llvm-6502/lib/Transforms/IPO/FunctionAttrs.cpp
Nick Lewycky 3b3b4e3f0f Any void readonly functions are provably dead, don't waste time adding
nocapture attributes to them.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61610 91177308-0d34-0410-b5e6-96231b3b80d8
2009-01-03 17:05:32 +00:00

332 lines
13 KiB
C++

//===- 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. In addition,
// it marks function arguments (of pointer type) 'nocapture' if a call
// to the function does not create any copies of the pointer value that
// outlive the call. This more or less means that the pointer is only
// dereferenced, and not returned from the function or stored in a global.
// This pass is implemented 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/PointerIntPair.h"
#include "llvm/ADT/SmallSet.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 - Return true if this pointer value may be captured.
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 - Return true if this pointer value may be captured.
bool FunctionAttrs::isCaptured(Function &F, Value *V) {
typedef PointerIntPair<Use*, 2> UseWithDepth;
SmallVector<UseWithDepth, 16> Worklist;
SmallSet<UseWithDepth, 16> Visited;
for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE;
++UI) {
UseWithDepth UD(&UI.getUse(), 0);
Visited.insert(UD);
Worklist.push_back(UD);
}
while (!Worklist.empty()) {
UseWithDepth UD = Worklist.pop_back_val();
Use *U = UD.getPointer();
Instruction *I = cast<Instruction>(U->getUser());
// The value V may have any type if it comes from tracking a load.
V = U->get();
// The depth represents the number of loads that need to be performed to
// get back the original pointer (or a bitcast etc of it). For example,
// if the pointer is stored to an alloca, then all uses of the alloca get
// depth 1: if the alloca is loaded then you get the original pointer back.
// If a load of the alloca is returned then the pointer has been captured.
// The depth is needed in order to know which loads dereference the original
// pointer (these do not capture), and which return a value which needs to
// be tracked because if it is captured then so is the original pointer.
unsigned Depth = UD.getInt();
if (isa<StoreInst>(I)) {
if (V == I->getOperand(0)) {
// Stored the pointer - it may be captured. If it is stored to a local
// object (alloca) then track that object. Otherwise give up.
Value *Target = I->getOperand(1)->getUnderlyingObject();
if (!isa<AllocaInst>(Target))
// Didn't store to an obviously local object - captured.
return true;
if (Depth >= 3)
// Alloca recursion too deep - give up.
return true;
// Analyze all uses of the alloca.
for (Value::use_iterator UI = Target->use_begin(),
UE = Target->use_end(); UI != UE; ++UI) {
UseWithDepth NUD(&UI.getUse(), Depth + 1);
if (Visited.insert(NUD))
Worklist.push_back(NUD);
}
}
// Storing to the pointee does not cause the pointer to be captured.
} else if (isa<FreeInst>(I)) {
// Freeing a pointer does not cause it to be captured.
} else if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
CallSite CS = CallSite::get(I);
// Not captured if the callee is readonly and doesn't return a copy
// through its return value.
if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
continue;
// Not captured if only passed via 'nocapture' arguments. Note that
// calling a function pointer does not in itself cause the pointer to
// be captured. 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 be
// captured, 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' - captured.
return true;
// Only passed via 'nocapture' arguments, or is the called function - not
// captured.
} else if (isa<BitCastInst>(I) || isa<LoadInst>(I) || isa<PHINode>(I) ||
// Play safe and exclude GEP indices.
(isa<GetElementPtrInst>(I) && V == I->getOperand(0)) ||
// Play safe and exclude the select condition.
(isa<SelectInst>(I) && V != I->getOperand(0))) {
// Usually loads can be ignored because they dereference the original
// pointer. However the loaded value needs to be tracked if loading
// from an object that the original pointer was stored to.
if (isa<LoadInst>(I)) {
if (Depth == 0)
// Loading the original pointer or a variation of it. This does not
// cause the pointer to be captured. Note that the loaded value might
// be the pointer itself (think of self-referential objects), but that
// is fine as long as it's not this function that stored it there.
continue;
// Loading a pointer to (a pointer to...) the original pointer or a
// variation of it. Track uses of the loaded value, noting that one
// dereference was performed. Note that the loaded value need not be
// of pointer type. For example, an alloca may have been bitcast to
// a pointer to another type, which was then loaded.
--Depth;
}
// The original value is not captured via this if the instruction isn't.
for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
UI != UE; ++UI) {
UseWithDepth UD(&UI.getUse(), Depth);
if (Visited.insert(UD))
Worklist.push_back(UD);
}
} else {
// Something else - be conservative and say it is captured.
return true;
}
}
// All uses examined - not captured.
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;
// 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;
}