lib/Transforms/IPO/IPConstantPropagation.cpp - external/github.com/llvm-mirror/llvm - Git at Google

 //===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This pass implements an _extremely_ simple interprocedural constant
 // propagation pass.  It could certainly be improved in many different ways,
 // like using a worklist.  This pass makes arguments dead, but does not remove
 // them.  The existing dead argument elimination pass should be run after this
 // to clean up the mess.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/Transforms/IPO.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/CallSite.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Module.h"
 #include "llvm/Pass.h"
 using namespace llvm;

 #define DEBUG_TYPE "ipconstprop"

 STATISTIC(NumArgumentsProped, "Number of args turned into constants");
 STATISTIC(NumReturnValProped, "Number of return values turned into constants");

 namespace {
   /// IPCP - The interprocedural constant propagation pass
   ///
   struct IPCP : public ModulePass {
     static char ID; // Pass identification, replacement for typeid
     IPCP() : ModulePass(ID) {
       initializeIPCPPass(*PassRegistry::getPassRegistry());
     }

     bool runOnModule(Module &M) override;
   private:
     bool PropagateConstantsIntoArguments(Function &F);
     bool PropagateConstantReturn(Function &F);
   };
 }

 char IPCP::ID = 0;
 INITIALIZE_PASS(IPCP, "ipconstprop",
                 "Interprocedural constant propagation", false, false)

 ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }

 bool IPCP::runOnModule(Module &M) {
   bool Changed = false;
   bool LocalChange = true;

   // FIXME: instead of using smart algorithms, we just iterate until we stop
   // making changes.
   while (LocalChange) {
     LocalChange = false;
     for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
       if (!I->isDeclaration()) {
         // Delete any klingons.
         I->removeDeadConstantUsers();
         if (I->hasLocalLinkage())
           LocalChange |= PropagateConstantsIntoArguments(*I);
         Changed |= PropagateConstantReturn(*I);
       }
     Changed |= LocalChange;
   }
   return Changed;
 }

 /// PropagateConstantsIntoArguments - Look at all uses of the specified
 /// function.  If all uses are direct call sites, and all pass a particular
 /// constant in for an argument, propagate that constant in as the argument.
 ///
 bool IPCP::PropagateConstantsIntoArguments(Function &F) {
   if (F.arg_empty() || F.use_empty()) return false; // No arguments? Early exit.

   // For each argument, keep track of its constant value and whether it is a
   // constant or not.  The bool is driven to true when found to be non-constant.
   SmallVector<std::pair<Constant*, bool>, 16> ArgumentConstants;
   ArgumentConstants.resize(F.arg_size());

   unsigned NumNonconstant = 0;
   for (Use &U : F.uses()) {
     User *UR = U.getUser();
     // Ignore blockaddress uses.
     if (isa<BlockAddress>(UR)) continue;

     // Used by a non-instruction, or not the callee of a function, do not
     // transform.
     if (!isa<CallInst>(UR) && !isa<InvokeInst>(UR))
       return false;

     CallSite CS(cast<Instruction>(UR));
     if (!CS.isCallee(&U))
       return false;

     // Check out all of the potentially constant arguments.  Note that we don't
     // inspect varargs here.
     CallSite::arg_iterator AI = CS.arg_begin();
     Function::arg_iterator Arg = F.arg_begin();
     for (unsigned i = 0, e = ArgumentConstants.size(); i != e;
          ++i, ++AI, ++Arg) {

       // If this argument is known non-constant, ignore it.
       if (ArgumentConstants[i].second)
         continue;

       Constant *C = dyn_cast<Constant>(*AI);
       if (C && ArgumentConstants[i].first == nullptr) {
         ArgumentConstants[i].first = C;   // First constant seen.
       } else if (C && ArgumentConstants[i].first == C) {
         // Still the constant value we think it is.
       } else if (*AI == &*Arg) {
         // Ignore recursive calls passing argument down.
       } else {
         // Argument became non-constant.  If all arguments are non-constant now,
         // give up on this function.
         if (++NumNonconstant == ArgumentConstants.size())
           return false;
         ArgumentConstants[i].second = true;
       }
     }
   }

   // If we got to this point, there is a constant argument!
   assert(NumNonconstant != ArgumentConstants.size());
   bool MadeChange = false;
   Function::arg_iterator AI = F.arg_begin();
   for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
     // Do we have a constant argument?
     if (ArgumentConstants[i].second || AI->use_empty() ||
         AI->hasInAllocaAttr() || (AI->hasByValAttr() && !F.onlyReadsMemory()))
       continue;

