llvm-6502/lib/Transforms/IPO/DeadArgumentElimination.cpp
2004-07-29 17:30:56 +00:00

579 lines
22 KiB
C++

//===-- DeadArgumentElimination.cpp - Eliminate dead arguments ------------===//
//
// 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 pass deletes dead arguments from internal functions. Dead argument
// elimination removes arguments which are directly dead, as well as arguments
// only passed into function calls as dead arguments of other functions. This
// pass also deletes dead arguments in a similar way.
//
// This pass is often useful as a cleanup pass to run after aggressive
// interprocedural passes, which add possibly-dead arguments.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Constant.h"
#include "llvm/Instructions.h"
#include "llvm/Support/CallSite.h"
#include "Support/Debug.h"
#include "Support/Statistic.h"
#include "Support/iterator"
#include <set>
using namespace llvm;
namespace {
Statistic<> NumArgumentsEliminated("deadargelim",
"Number of unread args removed");
Statistic<> NumRetValsEliminated("deadargelim",
"Number of unused return values removed");
/// DAE - The dead argument elimination pass.
///
class DAE : public Pass {
/// Liveness enum - During our initial pass over the program, we determine
/// that things are either definately alive, definately dead, or in need of
/// interprocedural analysis (MaybeLive).
///
enum Liveness { Live, MaybeLive, Dead };
/// LiveArguments, MaybeLiveArguments, DeadArguments - These sets contain
/// all of the arguments in the program. The Dead set contains arguments
/// which are completely dead (never used in the function). The MaybeLive
/// set contains arguments which are only passed into other function calls,
/// thus may be live and may be dead. The Live set contains arguments which
/// are known to be alive.
///
std::set<Argument*> DeadArguments, MaybeLiveArguments, LiveArguments;
/// DeadRetVal, MaybeLiveRetVal, LifeRetVal - These sets contain all of the
/// functions in the program. The Dead set contains functions whose return
/// value is known to be dead. The MaybeLive set contains functions whose
/// return values are only used by return instructions, and the Live set
/// contains functions whose return values are used, functions that are
/// external, and functions that already return void.
///
std::set<Function*> DeadRetVal, MaybeLiveRetVal, LiveRetVal;
/// InstructionsToInspect - As we mark arguments and return values
/// MaybeLive, we keep track of which instructions could make the values
/// live here. Once the entire program has had the return value and
/// arguments analyzed, this set is scanned to promote the MaybeLive objects
/// to be Live if they really are used.
std::vector<Instruction*> InstructionsToInspect;
/// CallSites - Keep track of the call sites of functions that have
/// MaybeLive arguments or return values.
std::multimap<Function*, CallSite> CallSites;
public:
bool run(Module &M);
virtual bool ShouldHackArguments() const { return false; }
private:
Liveness getArgumentLiveness(const Argument &A);
bool isMaybeLiveArgumentNowLive(Argument *Arg);
void SurveyFunction(Function &Fn);
void MarkArgumentLive(Argument *Arg);
void MarkRetValLive(Function *F);
void MarkReturnInstArgumentLive(ReturnInst *RI);
void RemoveDeadArgumentsFromFunction(Function *F);
};
RegisterOpt<DAE> X("deadargelim", "Dead Argument Elimination");
/// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but
/// deletes arguments to functions which are external. This is only for use
/// by bugpoint.
struct DAH : public DAE {
virtual bool ShouldHackArguments() const { return true; }
};
RegisterPass<DAH> Y("deadarghaX0r",
"Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)");
}
/// createDeadArgEliminationPass - This pass removes arguments from functions
/// which are not used by the body of the function.
///
Pass *llvm::createDeadArgEliminationPass() { return new DAE(); }
Pass *llvm::createDeadArgHackingPass() { return new DAH(); }
static inline bool CallPassesValueThoughVararg(Instruction *Call,
const Value *Arg) {
CallSite CS = CallSite::get(Call);
const Type *CalledValueTy = CS.getCalledValue()->getType();
const Type *FTy = cast<PointerType>(CalledValueTy)->getElementType();
unsigned NumFixedArgs = cast<FunctionType>(FTy)->getNumParams();
for (CallSite::arg_iterator AI = CS.arg_begin()+NumFixedArgs;
AI != CS.arg_end(); ++AI)
if (AI->get() == Arg)
return true;
return false;
}
// getArgumentLiveness - Inspect an argument, determining if is known Live
// (used in a computation), MaybeLive (only passed as an argument to a call), or
// Dead (not used).
