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llvm-6502/lib/Analysis/IPA/GlobalsModRef.cpp
Duncan Sands 34f2a0ca17 If a SCC has a node without a function, then the SCC 
analysis would bail out without removing function
records for other members of the SCC (which may exist
if those functions read or wrote global variables).
Since these are initialized to "readnone", this
resulted in incorrect alias analysis results.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55714 91177308-0d34-0410-b5e6-96231b3b80d8
2008-09-03 19:37:16 +00:00

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//===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This simple pass provides alias and mod/ref information for global values
// that do not have their address taken, and keeps track of whether functions
// read or write memory (are "pure"). For this simple (but very common) case,
// we can provide pretty accurate and useful information.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "globalsmodref-aa"
#include "llvm/Analysis/Passes.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Instructions.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/InstIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SCCIterator.h"
#include <set>
using namespace llvm;
STATISTIC(NumNonAddrTakenGlobalVars,
"Number of global vars without address taken");
STATISTIC(NumNonAddrTakenFunctions,"Number of functions without address taken");
STATISTIC(NumNoMemFunctions, "Number of functions that do not access memory");
STATISTIC(NumReadMemFunctions, "Number of functions that only read memory");
STATISTIC(NumIndirectGlobalVars, "Number of indirect global objects");
namespace {
/// FunctionRecord - One instance of this structure is stored for every
/// function in the program. Later, the entries for these functions are
/// removed if the function is found to call an external function (in which
/// case we know nothing about it.
struct VISIBILITY_HIDDEN FunctionRecord {
/// GlobalInfo - Maintain mod/ref info for all of the globals without
/// addresses taken that are read or written (transitively) by this
/// function.
std::map<GlobalValue*, unsigned> GlobalInfo;
unsigned getInfoForGlobal(GlobalValue *GV) const {
std::map<GlobalValue*, unsigned>::const_iterator I = GlobalInfo.find(GV);
if (I != GlobalInfo.end())
return I->second;
return 0;
}
/// FunctionEffect - Capture whether or not this function reads or writes to
/// ANY memory. If not, we can do a lot of aggressive analysis on it.
unsigned FunctionEffect;
FunctionRecord() : FunctionEffect(0) {}
};
/// GlobalsModRef - The actual analysis pass.
class VISIBILITY_HIDDEN GlobalsModRef
: public ModulePass, public AliasAnalysis {
/// NonAddressTakenGlobals - The globals that do not have their addresses
/// taken.
std::set<GlobalValue*> NonAddressTakenGlobals;
/// ReadGlobals - The globals without addresses taken that are read by
/// some function.
std::set<GlobalValue*> ReadGlobals;
/// IndirectGlobals - The memory pointed to by this global is known to be
/// 'owned' by the global.
std::set<GlobalValue*> IndirectGlobals;
/// AllocsForIndirectGlobals - If an instruction allocates memory for an
/// indirect global, this map indicates which one.
std::map<Value*, GlobalValue*> AllocsForIndirectGlobals;
/// FunctionInfo - For each function, keep track of what globals are
/// modified or read.
std::map<Function*, FunctionRecord> FunctionInfo;
public:
static char ID;
GlobalsModRef() : ModulePass((intptr_t)&ID) {}
bool runOnModule(Module &M) {
InitializeAliasAnalysis(this); // set up super class
AnalyzeGlobals(M); // find non-addr taken globals
AnalyzeCallGraph(getAnalysis<CallGraph>(), M); // Propagate on CG
return false;
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AliasAnalysis::getAnalysisUsage(AU);
AU.addRequired<CallGraph>();
AU.setPreservesAll(); // Does not transform code
}
//------------------------------------------------
// Implement the AliasAnalysis API
//
AliasResult alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size);
ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
return AliasAnalysis::getModRefInfo(CS1,CS2);
}
bool hasNoModRefInfoForCalls() const { return false; }
/// getModRefBehavior - Return the behavior of the specified function if
/// called from the specified call site. The call site may be null in which
/// case the most generic behavior of this function should be returned.
virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
std::vector<PointerAccessInfo> *Info) {
if (FunctionRecord *FR = getFunctionInfo(F)) {
if (FR->FunctionEffect == 0)
return DoesNotAccessMemory;
else if ((FR->FunctionEffect & Mod) == 0)
return OnlyReadsMemory;
}
return AliasAnalysis::getModRefBehavior(F, CS, Info);
}
virtual void deleteValue(Value *V);
virtual void copyValue(Value *From, Value *To);
private:
/// getFunctionInfo - Return the function info for the function, or null if
/// we don't have anything useful to say about it.
