1 //===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass implements an _extremely_ simple interprocedural constant
11 // propagation pass. It could certainly be improved in many different ways,
12 // like using a worklist. This pass makes arguments dead, but does not remove
13 // them. The existing dead argument elimination pass should be run after this
14 // to clean up the mess.
16 //===----------------------------------------------------------------------===//
18 #define DEBUG_TYPE "ipconstprop"
19 #include "llvm/Transforms/IPO.h"
20 #include "llvm/Constants.h"
21 #include "llvm/Instructions.h"
22 #include "llvm/Module.h"
23 #include "llvm/Pass.h"
24 #include "llvm/Analysis/ValueTracking.h"
25 #include "llvm/Support/CallSite.h"
26 #include "llvm/Support/Compiler.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/ADT/SmallVector.h"
31 STATISTIC(NumArgumentsProped, "Number of args turned into constants");
32 STATISTIC(NumReturnValProped, "Number of return values turned into constants");
35 /// IPCP - The interprocedural constant propagation pass
37 struct VISIBILITY_HIDDEN IPCP : public ModulePass {
38 static char ID; // Pass identification, replacement for typeid
39 IPCP() : ModulePass((intptr_t)&ID) {}
41 bool runOnModule(Module &M);
43 bool PropagateConstantsIntoArguments(Function &F);
44 bool PropagateConstantReturn(Function &F);
49 static RegisterPass<IPCP>
50 X("ipconstprop", "Interprocedural constant propagation");
52 ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
54 bool IPCP::runOnModule(Module &M) {
56 bool LocalChange = true;
58 // FIXME: instead of using smart algorithms, we just iterate until we stop
62 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
63 if (!I->isDeclaration()) {
64 // Delete any klingons.
65 I->removeDeadConstantUsers();
66 if (I->hasInternalLinkage())
67 LocalChange |= PropagateConstantsIntoArguments(*I);
68 Changed |= PropagateConstantReturn(*I);
70 Changed |= LocalChange;
75 /// PropagateConstantsIntoArguments - Look at all uses of the specified
76 /// function. If all uses are direct call sites, and all pass a particular
77 /// constant in for an argument, propagate that constant in as the argument.
79 bool IPCP::PropagateConstantsIntoArguments(Function &F) {
80 if (F.arg_empty() || F.use_empty()) return false; // No arguments? Early exit.
82 // For each argument, keep track of its constant value and whether it is a
83 // constant or not. The bool is driven to true when found to be non-constant.
84 SmallVector<std::pair<Constant*, bool>, 16> ArgumentConstants;
85 ArgumentConstants.resize(F.arg_size());
87 unsigned NumNonconstant = 0;
88 for (Value::use_iterator UI = F.use_begin(), E = F.use_end(); UI != E; ++UI) {
89 // Used by a non-instruction, or not the callee of a function, do not
91 if (UI.getOperandNo() != 0 ||
92 (!isa<CallInst>(*UI) && !isa<InvokeInst>(*UI)))
95 CallSite CS = CallSite::get(cast<Instruction>(*UI));
97 // Check out all of the potentially constant arguments. Note that we don't
98 // inspect varargs here.
99 CallSite::arg_iterator AI = CS.arg_begin();
100 Function::arg_iterator Arg = F.arg_begin();
101 for (unsigned i = 0, e = ArgumentConstants.size(); i != e;
104 // If this argument is known non-constant, ignore it.
105 if (ArgumentConstants[i].second)
108 Constant *C = dyn_cast<Constant>(*AI);
109 if (C && ArgumentConstants[i].first == 0) {
110 ArgumentConstants[i].first = C; // First constant seen.
111 } else if (C && ArgumentConstants[i].first == C) {
112 // Still the constant value we think it is.
113 } else if (*AI == &*Arg) {
114 // Ignore recursive calls passing argument down.
116 // Argument became non-constant. If all arguments are non-constant now,
117 // give up on this function.
118 if (++NumNonconstant == ArgumentConstants.size())
120 ArgumentConstants[i].second = true;
125 // If we got to this point, there is a constant argument!
126 assert(NumNonconstant != ArgumentConstants.size());
127 bool MadeChange = false;
128 Function::arg_iterator AI = F.arg_begin();
129 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
130 // Do we have a constant argument?