DAE::Liveness DAE::getArgumentLiveness(const Argument &A) {
if (A.use_empty()) return Dead; // First check, directly dead?
// Scan through all of the uses, looking for non-argument passing uses.
for (Value::use_const_iterator I = A.use_begin(), E = A.use_end(); I!=E;++I) {
// Return instructions do not immediately effect liveness.
if (isa<ReturnInst>(*I))
continue;
CallSite CS = CallSite::get(const_cast<User*>(*I));
if (!CS.getInstruction()) {
// If its used by something that is not a call or invoke, it's alive!
return Live;
}
// If it's an indirect call, mark it alive...
Function *Callee = CS.getCalledFunction();
if (!Callee) return Live;
// Check to see if it's passed through a va_arg area: if so, we cannot
// remove it.
if (CallPassesValueThoughVararg(CS.getInstruction(), &A))
return Live; // If passed through va_arg area, we cannot remove it
}
return MaybeLive; // It must be used, but only as argument to a function
}
// SurveyFunction - This performs the initial survey of the specified function,
// checking out whether or not it uses any of its incoming arguments or whether
// any callers use the return value. This fills in the
// (Dead|MaybeLive|Live)(Arguments|RetVal) sets.
//
// We consider arguments of non-internal functions to be intrinsically alive as
// well as arguments to functions which have their "address taken".
//
void DAE::SurveyFunction(Function &F) {
bool FunctionIntrinsicallyLive = false;
Liveness RetValLiveness = F.getReturnType() == Type::VoidTy ? Live : Dead;
if (!F.hasInternalLinkage() &&
(!ShouldHackArguments() || F.getIntrinsicID()))
FunctionIntrinsicallyLive = true;
else
for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I) {
// If this use is anything other than a call site, the function is alive.
CallSite CS = CallSite::get(*I);
Instruction *TheCall = CS.getInstruction();
if (!TheCall) { // Not a direct call site?
FunctionIntrinsicallyLive = true;
break;
}
// Check to see if the return value is used...
if (RetValLiveness != Live)
for (Value::use_iterator I = TheCall->use_begin(),
E = TheCall->use_end(); I != E; ++I)
if (isa<ReturnInst>(cast<Instruction>(*I))) {
RetValLiveness = MaybeLive;
} else if (isa<CallInst>(cast<Instruction>(*I)) ||
isa<InvokeInst>(cast<Instruction>(*I))) {
if (CallPassesValueThoughVararg(cast<Instruction>(*I), TheCall) ||
!CallSite::get(cast<Instruction>(*I)).getCalledFunction()) {
RetValLiveness = Live;
break;
} else {
RetValLiveness = MaybeLive;
}
} else {
RetValLiveness = Live;
break;
}
// If the function is PASSED IN as an argument, its address has been taken
for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
AI != E; ++AI)
if (AI->get() == &F) {
FunctionIntrinsicallyLive = true;
break;
}
if (FunctionIntrinsicallyLive) break;
}
if (FunctionIntrinsicallyLive) {
DEBUG(std::cerr << " Intrinsically live fn: " << F.getName() << "\n");
for (Function::aiterator AI = F.abegin(), E = F.aend(); AI != E; ++AI)
LiveArguments.insert(AI);
LiveRetVal.insert(&F);
return;
}
switch (RetValLiveness) {
case Live: LiveRetVal.insert(&F); break;
case MaybeLive: MaybeLiveRetVal.insert(&F); break;
case Dead: DeadRetVal.insert(&F); break;
}
DEBUG(std::cerr << " Inspecting args for fn: " << F.getName() << "\n");
// If it is not intrinsically alive, we know that all users of the
// function are call sites. Mark all of the arguments live which are
// directly used, and keep track of all of the call sites of this function
// if there are any arguments we assume that are dead.