FunctionRecord *getFunctionInfo(Function *F) {
std::map<Function*, FunctionRecord>::iterator I = FunctionInfo.find(F);
if (I != FunctionInfo.end())
return &I->second;
return 0;
}
void AnalyzeGlobals(Module &M);
void AnalyzeCallGraph(CallGraph &CG, Module &M);
bool AnalyzeUsesOfPointer(Value *V, std::vector<Function*> &Readers,
std::vector<Function*> &Writers,
GlobalValue *OkayStoreDest = 0);
bool AnalyzeIndirectGlobalMemory(GlobalValue *GV);
};
}
char GlobalsModRef::ID = 0;
static RegisterPass<GlobalsModRef>
X("globalsmodref-aa", "Simple mod/ref analysis for globals", false, true);
static RegisterAnalysisGroup<AliasAnalysis> Y(X);
Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); }
/// getUnderlyingObject - This traverses the use chain to figure out what object
/// the specified value points to. If the value points to, or is derived from,
/// a global object, return it.
static Value *getUnderlyingObject(Value *V) {
if (!isa<PointerType>(V->getType())) return V;
// If we are at some type of object... return it.
if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) return GV;
// Traverse through different addressing mechanisms.
if (Instruction *I = dyn_cast<Instruction>(V)) {
if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I))
return getUnderlyingObject(I->getOperand(0));
} else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
if (CE->getOpcode() == Instruction::BitCast ||
CE->getOpcode() == Instruction::GetElementPtr)
return getUnderlyingObject(CE->getOperand(0));
}
// Otherwise, we don't know what this is, return it as the base pointer.
return V;
}
/// AnalyzeGlobals - Scan through the users of all of the internal
/// GlobalValue's in the program. If none of them have their "address taken"
/// (really, their address passed to something nontrivial), record this fact,
/// and record the functions that they are used directly in.
void GlobalsModRef::AnalyzeGlobals(Module &M) {
std::vector<Function*> Readers, Writers;
for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
if (I->hasInternalLinkage()) {
if (!AnalyzeUsesOfPointer(I, Readers, Writers)) {
// Remember that we are tracking this global.
NonAddressTakenGlobals.insert(I);
++NumNonAddrTakenFunctions;
}
Readers.clear(); Writers.clear();
}
for (Module::global_iterator I = M.global_begin(), E = M.global_end();
I != E; ++I)
if (I->hasInternalLinkage()) {
if (!AnalyzeUsesOfPointer(I, Readers, Writers)) {
// Remember that we are tracking this global, and the mod/ref fns
NonAddressTakenGlobals.insert(I);
if (!Readers.empty())
// Some function read this global - remember that.
ReadGlobals.insert(I);
for (unsigned i = 0, e = Readers.size(); i != e; ++i)
FunctionInfo[Readers[i]].GlobalInfo[I] |= Ref;
if (!I->isConstant()) // No need to keep track of writers to constants
for (unsigned i = 0, e = Writers.size(); i != e; ++i)
FunctionInfo[Writers[i]].GlobalInfo[I] |= Mod;
++NumNonAddrTakenGlobalVars;
// If this global holds a pointer type, see if it is an indirect global.
if (isa<PointerType>(I->getType()->getElementType()) &&
AnalyzeIndirectGlobalMemory(I))
++NumIndirectGlobalVars;
}
Readers.clear(); Writers.clear();
}
}
/// AnalyzeUsesOfPointer - Look at all of the users of the specified pointer.
/// If this is used by anything complex (i.e., the address escapes), return
/// true. Also, while we are at it, keep track of those functions that read and
/// write to the value.
