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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
150 lines
4.9 KiB
C++
150 lines
4.9 KiB
C++
//===- AddReadAttrs.cpp - Pass which marks functions readnone or readonly -===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a simple interprocedural pass which walks the
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// call-graph, looking for functions which do not access or only read
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// non-local memory, and marking them readnone/readonly. It implements
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// this as a bottom-up traversal of the call-graph.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "addreadattrs"
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#include "llvm/Transforms/IPO.h"
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#include "llvm/CallGraphSCCPass.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/CallGraph.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Support/InstIterator.h"
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using namespace llvm;
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STATISTIC(NumReadNone, "Number of functions marked readnone");
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STATISTIC(NumReadOnly, "Number of functions marked readonly");
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namespace {
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struct VISIBILITY_HIDDEN AddReadAttrs : public CallGraphSCCPass {
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static char ID; // Pass identification, replacement for typeid
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AddReadAttrs() : CallGraphSCCPass(&ID) {}
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// runOnSCC - Analyze the SCC, performing the transformation if possible.
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bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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CallGraphSCCPass::getAnalysisUsage(AU);
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}
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};
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}
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char AddReadAttrs::ID = 0;
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static RegisterPass<AddReadAttrs>
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X("addreadattrs", "Mark functions readnone/readonly");
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Pass *llvm::createAddReadAttrsPass() { return new AddReadAttrs(); }
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bool AddReadAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
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SmallPtrSet<CallGraphNode *, 8> SCCNodes;
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CallGraph &CG = getAnalysis<CallGraph>();
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// Fill SCCNodes with the elements of the SCC. Used for quickly
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// looking up whether a given CallGraphNode is in this SCC.
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for (unsigned i = 0, e = SCC.size(); i != e; ++i)
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SCCNodes.insert(SCC[i]);
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// Check if any of the functions in the SCC read or write memory. If they
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// write memory then they can't be marked readnone or readonly.
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bool ReadsMemory = false;
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for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
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Function *F = SCC[i]->getFunction();
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if (F == 0)
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// External node - may write memory. Just give up.
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return false;
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if (F->doesNotAccessMemory())
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// Already perfect!
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continue;
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// Definitions with weak linkage may be overridden at linktime with
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// something that writes memory, so treat them like declarations.
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if (F->isDeclaration() || F->mayBeOverridden()) {
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if (!F->onlyReadsMemory())
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// May write memory. Just give up.
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return false;
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ReadsMemory = true;
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continue;
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}
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// Scan the function body for instructions that may read or write memory.
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for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
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Instruction *I = &*II;
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// Some instructions can be ignored even if they read or write memory.
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// Detect these now, skipping to the next instruction if one is found.
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CallSite CS = CallSite::get(I);
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if (CS.getInstruction()) {
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// Ignore calls to functions in the same SCC.
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if (SCCNodes.count(CG[CS.getCalledFunction()]))
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continue;
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} else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
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Value *Target = LI->getPointerOperand()->getUnderlyingObject();
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// Ignore loads from local memory.
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if (isa<AllocaInst>(Target))
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continue;
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} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
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Value *Target = SI->getPointerOperand()->getUnderlyingObject();
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// Ignore stores to local memory.
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if (isa<AllocaInst>(Target))
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continue;
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}
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// Any remaining instructions need to be taken seriously! Check if they
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// read or write memory.
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if (I->mayWriteToMemory())
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// Writes memory. Just give up.
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return false;
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// If this instruction may read memory, remember that.
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ReadsMemory |= I->mayReadFromMemory();
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}
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}
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// Success! Functions in this SCC do not access memory, or only read memory.
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// Give them the appropriate attribute.
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bool MadeChange = false;
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for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
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Function *F = SCC[i]->getFunction();
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if (F->doesNotAccessMemory())
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// Already perfect!
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continue;
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if (F->onlyReadsMemory() && ReadsMemory)
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// No change.
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continue;
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MadeChange = true;
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// Clear out any existing attributes.
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F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);
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// Add in the new attribute.
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F->addAttribute(~0, ReadsMemory ? Attribute::ReadOnly : Attribute::ReadNone);
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if (ReadsMemory)
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NumReadOnly++;
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else
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NumReadNone++;
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}
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return MadeChange;
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}
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