//
bool AnyMaybeLiveArgs = false;
for (Function::aiterator AI = F.abegin(), E = F.aend(); AI != E; ++AI)
switch (getArgumentLiveness(*AI)) {
case Live:
DEBUG(std::cerr << " Arg live by use: " << AI->getName() << "\n");
LiveArguments.insert(AI);
break;
case Dead:
DEBUG(std::cerr << " Arg definitely dead: " <<AI->getName()<<"\n");
DeadArguments.insert(AI);
break;
case MaybeLive:
DEBUG(std::cerr << " Arg only passed to calls: "
<< AI->getName() << "\n");
AnyMaybeLiveArgs = true;
MaybeLiveArguments.insert(AI);
break;
}
// If there are any "MaybeLive" arguments, we need to check callees of
// this function when/if they become alive. Record which functions are
// callees...
if (AnyMaybeLiveArgs || RetValLiveness == MaybeLive)
for (Value::use_iterator I = F.use_begin(), E = F.use_end();
I != E; ++I) {
if (AnyMaybeLiveArgs)
CallSites.insert(std::make_pair(&F, CallSite::get(*I)));
if (RetValLiveness == MaybeLive)
for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
UI != E; ++UI)
InstructionsToInspect.push_back(cast<Instruction>(*UI));
}
}
// isMaybeLiveArgumentNowLive - Check to see if Arg is alive. At this point, we
// know that the only uses of Arg are to be passed in as an argument to a
// function call or return. Check to see if the formal argument passed in is in
// the LiveArguments set. If so, return true.
//
bool DAE::isMaybeLiveArgumentNowLive(Argument *Arg) {
for (Value::use_iterator I = Arg->use_begin(), E = Arg->use_end(); I!=E; ++I){
if (isa<ReturnInst>(*I)) {
if (LiveRetVal.count(Arg->getParent())) return true;
continue;
}
CallSite CS = CallSite::get(*I);
// We know that this can only be used for direct calls...
Function *Callee = CS.getCalledFunction();
// Loop over all of the arguments (because Arg may be passed into the call
// multiple times) and check to see if any are now alive...
CallSite::arg_iterator CSAI = CS.arg_begin();
for (Function::aiterator AI = Callee->abegin(), E = Callee->aend();
AI != E; ++AI, ++CSAI)
// If this is the argument we are looking for, check to see if it's alive
if (*CSAI == Arg && LiveArguments.count(AI))
return true;
}
return false;
}
/// MarkArgumentLive - The MaybeLive argument 'Arg' is now known to be alive.
/// Mark it live in the specified sets and recursively mark arguments in callers
/// live that are needed to pass in a value.
///
void DAE::MarkArgumentLive(Argument *Arg) {
std::set<Argument*>::iterator It = MaybeLiveArguments.lower_bound(Arg);
if (It == MaybeLiveArguments.end() || *It != Arg) return;
DEBUG(std::cerr << " MaybeLive argument now live: " << Arg->getName()<<"\n");
MaybeLiveArguments.erase(It);
LiveArguments.insert(Arg);
// Loop over all of the call sites of the function, making any arguments
// passed in to provide a value for this argument live as necessary.
//
Function *Fn = Arg->getParent();
unsigned ArgNo = std::distance(Fn->abegin(), Function::aiterator(Arg));
std::multimap<Function*, CallSite>::iterator I = CallSites.lower_bound(Fn);
for (; I != CallSites.end() && I->first == Fn; ++I) {
CallSite CS = I->second;
Value *ArgVal = *(CS.arg_begin()+ArgNo);
if (Argument *ActualArg = dyn_cast<Argument>(ArgVal)) {
MarkArgumentLive(ActualArg);
} else {
// If the value passed in at this call site is a return value computed by
// some other call site, make sure to mark the return value at the other
// call site as being needed.
