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llvm-project/llvm/lib/Analysis/IPA/GlobalsModRef.cpp

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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.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/Passes.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Instructions.h"
#include "llvm/Constants.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Support/InstIterator.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SCCIterator.h"
#include <set>
using namespace llvm;
namespace {
Statistic<>
NumNonAddrTakenGlobalVars("globalsmodref-aa",
"Number of global vars without address taken");
Statistic<>
NumNonAddrTakenFunctions("globalsmodref-aa",
"Number of functions without address taken");
Statistic<>
NumNoMemFunctions("globalsmodref-aa",
"Number of functions that do not access memory");
Statistic<>
NumReadMemFunctions("globalsmodref-aa",
"Number of functions that only read memory");
/// 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 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 GlobalsModRef : public ModulePass, public AliasAnalysis {
/// NonAddressTakenGlobals - The globals that do not have their addresses
/// taken.
std::set<GlobalValue*> NonAddressTakenGlobals;
/// FunctionInfo - For each function, keep track of what globals are
/// modified or read.
std::map<Function*, FunctionRecord> FunctionInfo;
public:
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 AnalyzeUsesOfGlobal(Value *V, std::vector<Function*> &Readers,
std::vector<Function*> &Writers);
};
RegisterOpt<GlobalsModRef> X("globalsmodref-aa",
"Simple mod/ref analysis for globals");
RegisterAnalysisGroup<AliasAnalysis, GlobalsModRef> Y;
}
Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); }
/// AnalyzeGlobalUses - 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 (!AnalyzeUsesOfGlobal(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 (!AnalyzeUsesOfGlobal(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;
}
Readers.clear(); Writers.clear();
}
}
/// AnalyzeUsesOfGlobal - Look at all of the users of the specified global value
/// derived 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.
bool GlobalsModRef::AnalyzeUsesOfGlobal(Value *V,
std::vector<Function*> &Readers,
std::vector<Function*> &Writers) {
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(0)) return true; // Storing the pointer
Writers.push_back(SI->getParent()->getParent());
} else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
if (AnalyzeUsesOfGlobal(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::Cast) {
if (AnalyzeUsesOfGlobal(CE, Readers, Writers))
return true;
} else {
return true;
}
} else if (GlobalValue *GV = dyn_cast<GlobalValue>(*UI)) {
if (AnalyzeUsesOfGlobal(GV, Readers, Writers)) return true;
} else {
return true;
}
return false;
}
/// 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->isExternal()) {
// 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)->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, CallSite());
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;
}
/// 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 const GlobalValue *getUnderlyingObject(const Value *V) {
if (!isa<PointerType>(V->getType())) return 0;
// If we are at some type of object... return it.
if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) return GV;
// Traverse through different addressing mechanisms...
if (const Instruction *I = dyn_cast<Instruction>(V)) {
if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
return getUnderlyingObject(I->getOperand(0));
} else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
if (CE->getOpcode() == Instruction::Cast ||
CE->getOpcode() == Instruction::GetElementPtr)
return getUnderlyingObject(CE->getOperand(0));
}
return 0;
}
/// 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) {
GlobalValue *GV1 = const_cast<GlobalValue*>(getUnderlyingObject(V1));
GlobalValue *GV2 = const_cast<GlobalValue*>(getUnderlyingObject(V2));
// 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 ((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 = const_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))
NonAddressTakenGlobals.erase(GV);
}
void GlobalsModRef::copyValue(Value *From, Value *To) {
}
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