     Value *V = ArgumentConstants[i].first;
     if (!V) V = UndefValue::get(AI->getType());
     AI->replaceAllUsesWith(V);
     ++NumArgumentsProped;
     MadeChange = true;
   }
   return MadeChange;
 }


 // Check to see if this function returns one or more constants. If so, replace
 // all callers that use those return values with the constant value. This will
 // leave in the actual return values and instructions, but deadargelim will
 // clean that up.
 //
 // Additionally if a function always returns one of its arguments directly,
 // callers will be updated to use the value they pass in directly instead of
 // using the return value.
 bool IPCP::PropagateConstantReturn(Function &F) {
   if (F.getReturnType()->isVoidTy())
     return false; // No return value.

   // If this function could be overridden later in the link stage, we can't
   // propagate information about its results into callers.
   if (F.mayBeOverridden())
     return false;

   // Check to see if this function returns a constant.
   SmallVector<Value *,4> RetVals;
   StructType *STy = dyn_cast<StructType>(F.getReturnType());
   if (STy)
     for (unsigned i = 0, e = STy->getNumElements(); i < e; ++i)
       RetVals.push_back(UndefValue::get(STy->getElementType(i)));
   else
     RetVals.push_back(UndefValue::get(F.getReturnType()));

   unsigned NumNonConstant = 0;
   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
     if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
       for (unsigned i = 0, e = RetVals.size(); i != e; ++i) {
         // Already found conflicting return values?
         Value *RV = RetVals[i];
         if (!RV)
           continue;

         // Find the returned value
         Value *V;
         if (!STy)
           V = RI->getOperand(0);
         else
           V = FindInsertedValue(RI->getOperand(0), i);

         if (V) {
           // Ignore undefs, we can change them into anything
           if (isa<UndefValue>(V))
             continue;

           // Try to see if all the rets return the same constant or argument.
           if (isa<Constant>(V) || isa<Argument>(V)) {
             if (isa<UndefValue>(RV)) {
               // No value found yet? Try the current one.
               RetVals[i] = V;
               continue;
             }
             // Returning the same value? Good.
             if (RV == V)
               continue;
           }
         }
         // Different or no known return value? Don't propagate this return
         // value.
         RetVals[i] = nullptr;
         // All values non-constant? Stop looking.
         if (++NumNonConstant == RetVals.size())
           return false;
       }
     }

   // If we got here, the function returns at least one constant value.  Loop
   // over all users, replacing any uses of the return value with the returned
   // constant.
   bool MadeChange = false;
   for (Use &U : F.uses()) {
     CallSite CS(U.getUser());
     Instruction* Call = CS.getInstruction();

     // Not a call instruction or a call instruction that's not calling F
     // directly?
     if (!Call || !CS.isCallee(&U))
       continue;

     // Call result not used?
     if (Call->use_empty())
       continue;

     MadeChange = true;

     if (!STy) {
       Value* New = RetVals[0];
       if (Argument *A = dyn_cast<Argument>(New))
         // Was an argument returned? Then find the corresponding argument in
         // the call instruction and use that.
         New = CS.getArgument(A->getArgNo());
       Call->replaceAllUsesWith(New);
       continue;
     }

     for (auto I = Call->user_begin(), E = Call->user_end(); I != E;) {
       Instruction *Ins = cast<Instruction>(*I);

       // Increment now, so we can remove the use
       ++I;

       // Find the index of the retval to replace with
       int index = -1;
       if (ExtractValueInst *EV = dyn_cast<ExtractValueInst>(Ins))
         if (EV->hasIndices())
           index = *EV->idx_begin();

       // If this use uses a specific return value, and we have a replacement,
       // replace it.
       if (index != -1) {
         Value *New = RetVals[index];
         if (New) {
           if (Argument *A = dyn_cast<Argument>(New))
             // Was an argument returned? Then find the corresponding argument in
             // the call instruction and use that.
             New = CS.getArgument(A->getArgNo());
           Ins->replaceAllUsesWith(New);
           Ins->eraseFromParent();
         }
       }
     }
   }

   if (MadeChange) ++NumReturnValProped;
   return MadeChange;
 }
	//===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This pass implements an _extremely_ simple interprocedural constant
	// propagation pass. It could certainly be improved in many different ways,
	// like using a worklist. This pass makes arguments dead, but does not remove
	// them. The existing dead argument elimination pass should be run after this
	// to clean up the mess.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/Transforms/IPO.h"
	#include "llvm/ADT/SmallVector.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Analysis/ValueTracking.h"
	#include "llvm/IR/CallSite.h"
	#include "llvm/IR/Constants.h"
	#include "llvm/IR/Instructions.h"
	#include "llvm/IR/Module.h"
	#include "llvm/Pass.h"
	using namespace llvm;