CallSite ArgCS = CallSite::get(ArgVal);
if (ArgCS.getInstruction())
if (Function *Fn = ArgCS.getCalledFunction())
MarkRetValLive(Fn);
}
}
}
/// MarkArgumentLive - The MaybeLive return value for the specified function is
/// now known to be alive. Propagate this fact to the return instructions which
/// produce it.
void DAE::MarkRetValLive(Function *F) {
assert(F && "Shame shame, we can't have null pointers here!");
// Check to see if we already knew it was live
std::set<Function*>::iterator I = MaybeLiveRetVal.lower_bound(F);
if (I == MaybeLiveRetVal.end() || *I != F) return; // It's already alive!
DEBUG(std::cerr << " MaybeLive retval now live: " << F->getName() << "\n");
MaybeLiveRetVal.erase(I);
LiveRetVal.insert(F); // It is now known to be live!
// Loop over all of the functions, noticing that the return value is now live.
for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
MarkReturnInstArgumentLive(RI);
}
void DAE::MarkReturnInstArgumentLive(ReturnInst *RI) {
Value *Op = RI->getOperand(0);
if (Argument *A = dyn_cast<Argument>(Op)) {
MarkArgumentLive(A);
} else if (CallInst *CI = dyn_cast<CallInst>(Op)) {
if (Function *F = CI->getCalledFunction())
MarkRetValLive(F);
} else if (InvokeInst *II = dyn_cast<InvokeInst>(Op)) {
if (Function *F = II->getCalledFunction())
MarkRetValLive(F);
}
}
// RemoveDeadArgumentsFromFunction - We know that F has dead arguments, as
// specified by the DeadArguments list. Transform the function and all of the
// callees of the function to not have these arguments.
//
void DAE::RemoveDeadArgumentsFromFunction(Function *F) {
// Start by computing a new prototype for the function, which is the same as
// the old function, but has fewer arguments.
const FunctionType *FTy = F->getFunctionType();
std::vector<const Type*> Params;
for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
if (!DeadArguments.count(I))
Params.push_back(I->getType());
const Type *RetTy = FTy->getReturnType();
if (DeadRetVal.count(F)) {
RetTy = Type::VoidTy;
DeadRetVal.erase(F);
}
// Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
// have zero fixed arguments.
//
// FIXME: once this bug is fixed in the CWriter, this hack should be removed.
//
bool ExtraArgHack = false;
if (Params.empty() && FTy->isVarArg()) {
ExtraArgHack = true;
Params.push_back(Type::IntTy);
}
FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
// Create the new function body and insert it into the module...
Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
F->getParent()->getFunctionList().insert(F, NF);
// Loop over all of the callers of the function, transforming the call sites
// to pass in a smaller number of arguments into the new function.
//
std::vector<Value*> Args;
while (!F->use_empty()) {
CallSite CS = CallSite::get(F->use_back());
Instruction *Call = CS.getInstruction();
// Loop over the operands, deleting dead ones...
CallSite::arg_iterator AI = CS.arg_begin();
for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I, ++AI)
if (!DeadArguments.count(I)) // Remove operands for dead arguments
Args.push_back(*AI);
if (ExtraArgHack)
Args.push_back(Constant::getNullValue(Type::IntTy));
// Push any varargs arguments on the list
for (; AI != CS.arg_end(); ++AI)
Args.push_back(*AI);
Instruction *New;
if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
Args, "", Call);
} else {
New = new CallInst(NF, Args, "", Call);
}
Args.clear();
if (!Call->use_empty()) {
if (New->getType() == Type::VoidTy)
Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
else {
Call->replaceAllUsesWith(New);
std::string Name = Call->getName();
Call->setName("");
New->setName(Name);
}
}
// Finally, remove the old call from the program, reducing the use-count of
// F.
Call->getParent()->getInstList().erase(Call);
}
// Since we have now created the new function, splice the body of the old
// function right into the new function, leaving the old rotting hulk of the
// function empty.
NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
// Loop over the argument list, transfering uses of the old arguments over to
// the new arguments, also transfering over the names as well. While we're at
// it, remove the dead arguments from the DeadArguments list.
//
for (Function::aiterator I = F->abegin(), E = F->aend(), I2 = NF->abegin();
I != E; ++I)
if (!DeadArguments.count(I)) {
// If this is a live argument, move the name and users over to the new
// version.