///
/// If OkayStoreDest is non-null, stores into this global are allowed.
bool GlobalsModRef::AnalyzeUsesOfPointer(Value *V,
std::vector<Function*> &Readers,
std::vector<Function*> &Writers,
GlobalValue *OkayStoreDest) {
if (!isa<PointerType>(V->getType())) return true;
for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
Readers.push_back(LI->getParent()->getParent());
} else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
if (V == SI->getOperand(1)) {
Writers.push_back(SI->getParent()->getParent());
} else if (SI->getOperand(1) != OkayStoreDest) {
return true; // Storing the pointer
}
} else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
if (AnalyzeUsesOfPointer(GEP, Readers, Writers)) return true;
} else if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
// Make sure that this is just the function being called, not that it is
// passing into the function.
for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
if (CI->getOperand(i) == V) return true;
} else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) {
// Make sure that this is just the function being called, not that it is
// passing into the function.
for (unsigned i = 3, e = II->getNumOperands(); i != e; ++i)
if (II->getOperand(i) == V) return true;
} else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(*UI)) {
if (CE->getOpcode() == Instruction::GetElementPtr ||
CE->getOpcode() == Instruction::BitCast) {
if (AnalyzeUsesOfPointer(CE, Readers, Writers))
return true;
} else {
return true;
}
} else if (ICmpInst *ICI = dyn_cast<ICmpInst>(*UI)) {
if (!isa<ConstantPointerNull>(ICI->getOperand(1)))
return true; // Allow comparison against null.
} else if (FreeInst *F = dyn_cast<FreeInst>(*UI)) {
Writers.push_back(F->getParent()->getParent());
} else {
return true;
}
return false;
}
/// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable
/// which holds a pointer type. See if the global always points to non-aliased
/// heap memory: that is, all initializers of the globals are allocations, and
/// those allocations have no use other than initialization of the global.
/// Further, all loads out of GV must directly use the memory, not store the
/// pointer somewhere. If this is true, we consider the memory pointed to by
/// GV to be owned by GV and can disambiguate other pointers from it.
bool GlobalsModRef::AnalyzeIndirectGlobalMemory(GlobalValue *GV) {
// Keep track of values related to the allocation of the memory, f.e. the
// value produced by the malloc call and any casts.
std::vector<Value*> AllocRelatedValues;
// Walk the user list of the global. If we find anything other than a direct
// load or store, bail out.
for (Value::use_iterator I = GV->use_begin(), E = GV->use_end(); I != E; ++I){
if (LoadInst *LI = dyn_cast<LoadInst>(*I)) {
// The pointer loaded from the global can only be used in simple ways:
// we allow addressing of it and loading storing to it. We do *not* allow
// storing the loaded pointer somewhere else or passing to a function.
std::vector<Function*> ReadersWriters;
if (AnalyzeUsesOfPointer(LI, ReadersWriters, ReadersWriters))
return false; // Loaded pointer escapes.
// TODO: Could try some IP mod/ref of the loaded pointer.
} else if (StoreInst *SI = dyn_cast<StoreInst>(*I)) {
// Storing the global itself.
if (SI->getOperand(0) == GV) return false;
// If storing the null pointer, ignore it.
if (isa<ConstantPointerNull>(SI->getOperand(0)))
continue;
// Check the value being stored.
Value *Ptr = getUnderlyingObject(SI->getOperand(0));
if (isa<MallocInst>(Ptr)) {
// Okay, easy case.
} else if (CallInst *CI = dyn_cast<CallInst>(Ptr)) {
Function *F = CI->getCalledFunction();
if (!F || !F->isDeclaration()) return false; // Too hard to analyze.
if (F->getName() != "calloc") return false; // Not calloc.
} else {
return false; // Too hard to analyze.
}
// Analyze all uses of the allocation. If any of them are used in a
// non-simple way (e.g. stored to another global) bail out.
std::vector<Function*> ReadersWriters;
if (AnalyzeUsesOfPointer(Ptr, ReadersWriters, ReadersWriters, GV))
return false; // Loaded pointer escapes.
// Remember that this allocation is related to the indirect global.