	#define DEBUG_TYPE "ipconstprop"

	STATISTIC(NumArgumentsProped, "Number of args turned into constants");
	STATISTIC(NumReturnValProped, "Number of return values turned into constants");

	namespace {
	/// IPCP - The interprocedural constant propagation pass
	///
	struct IPCP : public ModulePass {
	static char ID; // Pass identification, replacement for typeid
	IPCP() : ModulePass(ID) {
	initializeIPCPPass(*PassRegistry::getPassRegistry());
	}

	bool runOnModule(Module &M) override;
	private:
	bool PropagateConstantsIntoArguments(Function &F);
	bool PropagateConstantReturn(Function &F);
	};
	}

	char IPCP::ID = 0;
	INITIALIZE_PASS(IPCP, "ipconstprop",
	"Interprocedural constant propagation", false, false)

	ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }

	bool IPCP::runOnModule(Module &M) {
	bool Changed = false;
	bool LocalChange = true;

	// FIXME: instead of using smart algorithms, we just iterate until we stop
	// making changes.
	while (LocalChange) {
	LocalChange = false;
	for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
	if (!I->isDeclaration()) {
	// Delete any klingons.
	I->removeDeadConstantUsers();
	if (I->hasLocalLinkage())
	LocalChange \|= PropagateConstantsIntoArguments(*I);
	Changed \|= PropagateConstantReturn(*I);
	}
	Changed \|= LocalChange;
	}
	return Changed;
	}

	/// PropagateConstantsIntoArguments - Look at all uses of the specified
	/// function. If all uses are direct call sites, and all pass a particular
	/// constant in for an argument, propagate that constant in as the argument.
	///
	bool IPCP::PropagateConstantsIntoArguments(Function &F) {
	if (F.arg_empty() \|\| F.use_empty()) return false; // No arguments? Early exit.

	// For each argument, keep track of its constant value and whether it is a
	// constant or not. The bool is driven to true when found to be non-constant.
	SmallVector<std::pair<Constant*, bool>, 16> ArgumentConstants;
	ArgumentConstants.resize(F.arg_size());

	unsigned NumNonconstant = 0;
	for (Use &U : F.uses()) {
	User *UR = U.getUser();
	// Ignore blockaddress uses.
	if (isa<BlockAddress>(UR)) continue;

	// Used by a non-instruction, or not the callee of a function, do not
	// transform.
	if (!isa<CallInst>(UR) && !isa<InvokeInst>(UR))
	return false;

	CallSite CS(cast<Instruction>(UR));
	if (!CS.isCallee(&U))
	return false;

	// Check out all of the potentially constant arguments. Note that we don't
	// inspect varargs here.
	CallSite::arg_iterator AI = CS.arg_begin();
	Function::arg_iterator Arg = F.arg_begin();
	for (unsigned i = 0, e = ArgumentConstants.size(); i != e;
	++i, ++AI, ++Arg) {

	// If this argument is known non-constant, ignore it.
	if (ArgumentConstants[i].second)
	continue;

	Constant C = dyn_cast<Constant>(AI);
	if (C && ArgumentConstants[i].first == nullptr) {
	ArgumentConstants[i].first = C; // First constant seen.
	} else if (C && ArgumentConstants[i].first == C) {
	// Still the constant value we think it is.
	} else if (AI == &Arg) {
	// Ignore recursive calls passing argument down.
	} else {
	// Argument became non-constant. If all arguments are non-constant now,
	// give up on this function.
	if (++NumNonconstant == ArgumentConstants.size())
	return false;
	ArgumentConstants[i].second = true;
	}
	}
	}

