lib/Transforms/IPO/FunctionAttrs.cpp - platform/external/llvm - Gitiles

 //===- FunctionAttrs.cpp - Pass which marks functions readnone or readonly ===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements a simple interprocedural pass which walks the
 // call-graph, looking for functions which do not access or only read
 // non-local memory, and marking them readnone/readonly.  It addition,
 // it deduces which function arguments (of pointer type) do not escape,
 // and marks them nocapture.  It implements this as a bottom-up traversal
 // of the call-graph.
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "functionattrs"
 #include "llvm/Transforms/IPO.h"
 #include "llvm/CallGraphSCCPass.h"
 #include "llvm/GlobalVariable.h"
 #include "llvm/Instructions.h"
 #include "llvm/Analysis/CallGraph.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/InstIterator.h"
 using namespace llvm;

 STATISTIC(NumReadNone, "Number of functions marked readnone");
 STATISTIC(NumReadOnly, "Number of functions marked readonly");
 STATISTIC(NumNoCapture, "Number of arguments marked nocapture");

 namespace {
   struct VISIBILITY_HIDDEN FunctionAttrs : public CallGraphSCCPass {
     static char ID; // Pass identification, replacement for typeid
     FunctionAttrs() : CallGraphSCCPass(&ID) {}

     // runOnSCC - Analyze the SCC, performing the transformation if possible.
     bool runOnSCC(const std::vector<CallGraphNode *> &SCC);

     // AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
     bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC);

     // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
     bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC);

     // isCaptured - Returns true if this pointer value escapes.
     bool isCaptured(Function &F, Value *V);

     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.setPreservesCFG();
       CallGraphSCCPass::getAnalysisUsage(AU);
     }

     bool PointsToLocalMemory(Value *V);
   };
 }

 char FunctionAttrs::ID = 0;
 static RegisterPass<FunctionAttrs>
 X("functionattrs", "Deduce function attributes");

 Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }


 /// PointsToLocalMemory - Returns whether the given pointer value points to
 /// memory that is local to the function.  Global constants are considered
 /// local to all functions.
 bool FunctionAttrs::PointsToLocalMemory(Value *V) {
   V = V->getUnderlyingObject();
   // An alloca instruction defines local memory.
   if (isa<AllocaInst>(V))
     return true;
   // A global constant counts as local memory for our purposes.
   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
     return GV->isConstant();
   // Could look through phi nodes and selects here, but it doesn't seem
   // to be useful in practice.
   return false;
 }

 /// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
 bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) {
   SmallPtrSet<CallGraphNode*, 8> SCCNodes;
   CallGraph &CG = getAnalysis<CallGraph>();

   // Fill SCCNodes with the elements of the SCC.  Used for quickly
   // looking up whether a given CallGraphNode is in this SCC.
   for (unsigned i = 0, e = SCC.size(); i != e; ++i)
     SCCNodes.insert(SCC[i]);

   // Check if any of the functions in the SCC read or write memory.  If they
   // write memory then they can't be marked readnone or readonly.
   bool ReadsMemory = false;
   for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
     Function *F = SCC[i]->getFunction();

     if (F == 0)
       // External node - may write memory.  Just give up.
       return false;

     if (F->doesNotAccessMemory())
       // Already perfect!
       continue;

     // Definitions with weak linkage may be overridden at linktime with
     // something that writes memory, so treat them like declarations.
     if (F->isDeclaration() || F->mayBeOverridden()) {
       if (!F->onlyReadsMemory())
         // May write memory.  Just give up.
         return false;

       ReadsMemory = true;
       continue;
     }

     // Scan the function body for instructions that may read or write memory.
     for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
       Instruction *I = &*II;

       // Some instructions can be ignored even if they read or write memory.
       // Detect these now, skipping to the next instruction if one is found.
       CallSite CS = CallSite::get(I);
       if (CS.getInstruction()) {
         // Ignore calls to functions in the same SCC.
         if (SCCNodes.count(CG[CS.getCalledFunction()]))
           continue;
       } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
         // Ignore loads from local memory.
         if (PointsToLocalMemory(LI->getPointerOperand()))
           continue;
       } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
         // Ignore stores to local memory.
         if (PointsToLocalMemory(SI->getPointerOperand()))
           continue;
       }

       // Any remaining instructions need to be taken seriously!  Check if they
       // read or write memory.
       if (I->mayWriteToMemory())
         // Writes memory.  Just give up.
         return false;
       // If this instruction may read memory, remember that.
       ReadsMemory |= I->mayReadFromMemory();
     }
   }

