Rename AddReadAttrs to FunctionAttrs, and teach it how

to work out (in a very simplistic way) which function
arguments (pointer arguments only) are only dereferenced
and so do not escape.  Mark such arguments 'nocapture'.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61525 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 280 insertions, 0 deletions

diff --git a/lib/Transforms/IPO/FunctionAttrs.cpp b/lib/Transforms/IPO/FunctionAttrs.cpp
new file mode 100644
index 0000000000..9ed605cf9f
--- /dev/null
+++ b/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -0,0 +1,280 @@
+//===- 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.  It implements
+// this 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/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");
+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 - Returns whether this pointer value is 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 - Returns whether this pointer value is captured.
+bool FunctionAttrs::isCaptured(Function &F, Value *V) {
+  SmallVector<Use*, 16> Worklist;
+  SmallPtrSet<Use*, 16> Visited;
+
+  for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE;
+       ++UI) {
+    Use *U = &UI.getUse();
+    Visited.insert(U);
+    Worklist.push_back(U);
+  }
+
+  while (!Worklist.empty()) {
+    Use *U = Worklist.pop_back_val();
+    Instruction *I = cast<Instruction>(U->getUser());
+    V = U->get();
+
+    if (isa<LoadInst>(I)) {
+      // Loading a pointer does not cause it to escape.
+      continue;
+    }
+
+    if (isa<StoreInst>(I)) {
+      if (V == I->getOperand(0))
+        // Stored the pointer - escapes.  TODO: improve this.
+        return true;
+      // Storing to the pointee does not cause the pointer to escape.
+      continue;
+    }
+
+    CallSite CS = CallSite::get(I);
+    if (CS.getInstruction()) {
+      // Does not escape if only passed via 'nocapture' arguments.  Note
+      // that calling a function pointer does not in itself cause that
+      // function pointer to escape.  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 escape, 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' - escapes.
+          return true;
+      // Only passed via 'nocapture' arguments, or is the called function.
+      // Does not escape.
+      continue;
+    }
+
+    if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I)) {
+      // Type conversion or calculating an offset.  Does not escape if the new
+      // value doesn't.
+      for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
+           UI != UE; ++UI) {
+        Use *U = &UI.getUse();
+        if (Visited.insert(U))
+          Worklist.push_back(U);
+      }
+      continue;
+    }
+
+    // Something else - be conservative and say it escapes.
+    return true;
+  }
+
+  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()) && !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;
+}