llvm-6502/lib/Transforms/IPO/AddReadAttrs.cpp
Duncan Sands c82b6a1ed2 Ignore loads from and stores to local memory (i.e. allocas)
when deciding whether to mark a function readnone/readonly.
Since the pass is currently run before SROA, this may be
quite helpful.  Requested by Chris on IRC.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57050 91177308-0d34-0410-b5e6-96231b3b80d8
2008-10-04 13:24:24 +00:00

150 lines
4.9 KiB
C++

//===- AddReadAttrs.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. It implements
// this as a bottom-up traversal of the call-graph.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "addreadattrs"
#include "llvm/Transforms/IPO.h"
#include "llvm/CallGraphSCCPass.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/ADT/SmallPtrSet.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");
namespace {
struct VISIBILITY_HIDDEN AddReadAttrs : public CallGraphSCCPass {
static char ID; // Pass identification, replacement for typeid
AddReadAttrs() : CallGraphSCCPass(&ID) {}
// runOnSCC - Analyze the SCC, performing the transformation if possible.
bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
CallGraphSCCPass::getAnalysisUsage(AU);
}
};
}
char AddReadAttrs::ID = 0;
static RegisterPass<AddReadAttrs>
X("addreadattrs", "Mark functions readnone/readonly");
Pass *llvm::createAddReadAttrsPass() { return new AddReadAttrs(); }
bool AddReadAttrs::runOnSCC(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)) {
Value *Target = LI->getPointerOperand()->getUnderlyingObject();
// Ignore loads from local memory.
if (isa<AllocaInst>(Target))
continue;
} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
Value *Target = SI->getPointerOperand()->getUnderlyingObject();
// Ignore stores to local memory.
if (isa<AllocaInst>(Target))
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;
}