AllocRelatedValues.push_back(Ptr);
} else {
// Something complex, bail out.
return false;
}
}
// Okay, this is an indirect global. Remember all of the allocations for
// this global in AllocsForIndirectGlobals.
while (!AllocRelatedValues.empty()) {
AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV;
AllocRelatedValues.pop_back();
}
IndirectGlobals.insert(GV);
return true;
}
/// AnalyzeCallGraph - At this point, we know the functions where globals are
/// immediately stored to and read from. Propagate this information up the call
/// graph to all callers and compute the mod/ref info for all memory for each
/// function.
void GlobalsModRef::AnalyzeCallGraph(CallGraph &CG, Module &M) {
// We do a bottom-up SCC traversal of the call graph. In other words, we
// visit all callees before callers (leaf-first).
for (scc_iterator<CallGraph*> I = scc_begin(&CG), E = scc_end(&CG); I != E;
++I) {
std::vector<CallGraphNode *> &SCC = *I;
FunctionRecord *FR = 0;
// Find a function record from the SCC (it doesn't matter which one).
for (unsigned i = 0, e = SCC.size(); i != e; ++i)
if (Function *F = SCC[i]->getFunction()) {
FR = &FunctionInfo[F];
break;
}
if (!FR)
// Nothing to do.
continue;
bool KnowNothing = false;
unsigned FunctionEffect = 0;
// Collect the mod/ref properties due to called functions. We only compute
// one mod-ref set.
for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
Function *F = SCC[i]->getFunction();
if (!F) {
KnowNothing = true;
break;
}
if (F->isDeclaration()) {
// Try to get mod/ref behaviour from function attributes.
if (F->doesNotAccessMemory()) {
// Can't do better than that!
} else if (F->onlyReadsMemory()) {
FunctionEffect |= Ref;
// This function might call back into the module and read a global, so
// mark all globals read somewhere as being read by this function.
for (std::set<GlobalValue*>::iterator GI = ReadGlobals.begin(),
E = ReadGlobals.end(); GI != E; ++GI)
FR->GlobalInfo[*GI] |= Ref;
} else {
// Can't say anything useful.
KnowNothing = true;
}
continue;
}
for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
CI != E; ++CI)
if (Function *Callee = CI->second->getFunction()) {
if (FunctionRecord *CalleeFR = getFunctionInfo(Callee)) {
// Propagate function effect up.
FunctionEffect |= CalleeFR->FunctionEffect;
// Incorporate callee's effects on globals into our info.
for (std::map<GlobalValue*, unsigned>::iterator GI =
CalleeFR->GlobalInfo.begin(), E = CalleeFR->GlobalInfo.end();
GI != E; ++GI)
FR->GlobalInfo[GI->first] |= GI->second;
} else {
// Can't say anything about it. However, if it is inside our SCC,
// then nothing needs to be done.
CallGraphNode *CalleeNode = CG[Callee];
if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end())
KnowNothing = true;
}
} else {
KnowNothing = true;
}
}
// If we can't say anything useful about this SCC, remove all SCC functions
// from the FunctionInfo map.
if (KnowNothing) {
for (unsigned i = 0, e = SCC.size(); i != e; ++i)
FunctionInfo.erase(SCC[i]->getFunction());
continue;
}
// Scan the function bodies for explicit loads or stores.
for (unsigned i = 0, e = SCC.size(); i != e && FunctionEffect != ModRef;++i)
for (inst_iterator II = inst_begin(SCC[i]->getFunction()),
E = inst_end(SCC[i]->getFunction());
II != E && FunctionEffect != ModRef; ++II)
if (isa<LoadInst>(*II))
FunctionEffect |= Ref;
else if (isa<StoreInst>(*II))
FunctionEffect |= Mod;
else if (isa<MallocInst>(*II) || isa<FreeInst>(*II))
FunctionEffect |= ModRef;
if ((FunctionEffect & Mod) == 0)
++NumReadMemFunctions;
if (FunctionEffect == 0)
++NumNoMemFunctions;
FR->FunctionEffect = FunctionEffect;
// Finally, now that we know the full effect on this SCC, clone the
// information to each function in the SCC.
for (unsigned i = 1, e = SCC.size(); i != e; ++i)
FunctionInfo[SCC[i]->getFunction()] = *FR;
}
}
/// alias - If one of the pointers is to a global that we are tracking, and the
/// other is some random pointer, we know there cannot be an alias, because the
/// address of the global isn't taken.
