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llvm-6502/lib/Analysis/IPA/GlobalsModRef.cpp
Duncan Sands dff6710717 Integrate the readonly/readnone logic more deeply 
into alias analysis.  This meant updating the API
which now has versions of the getModRefBehavior,
doesNotAccessMemory and onlyReadsMemory methods
which take a callsite parameter.  These should be
used unless the callsite is not known, since in
general they can do a better job than the versions
that take a function.  Also, users should no longer
call the version of getModRefBehavior that takes
both a function and a callsite.  To reduce the
chance of misuse it is now protected.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44487 91177308-0d34-0410-b5e6-96231b3b80d8
2007-12-01 07:51:45 +00:00

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//===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and 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;
/// 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
/// the function calls an external function (in which case 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);
void AnalyzeSCC(std::vector<CallGraphNode *> &SCC);
bool AnalyzeUsesOfPointer(Value *V, std::vector<Function*> &Readers,
std::vector<Function*> &Writers,
GlobalValue *OkayStoreDest = 0);
bool AnalyzeIndirectGlobalMemory(GlobalValue *GV);
};
char GlobalsModRef::ID = 0;
RegisterPass<GlobalsModRef> X("globalsmodref-aa",
"Simple mod/ref analysis for globals");
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));
}
// Othewise, 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);
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)
if ((*I).size() != 1) {
AnalyzeSCC(*I);
} else if (Function *F = (*I)[0]->getFunction()) {
if (!F->isDeclaration()) {
// Nonexternal function.
AnalyzeSCC(*I);
} else {
// Otherwise external function. Handle intrinsics and other special
// cases here.
if (getAnalysis<AliasAnalysis>().doesNotAccessMemory(F))
// If it does not access memory, process the function, causing us to
// realize it doesn't do anything (the body is empty).
AnalyzeSCC(*I);
else {
// Otherwise, don't process it. This will cause us to conservatively
// assume the worst.
}
}
} else {
// Do not process the external node, assume the worst.
}
}
void GlobalsModRef::AnalyzeSCC(std::vector<CallGraphNode *> &SCC) {
assert(!SCC.empty() && "SCC with no functions?");
FunctionRecord &FR = FunctionInfo[SCC[0]->getFunction()];
bool CallsExternal = 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 && !CallsExternal; ++i)
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 {
// Okay, if we can't say anything about it, maybe some other alias
// analysis can.
ModRefBehavior MRB =
AliasAnalysis::getModRefBehavior(Callee);
if (MRB != DoesNotAccessMemory) {
// FIXME: could make this more aggressive for functions that just
// read memory. We should just say they read all globals.
CallsExternal = true;
break;
}
}
} else {
CallsExternal = true;
break;
}
// If this SCC calls an external function, we can't say anything about it, so
// remove all SCC functions from the FunctionInfo map.
if (CallsExternal) {
for (unsigned i = 0, e = SCC.size(); i != e; ++i)
FunctionInfo.erase(SCC[i]->getFunction());
return;
}
// Otherwise, unless we already know that this function mod/refs memory, scan
// the function bodies to see if there are any explicit loads or stores.
if (FunctionEffect != ModRef) {
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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