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/Analysis/ConstantFolding.h"
16 #include "llvm/Transforms/Utils/Local.h"
17 #include "llvm/Constants.h"
18 #include "llvm/DerivedTypes.h"
19 #include "llvm/Instructions.h"
20 #include "llvm/Intrinsics.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::GetElementPtr:
106 std::vector<Constant*> IdxList;
107 IdxList.reserve(I->getNumOperands()-1);
108 if (Op1) IdxList.push_back(Op1);
109 for (unsigned i = 2, e = I->getNumOperands(); i != e; ++i)
110 if (Constant *C = dyn_cast<Constant>(I->getOperand(i)))
111 IdxList.push_back(C);
113 return 0; // Non-constant operand
114 return ConstantExpr::getGetElementPtr(Op0, IdxList);
118 // ConstantFoldTerminator - If a terminator instruction is predicated on a
119 // constant value, convert it into an unconditional branch to the constant
122 bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
123 TerminatorInst *T = BB->getTerminator();
125 // Branch - See if we are conditional jumping on constant
126 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
127 if (BI->isUnconditional()) return false; // Can't optimize uncond branch
128 BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
129 BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
131 if (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
132 // Are we branching on constant?
133 // YES. Change to unconditional branch...
134 BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
135 BasicBlock *OldDest = Cond->getValue() ? Dest2 : Dest1;
137 //cerr << "Function: " << T->getParent()->getParent()
138 // << "\nRemoving branch from " << T->getParent()
139 // << "\n\nTo: " << OldDest << endl;
141 // Let the basic block know that we are letting go of it. Based on this,
142 // it will adjust it's PHI nodes.
143 assert(BI->getParent() && "Terminator not inserted in block!");
144 OldDest->removePredecessor(BI->getParent());
146 // Set the unconditional destination, and change the insn to be an
147 // unconditional branch.
148 BI->setUnconditionalDest(Destination);
150 } else if (Dest2 == Dest1) { // Conditional branch to same location?
151 // This branch matches something like this:
152 // br bool %cond, label %Dest, label %Dest
153 // and changes it into: br label %Dest
155 // Let the basic block know that we are letting go of one copy of it.
156 assert(BI->getParent() && "Terminator not inserted in block!");
157 Dest1->removePredecessor(BI->getParent());
159 // Change a conditional branch to unconditional.
160 BI->setUnconditionalDest(Dest1);
163 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
164 // If we are switching on a constant, we can convert the switch into a
165 // single branch instruction!
166 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
167 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest
168 BasicBlock *DefaultDest = TheOnlyDest;
169 assert(TheOnlyDest == SI->getDefaultDest() &&
170 "Default destination is not successor #0?");
172 // Figure out which case it goes to...
173 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
174 // Found case matching a constant operand?
175 if (SI->getSuccessorValue(i) == CI) {
176 TheOnlyDest = SI->getSuccessor(i);
180 // Check to see if this branch is going to the same place as the default
181 // dest. If so, eliminate it as an explicit compare.
182 if (SI->getSuccessor(i) == DefaultDest) {
183 // Remove this entry...
184 DefaultDest->removePredecessor(SI->getParent());
186 --i; --e; // Don't skip an entry...
190 // Otherwise, check to see if the switch only branches to one destination.
191 // We do this by reseting "TheOnlyDest" to null when we find two non-equal
193 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
196 if (CI && !TheOnlyDest) {
197 // Branching on a constant, but not any of the cases, go to the default
199 TheOnlyDest = SI->getDefaultDest();
202 // If we found a single destination that we can fold the switch into, do so
205 // Insert the new branch..
206 new BranchInst(TheOnlyDest, SI);
207 BasicBlock *BB = SI->getParent();
209 // Remove entries from PHI nodes which we no longer branch to...
210 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
211 // Found case matching a constant operand?
212 BasicBlock *Succ = SI->getSuccessor(i);
213 if (Succ == TheOnlyDest)
214 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest
216 Succ->removePredecessor(BB);
219 // Delete the old switch...
220 BB->getInstList().erase(SI);
222 } else if (SI->getNumSuccessors() == 2) {
223 // Otherwise, we can fold this switch into a conditional branch
224 // instruction if it has only one non-default destination.
225 Value *Cond = new SetCondInst(Instruction::SetEQ, SI->getCondition(),
226 SI->getSuccessorValue(1), "cond", SI);
227 // Insert the new branch...
228 new BranchInst(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
230 // Delete the old switch...
231 SI->getParent()->getInstList().erase(SI);
238 /// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a
239 /// getelementptr constantexpr, return the constant value being addressed by the
240 /// constant expression, or null if something is funny and we can't decide.
241 Constant *llvm::ConstantFoldLoadThroughGEPConstantExpr(Constant *C,
243 if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
244 return 0; // Do not allow stepping over the value!
246 // Loop over all of the operands, tracking down which value we are
248 gep_type_iterator I = gep_type_begin(CE), E = gep_type_end(CE);
249 for (++I; I != E; ++I)
250 if (const StructType *STy = dyn_cast<StructType>(*I)) {
251 ConstantUInt *CU = cast<ConstantUInt>(I.getOperand());
252 assert(CU->getValue() < STy->getNumElements() &&
253 "Struct index out of range!");
254 unsigned El = (unsigned)CU->getValue();
255 if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) {
256 C = CS->getOperand(El);
257 } else if (isa<ConstantAggregateZero>(C)) {
258 C = Constant::getNullValue(STy->getElementType(El));
259 } else if (isa<UndefValue>(C)) {
260 C = UndefValue::get(STy->getElementType(El));
264 } else if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) {
265 const ArrayType *ATy = cast<ArrayType>(*I);
266 if ((uint64_t)CI->getRawValue() >= ATy->getNumElements()) return 0;
267 if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
268 C = CA->getOperand((unsigned)CI->getRawValue());
269 else if (isa<ConstantAggregateZero>(C))
270 C = Constant::getNullValue(ATy->getElementType());
271 else if (isa<UndefValue>(C))
272 C = UndefValue::get(ATy->getElementType());
282 //===----------------------------------------------------------------------===//
283 // Local dead code elimination...
286 bool llvm::isInstructionTriviallyDead(Instruction *I) {
287 if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
289 if (!I->mayWriteToMemory()) return true;
291 if (CallInst *CI = dyn_cast<CallInst>(I))
292 if (Function *F = CI->getCalledFunction())
293 switch (F->getIntrinsicID()) {
295 case Intrinsic::returnaddress:
296 case Intrinsic::frameaddress:
297 case Intrinsic::isunordered:
298 case Intrinsic::ctpop:
299 case Intrinsic::ctlz:
300 case Intrinsic::cttz:
301 case Intrinsic::sqrt:
302 return true; // These intrinsics have no side effects.
307 // dceInstruction - Inspect the instruction at *BBI and figure out if it's
308 // [trivially] dead. If so, remove the instruction and update the iterator
309 // to point to the instruction that immediately succeeded the original
312 bool llvm::dceInstruction(BasicBlock::iterator &BBI) {
313 // Look for un"used" definitions...
314 if (isInstructionTriviallyDead(BBI)) {
315 BBI = BBI->getParent()->getInstList().erase(BBI); // Bye bye