   // Success!  Functions in this SCC do not access memory, or only read memory.
   // Give them the appropriate attribute.
   bool MadeChange = false;
   for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
     Function *F = SCC[i]->getFunction();

     if (F->doesNotAccessMemory())
       // Already perfect!
       continue;

     if (F->onlyReadsMemory() && ReadsMemory)
       // No change.
       continue;

     MadeChange = true;

     // Clear out any existing attributes.
     F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);

     // Add in the new attribute.
     F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);

     if (ReadsMemory)
       NumReadOnly++;
     else
       NumReadNone++;
   }

   return MadeChange;
 }

 /// isCaptured - Returns whether this pointer value is captured.
 bool FunctionAttrs::isCaptured(Function &F, Value *V) {
   SmallVector<Use*, 16> Worklist;
   SmallPtrSet<Use*, 16> Visited;

   for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE;
        ++UI) {
     Use *U = &UI.getUse();
     Visited.insert(U);
     Worklist.push_back(U);
   }

   while (!Worklist.empty()) {
     Use *U = Worklist.pop_back_val();
     Instruction *I = cast<Instruction>(U->getUser());
     V = U->get();

     if (isa<LoadInst>(I)) {
       // Loading a pointer does not cause it to escape.
       continue;
     }

     if (isa<StoreInst>(I)) {
       if (V == I->getOperand(0))
         // Stored the pointer - escapes.  TODO: improve this.
         return true;
       // Storing to the pointee does not cause the pointer to escape.
       continue;
     }

     if (isa<FreeInst>(I)) {
       // Freeing a pointer does not cause it to escape.
       continue;
     }

     CallSite CS = CallSite::get(I);
     if (CS.getInstruction()) {
       // Does not escape if the callee is readonly and doesn't return a
       // copy through its own return value.
       if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
         continue;

       // Does not escape if passed via 'nocapture' arguments.  Note that
       // calling a function pointer does not in itself cause that
       // function pointer to escape.  This is a subtle point considering
       // that (for example) the callee might return its own address.  It
       // is analogous to saying that loading a value from a pointer does
       // not cause the pointer to escape, even though the loaded value
       // might be the pointer itself (think of self-referential objects).
       CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
       for (CallSite::arg_iterator A = B; A != E; ++A)
         if (A->get() == V && !CS.paramHasAttr(A-B+1, Attribute::NoCapture))
           // The parameter is not marked 'nocapture' - escapes.
           return true;
       // Only passed via 'nocapture' arguments, or is the called function.
       // Does not escape.
       continue;
     }

     if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I) ||
         isa<PHINode>(I) || isa<SelectInst>(I)) {
       // Type conversion, calculating an offset, or merging values.
       // The original value does not escape via this if the new value doesn't.
       // Note that in the case of a select instruction it is important that
       // the value not be used as the condition, since otherwise one bit of
       // information might escape.  It cannot be the condition because it has
       // the wrong type.
       for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
            UI != UE; ++UI) {
         Use *U = &UI.getUse();
         if (Visited.insert(U))
           Worklist.push_back(U);
       }
       continue;
     }

     // Something else - be conservative and say it escapes.
     return true;
   }

   return false;
 }

 /// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
 bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) {
   bool Changed = false;

   // Check each function in turn, determining which pointer arguments are not
   // captured.
   for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
     Function *F = SCC[i]->getFunction();

     if (F == 0)
       // External node - skip it;
       continue;

     // If the function is readonly and doesn't return any value, we
     // know that the pointer value can't escape. Mark all of its pointer
     // arguments nocapture.
     if (F->onlyReadsMemory() && F->getReturnType() == Type::VoidTy) {
       for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end();
            A != E; ++A)
         if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr()) {
           A->addAttr(Attribute::NoCapture);
           ++NumNoCapture;
           Changed = true;
         }
       continue;
     }