I->replaceAllUsesWith(I2);
I2->setName(I->getName());
++I2;
} else {
// If this argument is dead, replace any uses of it with null constants
// (these are guaranteed to only be operands to call instructions which
// will later be simplified).
I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
DeadArguments.erase(I);
}
// If we change the return value of the function we must rewrite any return
// instructions. Check this now.
if (F->getReturnType() != NF->getReturnType())
for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
new ReturnInst(0, RI);
BB->getInstList().erase(RI);
}
// Now that the old function is dead, delete it.
F->getParent()->getFunctionList().erase(F);
}
bool DAE::run(Module &M) {
// First phase: loop through the module, determining which arguments are live.
// We assume all arguments are dead unless proven otherwise (allowing us to
// determine that dead arguments passed into recursive functions are dead).
//
DEBUG(std::cerr << "DAE - Determining liveness\n");
for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
SurveyFunction(*I);
// Loop over the instructions to inspect, propagating liveness among arguments
// and return values which are MaybeLive.
while (!InstructionsToInspect.empty()) {
Instruction *I = InstructionsToInspect.back();
InstructionsToInspect.pop_back();
if (ReturnInst *RI = dyn_cast<ReturnInst>(I)) {
// For return instructions, we just have to check to see if the return
// value for the current function is known now to be alive. If so, any
// arguments used by it are now alive, and any call instruction return
// value is alive as well.
if (LiveRetVal.count(RI->getParent()->getParent()))
MarkReturnInstArgumentLive(RI);
} else {
CallSite CS = CallSite::get(I);
assert(CS.getInstruction() && "Unknown instruction for the I2I list!");
Function *Callee = CS.getCalledFunction();
// If we found a call or invoke instruction on this list, that means that
// an argument of the function is a call instruction. If the argument is
// live, then the return value of the called instruction is now live.
//
CallSite::arg_iterator AI = CS.arg_begin(); // ActualIterator
for (Function::aiterator FI = Callee->abegin(), E = Callee->aend();
FI != E; ++AI, ++FI) {
// If this argument is another call...
CallSite ArgCS = CallSite::get(*AI);
if (ArgCS.getInstruction() && LiveArguments.count(FI))
if (Function *Callee = ArgCS.getCalledFunction())
MarkRetValLive(Callee);
}
}
}
// Now we loop over all of the MaybeLive arguments, promoting them to be live
// arguments if one of the calls that uses the arguments to the calls they are
// passed into requires them to be live. Of course this could make other
// arguments live, so process callers recursively.
//
// Because elements can be removed from the MaybeLiveArguments set, copy it to
// a temporary vector.
//
std::vector<Argument*> TmpArgList(MaybeLiveArguments.begin(),
MaybeLiveArguments.end());
for (unsigned i = 0, e = TmpArgList.size(); i != e; ++i) {
Argument *MLA = TmpArgList[i];
if (MaybeLiveArguments.count(MLA) &&
isMaybeLiveArgumentNowLive(MLA))
MarkArgumentLive(MLA);
}
// Recover memory early...
CallSites.clear();
// At this point, we know that all arguments in DeadArguments and
// MaybeLiveArguments are dead. If the two sets are empty, there is nothing
// to do.
if (MaybeLiveArguments.empty() && DeadArguments.empty() &&
MaybeLiveRetVal.empty() && DeadRetVal.empty())
return false;
// Otherwise, compact into one set, and start eliminating the arguments from
// the functions.
DeadArguments.insert(MaybeLiveArguments.begin(), MaybeLiveArguments.end());
MaybeLiveArguments.clear();
DeadRetVal.insert(MaybeLiveRetVal.begin(), MaybeLiveRetVal.end());
MaybeLiveRetVal.clear();
LiveArguments.clear();
LiveRetVal.clear();
NumArgumentsEliminated += DeadArguments.size();
NumRetValsEliminated += DeadRetVal.size();
while (!DeadArguments.empty())
RemoveDeadArgumentsFromFunction((*DeadArguments.begin())->getParent());
while (!DeadRetVal.empty())
RemoveDeadArgumentsFromFunction(*DeadRetVal.begin());
return true;
}