionTriviallyDead(Instruction *I) {
289 if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
291 if (!I->mayWriteToMemory()) return true;
293 if (CallInst *CI = dyn_cast<CallInst>(I))
294 if (Function *F = CI->getCalledFunction())
295 switch (F->getIntrinsicID()) {
297 case Intrinsic::returnaddress:
298 case Intrinsic::frameaddress:
299 case Intrinsic::stacksave:
300 case Intrinsic::isunordered:
301 case Intrinsic::ctpop:
302 case Intrinsic::ctlz:
303 case Intrinsic::cttz:
304 case Intrinsic::sqrt:
305 return true; // These intrinsics have no side effects.
310 // dceInstruction - Inspect the instruction at *BBI and figure out if it's
311 // [trivially] dead. If so, remove the instruction and update the iterator
312 // to point to the instruction that immediately succeeded the original
315 bool llvm::dceInstruction(BasicBlock::iterator &BBI) {
316 // Look for un"used" definitions...
317 if (isInstructionTriviallyDead(BBI)) {
318 BBI = BBI->getParent()->getInstList().erase(BBI); // Bye bye