AliasAnalysis::AliasResult
GlobalsModRef::alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size) {
// Get the base object these pointers point to.
Value *UV1 = getUnderlyingObject(const_cast<Value*>(V1));
Value *UV2 = getUnderlyingObject(const_cast<Value*>(V2));
// If either of the underlying values is a global, they may be non-addr-taken
// globals, which we can answer queries about.
GlobalValue *GV1 = dyn_cast<GlobalValue>(UV1);
GlobalValue *GV2 = dyn_cast<GlobalValue>(UV2);
if (GV1 || GV2) {
// If the global's address is taken, pretend we don't know it's a pointer to
// the global.
if (GV1 && !NonAddressTakenGlobals.count(GV1)) GV1 = 0;
if (GV2 && !NonAddressTakenGlobals.count(GV2)) GV2 = 0;
// If the the two pointers are derived from two different non-addr-taken
// globals, or if one is and the other isn't, we know these can't alias.
if ((GV1 || GV2) && GV1 != GV2)
return NoAlias;
// Otherwise if they are both derived from the same addr-taken global, we
// can't know the two accesses don't overlap.
}
// These pointers may be based on the memory owned by an indirect global. If
// so, we may be able to handle this. First check to see if the base pointer
// is a direct load from an indirect global.
GV1 = GV2 = 0;
if (LoadInst *LI = dyn_cast<LoadInst>(UV1))
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
if (IndirectGlobals.count(GV))
GV1 = GV;
if (LoadInst *LI = dyn_cast<LoadInst>(UV2))
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
if (IndirectGlobals.count(GV))
GV2 = GV;
// These pointers may also be from an allocation for the indirect global. If
// so, also handle them.
if (AllocsForIndirectGlobals.count(UV1))
GV1 = AllocsForIndirectGlobals[UV1];
if (AllocsForIndirectGlobals.count(UV2))
GV2 = AllocsForIndirectGlobals[UV2];
// Now that we know whether the two pointers are related to indirect globals,
// use this to disambiguate the pointers. If either pointer is based on an
// indirect global and if they are not both based on the same indirect global,
// they cannot alias.
if ((GV1 || GV2) && GV1 != GV2)
return NoAlias;
return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
}
AliasAnalysis::ModRefResult
GlobalsModRef::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
unsigned Known = ModRef;
// If we are asking for mod/ref info of a direct call with a pointer to a
// global we are tracking, return information if we have it.
if (GlobalValue *GV = dyn_cast<GlobalValue>(getUnderlyingObject(P)))
if (GV->hasInternalLinkage())
if (Function *F = CS.getCalledFunction())
if (NonAddressTakenGlobals.count(GV))
if (FunctionRecord *FR = getFunctionInfo(F))
Known = FR->getInfoForGlobal(GV);
if (Known == NoModRef)
return NoModRef; // No need to query other mod/ref analyses
return ModRefResult(Known & AliasAnalysis::getModRefInfo(CS, P, Size));
}
//===----------------------------------------------------------------------===//
// Methods to update the analysis as a result of the client transformation.
//
void GlobalsModRef::deleteValue(Value *V) {
if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
if (NonAddressTakenGlobals.erase(GV)) {
// This global might be an indirect global. If so, remove it and remove
// any AllocRelatedValues for it.
if (IndirectGlobals.erase(GV)) {
// Remove any entries in AllocsForIndirectGlobals for this global.
for (std::map<Value*, GlobalValue*>::iterator
I = AllocsForIndirectGlobals.begin(),
E = AllocsForIndirectGlobals.end(); I != E; ) {
if (I->second == GV) {
AllocsForIndirectGlobals.erase(I++);
} else {
++I;
}
}
}
}
}
// Otherwise, if this is an allocation related to an indirect global, remove
// it.
AllocsForIndirectGlobals.erase(V);
AliasAnalysis::deleteValue(V);
}
void GlobalsModRef::copyValue(Value *From, Value *To) {
AliasAnalysis::copyValue(From, To);
}
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