131 if (ArgumentConstants[i].second || AI->use_empty())
134 Value *V = ArgumentConstants[i].first;
135 if (V == 0) V = UndefValue::get(AI->getType());
136 AI->replaceAllUsesWith(V);
137 ++NumArgumentsProped;
144 // Check to see if this function returns one or more constants. If so, replace
145 // all callers that use those return values with the constant value. This will
146 // leave in the actual return values and instructions, but deadargelim will
149 // Additionally if a function always returns one of its arguments directly,
150 // callers will be updated to use the value they pass in directly instead of
151 // using the return value.
152 bool IPCP::PropagateConstantReturn(Function &F) {
153 if (F.getReturnType() == Type::VoidTy)
154 return false; // No return value.
156 // If this function could be overridden later in the link stage, we can't
157 // propagate information about its results into callers.
158 if (F.hasLinkOnceLinkage() || F.hasWeakLinkage())
161 // Check to see if this function returns a constant.
162 SmallVector<Value *,4> RetVals;
163 const StructType *STy = dyn_cast<StructType>(F.getReturnType());
165 for (unsigned i = 0, e = STy->getNumElements(); i < e; ++i)
166 RetVals.push_back(UndefValue::get(STy->getElementType(i)));
168 RetVals.push_back(UndefValue::get(F.getReturnType()));
170 unsigned NumNonConstant = 0;
171 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
172 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
173 // Return type does not match operand type, this is an old style multiple
175 bool OldReturn = (F.getReturnType() != RI->getOperand(0)->getType());
177 for (unsigned i = 0, e = RetVals.size(); i != e; ++i) {
178 // Already found conflicting return values?
179 Value *RV = RetVals[i];
183 // Find the returned value
185 if (!STy || OldReturn)
186 V = RI->getOperand(i);
188 V = FindInsertedValue(RI->getOperand(0), i);
191 // Ignore undefs, we can change them into anything
192 if (isa<UndefValue>(V))
195 // Try to see if all the rets return the same constant or argument.
196 if (isa<Constant>(V) || isa<Argument>(V)) {
197 if (isa<UndefValue>(RV)) {
198 // No value found yet? Try the current one.
202 // Returning the same value? Good.
207 // Different or no known return value? Don't propagate this return
210 // All values non constant? Stop looking.
211 if (++NumNonConstant == RetVals.size())
216 // If we got here, the function returns at least one constant value. Loop
217 // over all users, replacing any uses of the return value with the returned
219 bool MadeChange = false;
220 for (Value::use_iterator UI = F.use_begin(), E = F.use_end(); UI != E; ++UI) {
221 // Make sure this is an invoke or call and that the use is for the callee.
222 if (!(isa<InvokeInst>(*UI) || isa<CallInst>(*UI)) ||
223 UI.getOperandNo() != 0) {
227 Instruction *Call = cast<Instruction>(*UI);
228 if (Call->use_empty())
234 Value* New = RetVals[0];
235 if (Argument *A = dyn_cast<Argument>(New))
236 // Was an argument returned? Then find the corresponding argument in
237 // the call instruction and use that. Add 1 to the argument number
238 // to skip the first argument (the function itself).
239 New = Call->getOperand(A->getArgNo() + 1);
240 Call->replaceAllUsesWith(New);
244 for (Value::use_iterator I = Call->use_begin(), E = Call->use_end();
246 Instruction *Ins = dyn_cast<Instruction>(*I);
248 // Increment now, so we can remove the use
251 // Not an instruction? Ignore
255 // Find the index of the retval to replace with
257 if (GetResultInst *GR = dyn_cast<GetResultInst>(Ins))
258 index = GR->getIndex();
259 else if (ExtractValueInst *EV = dyn_cast<ExtractValueInst>(Ins))
260 if (EV->hasIndices())
261 index = *EV->idx_begin();
263 // If this use uses a specific return value, and we have a replacement,
266 Value *New = RetVals[index];
268 if (Argument *A = dyn_cast<Argument>(New))
269 // Was an argument returned? Then find the corresponding argument in
270 // the call instruction and use that. Add 1 to the argument number
271 // to skip the first argument (the function itself).
272 New = Call->getOperand(A->getArgNo() + 1);
273 Ins->replaceAllUsesWith(New);
274 Ins->eraseFromParent();
280 if (MadeChange) ++NumReturnValProped;