     // Definitions with weak linkage may be overridden at linktime with
     // something that writes memory, so treat them like declarations.
     if (F->isDeclaration() || F->mayBeOverridden())
       continue;

     for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
       if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() &&
           !isCaptured(*F, A)) {
         A->addAttr(Attribute::NoCapture);
         ++NumNoCapture;
         Changed = true;
       }
   }

   return Changed;
 }

 bool FunctionAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
   bool Changed = AddReadAttrs(SCC);
   Changed |= AddNoCaptureAttrs(SCC);
   return Changed;
 }
	//===- FunctionAttrs.cpp - Pass which marks functions readnone or readonly ===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This file implements a simple interprocedural pass which walks the
	// call-graph, looking for functions which do not access or only read
	// non-local memory, and marking them readnone/readonly. It addition,
	// it deduces which function arguments (of pointer type) do not escape,
	// and marks them nocapture. It implements this as a bottom-up traversal
	// of the call-graph.
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "functionattrs"
	#include "llvm/Transforms/IPO.h"
	#include "llvm/CallGraphSCCPass.h"
	#include "llvm/GlobalVariable.h"
	#include "llvm/Instructions.h"
	#include "llvm/Analysis/CallGraph.h"
	#include "llvm/ADT/SmallPtrSet.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Support/Compiler.h"
	#include "llvm/Support/InstIterator.h"
	using namespace llvm;

	STATISTIC(NumReadNone, "Number of functions marked readnone");
	STATISTIC(NumReadOnly, "Number of functions marked readonly");
	STATISTIC(NumNoCapture, "Number of arguments marked nocapture");

	namespace {
	struct VISIBILITY_HIDDEN FunctionAttrs : public CallGraphSCCPass {
	static char ID; // Pass identification, replacement for typeid
	FunctionAttrs() : CallGraphSCCPass(&ID) {}

	// runOnSCC - Analyze the SCC, performing the transformation if possible.
	bool runOnSCC(const std::vector<CallGraphNode *> &SCC);

	// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
	bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC);

	// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
	bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC);

	// isCaptured - Returns true if this pointer value escapes.
	bool isCaptured(Function &F, Value *V);

	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	AU.setPreservesCFG();
	CallGraphSCCPass::getAnalysisUsage(AU);
	}

	bool PointsToLocalMemory(Value *V);
	};
	}

	char FunctionAttrs::ID = 0;
	static RegisterPass<FunctionAttrs>
	X("functionattrs", "Deduce function attributes");

	Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }


	/// PointsToLocalMemory - Returns whether the given pointer value points to
	/// memory that is local to the function. Global constants are considered
	/// local to all functions.
	bool FunctionAttrs::PointsToLocalMemory(Value *V) {
	V = V->getUnderlyingObject();
	// An alloca instruction defines local memory.
	if (isa<AllocaInst>(V))
	return true;
	// A global constant counts as local memory for our purposes.
	if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
	return GV->isConstant();
	// Could look through phi nodes and selects here, but it doesn't seem
	// to be useful in practice.
	return false;
	}

	/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
	bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) {
	SmallPtrSet<CallGraphNode*, 8> SCCNodes;
	CallGraph &CG = getAnalysis<CallGraph>();

	// Fill SCCNodes with the elements of the SCC. Used for quickly
	// looking up whether a given CallGraphNode is in this SCC.
	for (unsigned i = 0, e = SCC.size(); i != e; ++i)
	SCCNodes.insert(SCC[i]);

	// Check if any of the functions in the SCC read or write memory. If they
	// write memory then they can't be marked readnone or readonly.
	bool ReadsMemory = false;
	for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
	Function *F = SCC[i]->getFunction();

	if (F == 0)
	// External node - may write memory. Just give up.
	return false;

	if (F->doesNotAccessMemory())
	// Already perfect!
	continue;

	// Definitions with weak linkage may be overridden at linktime with
	// something that writes memory, so treat them like declarations.
	if (F->isDeclaration() \|\| F->mayBeOverridden()) {
	if (!F->onlyReadsMemory())
	// May write memory. Just give up.
	return false;

	ReadsMemory = true;
	continue;
	}

	// Scan the function body for instructions that may read or write memory.
	for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
	Instruction I = &II;

	// Some instructions can be ignored even if they read or write memory.
	// Detect these now, skipping to the next instruction if one is found.
	CallSite CS = CallSite::get(I);
	if (CS.getInstruction()) {
	// Ignore calls to functions in the same SCC.
	if (SCCNodes.count(CG[CS.getCalledFunction()]))
	continue;
	} else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
	// Ignore loads from local memory.
	if (PointsToLocalMemory(LI->getPointerOperand()))
	continue;
	} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
	// Ignore stores to local memory.
	if (PointsToLocalMemory(SI->getPointerOperand()))
	continue;
	}

	// Any remaining instructions need to be taken seriously! Check if they
	// read or write memory.
	if (I->mayWriteToMemory())
	// Writes memory. Just give up.
	return false;
	// If this instruction may read memory, remember that.
	ReadsMemory \|= I->mayReadFromMemory();
	}
	}