	// If we got to this point, there is a constant argument!
	assert(NumNonconstant != ArgumentConstants.size());
	bool MadeChange = false;
	Function::arg_iterator AI = F.arg_begin();
	for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
	// Do we have a constant argument?
	if (ArgumentConstants[i].second \|\| AI->use_empty() \|\|
	AI->hasInAllocaAttr() \|\| (AI->hasByValAttr() && !F.onlyReadsMemory()))
	continue;

	Value *V = ArgumentConstants[i].first;
	if (!V) V = UndefValue::get(AI->getType());
	AI->replaceAllUsesWith(V);
	++NumArgumentsProped;
	MadeChange = true;
	}
	return MadeChange;
	}


	// Check to see if this function returns one or more constants. If so, replace
	// all callers that use those return values with the constant value. This will
	// leave in the actual return values and instructions, but deadargelim will
	// clean that up.
	//
	// Additionally if a function always returns one of its arguments directly,
	// callers will be updated to use the value they pass in directly instead of
	// using the return value.
	bool IPCP::PropagateConstantReturn(Function &F) {
	if (F.getReturnType()->isVoidTy())
	return false; // No return value.

	// If this function could be overridden later in the link stage, we can't
	// propagate information about its results into callers.
	if (F.mayBeOverridden())
	return false;

	// Check to see if this function returns a constant.
	SmallVector<Value *,4> RetVals;
	StructType *STy = dyn_cast<StructType>(F.getReturnType());
	if (STy)
	for (unsigned i = 0, e = STy->getNumElements(); i < e; ++i)
	RetVals.push_back(UndefValue::get(STy->getElementType(i)));
	else
	RetVals.push_back(UndefValue::get(F.getReturnType()));

	unsigned NumNonConstant = 0;
	for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
	if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
	for (unsigned i = 0, e = RetVals.size(); i != e; ++i) {
	// Already found conflicting return values?
	Value *RV = RetVals[i];
	if (!RV)
	continue;

	// Find the returned value
	Value *V;
	if (!STy)
	V = RI->getOperand(0);
	else
	V = FindInsertedValue(RI->getOperand(0), i);

	if (V) {
	// Ignore undefs, we can change them into anything
	if (isa<UndefValue>(V))
	continue;

	// Try to see if all the rets return the same constant or argument.
	if (isa<Constant>(V) \|\| isa<Argument>(V)) {
	if (isa<UndefValue>(RV)) {
	// No value found yet? Try the current one.
	RetVals[i] = V;
	continue;
	}
	// Returning the same value? Good.
	if (RV == V)
	continue;
	}
	}
	// Different or no known return value? Don't propagate this return
	// value.
	RetVals[i] = nullptr;
	// All values non-constant? Stop looking.
	if (++NumNonConstant == RetVals.size())
	return false;
	}
	}

	// If we got here, the function returns at least one constant value. Loop
	// over all users, replacing any uses of the return value with the returned
	// constant.
	bool MadeChange = false;
	for (Use &U : F.uses()) {
	CallSite CS(U.getUser());
	Instruction* Call = CS.getInstruction();

	// Not a call instruction or a call instruction that's not calling F
	// directly?
	if (!Call \|\| !CS.isCallee(&U))
	continue;

	// Call result not used?
	if (Call->use_empty())
	continue;

	MadeChange = true;

	if (!STy) {
	Value* New = RetVals[0];
	if (Argument *A = dyn_cast<Argument>(New))
	// Was an argument returned? Then find the corresponding argument in
	// the call instruction and use that.
	New = CS.getArgument(A->getArgNo());
	Call->replaceAllUsesWith(New);
	continue;
	}

	for (auto I = Call->user_begin(), E = Call->user_end(); I != E;) {
	Instruction Ins = cast<Instruction>(I);

	// Increment now, so we can remove the use
	++I;

	// Find the index of the retval to replace with
	int index = -1;
	if (ExtractValueInst *EV = dyn_cast<ExtractValueInst>(Ins))
	if (EV->hasIndices())
	index = *EV->idx_begin();

	// If this use uses a specific return value, and we have a replacement,
	// replace it.
	if (index != -1) {
	Value *New = RetVals[index];
	if (New) {
	if (Argument *A = dyn_cast<Argument>(New))
	// Was an argument returned? Then find the corresponding argument in
	// the call instruction and use that.
	New = CS.getArgument(A->getArgNo());
	Ins->replaceAllUsesWith(New);
	Ins->eraseFromParent();
	}
	}
	}
	}

	if (MadeChange) ++NumReturnValProped;
	return MadeChange;
	}