	// Success! Functions in this SCC do not access memory, or only read memory.
	// Give them the appropriate attribute.
	bool MadeChange = false;
	for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
	Function *F = SCC[i]->getFunction();

	if (F->doesNotAccessMemory())
	// Already perfect!
	continue;

	if (F->onlyReadsMemory() && ReadsMemory)
	// No change.
	continue;

	MadeChange = true;

	// Clear out any existing attributes.
	F->removeAttribute(~0, Attribute::ReadOnly \| Attribute::ReadNone);

	// Add in the new attribute.
	F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);

	if (ReadsMemory)
	NumReadOnly++;
	else
	NumReadNone++;
	}

	return MadeChange;
	}

	/// isCaptured - Returns whether this pointer value is captured.
	bool FunctionAttrs::isCaptured(Function &F, Value *V) {
	SmallVector<Use*, 16> Worklist;
	SmallPtrSet<Use*, 16> Visited;

	for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE;
	++UI) {
	Use *U = &UI.getUse();
	Visited.insert(U);
	Worklist.push_back(U);
	}

	while (!Worklist.empty()) {
	Use *U = Worklist.pop_back_val();
	Instruction *I = cast<Instruction>(U->getUser());
	V = U->get();

	if (isa<LoadInst>(I)) {
	// Loading a pointer does not cause it to escape.
	continue;
	}

	if (isa<StoreInst>(I)) {
	if (V == I->getOperand(0))
	// Stored the pointer - escapes. TODO: improve this.
	return true;
	// Storing to the pointee does not cause the pointer to escape.
	continue;
	}

	if (isa<FreeInst>(I)) {
	// Freeing a pointer does not cause it to escape.
	continue;
	}

	CallSite CS = CallSite::get(I);
	if (CS.getInstruction()) {
	// Does not escape if the callee is readonly and doesn't return a
	// copy through its own return value.
	if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
	continue;

	// Does not escape if passed via 'nocapture' arguments. Note that
	// calling a function pointer does not in itself cause that
	// function pointer to escape. This is a subtle point considering
	// that (for example) the callee might return its own address. It
	// is analogous to saying that loading a value from a pointer does
	// not cause the pointer to escape, even though the loaded value
	// might be the pointer itself (think of self-referential objects).
	CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
	for (CallSite::arg_iterator A = B; A != E; ++A)
	if (A->get() == V && !CS.paramHasAttr(A-B+1, Attribute::NoCapture))
	// The parameter is not marked 'nocapture' - escapes.
	return true;
	// Only passed via 'nocapture' arguments, or is the called function.
	// Does not escape.
	continue;
	}

	if (isa<BitCastInst>(I) \|\| isa<GetElementPtrInst>(I) \|\|
	isa<PHINode>(I) \|\| isa<SelectInst>(I)) {
	// Type conversion, calculating an offset, or merging values.
	// The original value does not escape via this if the new value doesn't.
	// Note that in the case of a select instruction it is important that
	// the value not be used as the condition, since otherwise one bit of
	// information might escape. It cannot be the condition because it has
	// the wrong type.
	for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
	UI != UE; ++UI) {
	Use *U = &UI.getUse();
	if (Visited.insert(U))
	Worklist.push_back(U);
	}
	continue;
	}

	// Something else - be conservative and say it escapes.
	return true;
	}

	return false;
	}

	/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
	bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) {
	bool Changed = false;

	// Check each function in turn, determining which pointer arguments are not
	// captured.
	for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
	Function *F = SCC[i]->getFunction();

	if (F == 0)
	// External node - skip it;
	continue;

	// If the function is readonly and doesn't return any value, we
	// know that the pointer value can't escape. Mark all of its pointer
	// arguments nocapture.
	if (F->onlyReadsMemory() && F->getReturnType() == Type::VoidTy) {
	for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end();
	A != E; ++A)
	if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr()) {
	A->addAttr(Attribute::NoCapture);
	++NumNoCapture;
	Changed = true;
	}
	continue;
	}

	// Definitions with weak linkage may be overridden at linktime with
	// something that writes memory, so treat them like declarations.
	if (F->isDeclaration() \|\| F->mayBeOverridden())
	continue;

	for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
	if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() &&
	!isCaptured(*F, A)) {
	A->addAttr(Attribute::NoCapture);
	++NumNoCapture;
	Changed = true;
	}
	}

	return Changed;
	}

	bool FunctionAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
	bool Changed = AddReadAttrs(SCC);
	Changed \|= AddNoCaptureAttrs(SCC);
	return Changed;
	}