Sketch up a preliminary Type-Based Alias Analysis implementation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110077 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 191 insertions, 0 deletions

diff --git a/lib/Analysis/TypeBasedAliasAnalysis.cpp b/lib/Analysis/TypeBasedAliasAnalysis.cpp
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+//===- TypeBasedAliasAnalysis.cpp - Type-Based Alias Analysis -------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the TypeBasedAliasAnalysis pass, which implements
+// metadata-based TBAA.
+//
+// In LLVM IR, memory does not have types, so LLVM's own type system is not
+// suitable for doing TBAA. Instead, metadata is added to the IR to describe
+// a type system of a higher level language.
+//
+// This pass is language-independent. The type system is encoded in
+// metadata. This allows this pass to support typical C and C++ TBAA, but
+// it can also support custom aliasing behavior for other languages.
+//
+// This is a work-in-progress. It doesn't work yet, and the metadata
+// format isn't stable.
+//
+// TODO: getModRefBehavior. The AliasAnalysis infrastructure will need to
+//       be extended.
+// TODO: AA chaining
+// TODO: struct fields
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/Passes.h"
+#include "llvm/Module.h"
+#include "llvm/Metadata.h"
+#include "llvm/Pass.h"
+using namespace llvm;
+
+namespace {
+  /// TBAANode - This is a simple wrapper around an MDNode which provides a
+  /// higher-level interface by hiding the details of how alias analysis
+  /// information is encoded in its operands.
+  class TBAANode {
+    const MDNode *Node;
+
+  public:
+    TBAANode() : Node(0) {}
+    explicit TBAANode(MDNode *N) : Node(N) {}
+
+    /// getNode - Get the MDNode for this TBAANode.
+    const MDNode *getNode() const { return Node; }
+
+    /// getParent - Get this TBAANode's Alias DAG parent.
+    TBAANode getParent() const {
+      if (Node->getNumOperands() < 2)
+        return TBAANode();
+      MDNode *P = dyn_cast<MDNode>(Node->getOperand(1));
+      if (!P)
+        return TBAANode();
+      // Ok, this node has a valid parent. Return it.
+      return TBAANode(P);
+    }
+
+    /// TypeIsImmutable - Test if this TBAANode represents a type for objects
+    /// which are not modified (by any means) in the context where this
+    /// AliasAnalysis is relevant.
+    bool TypeIsImmutable() const {
+      if (Node->getNumOperands() < 3)
+        return false;
+      ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(2));
+      if (!CI)
+        return false;
+      // TODO: Think about the encoding.
+      return CI->isOne();
+    }
+  };
+}
+
+namespace {
+  /// TypeBasedAliasAnalysis - This is a simple alias analysis
+  /// implementation that uses TypeBased to answer queries.
+  class TypeBasedAliasAnalysis : public ImmutablePass,
+                                 public AliasAnalysis {
+  public:
+    static char ID; // Class identification, replacement for typeinfo
+    TypeBasedAliasAnalysis() : ImmutablePass(&ID) {}
+
+    /// getAdjustedAnalysisPointer - This method is used when a pass implements
+    /// an analysis interface through multiple inheritance.  If needed, it
+    /// should override this to adjust the this pointer as needed for the
+    /// specified pass info.
+    virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) {
+      if (PI->isPassID(&AliasAnalysis::ID))
+        return (AliasAnalysis*)this;
+      return this;
+    }
+
+  private:
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
+    virtual AliasResult alias(const Value *V1, unsigned V1Size,
+                              const Value *V2, unsigned V2Size);
+    virtual bool pointsToConstantMemory(const Value *P);
+  };
+}  // End of anonymous namespace
+
+// Register this pass...
+char TypeBasedAliasAnalysis::ID = 0;
+INITIALIZE_AG_PASS(TypeBasedAliasAnalysis, AliasAnalysis, "tbaa",
+                   "Type-Based Alias Analysis", false, true, false);
+
+ImmutablePass *llvm::createTypeBasedAliasAnalysisPass() {
+  return new TypeBasedAliasAnalysis();
+}
+
+void
+TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.setPreservesAll();
+  AliasAnalysis::getAnalysisUsage(AU);
+}
+
+AliasAnalysis::AliasResult
+TypeBasedAliasAnalysis::alias(const Value *A, unsigned ASize,
+                              const Value *B, unsigned BSize) {
+  // Currently, metadata can only be attached to Instructions.
+  const Instruction *AI = dyn_cast<Instruction>(A);
+  if (!AI) return MayAlias;
+  const Instruction *BI = dyn_cast<Instruction>(B);
+  if (!BI) return MayAlias;
+
+  // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
+  // be conservative.
+  MDNode *AM =
+    AI->getMetadata(AI->getParent()->getParent()->getParent()
+                      ->getMDKindID("tbaa"));
+  if (!AM) return MayAlias;
+  MDNode *BM =
+    AI->getMetadata(BI->getParent()->getParent()->getParent()
+                      ->getMDKindID("tbaa"));
+  if (!BM) return MayAlias;
+
+  // Keep track of the root node for A and B.
+  TBAANode RootA, RootB;
+
+  // Climb the DAG from A to see if we reach B.
+  for (TBAANode T(AM); ; ) {
+    if (T.getNode() == BM)
+      // B is an ancestor of A.
+      return MayAlias;
+
+    RootA = T;
+    T = T.getParent();
+    if (!T.getNode())
+      break;
+  }
+
+  // Climb the DAG from B to see if we reach A.
+  for (TBAANode T(BM); ; ) {
+    if (T.getNode() == AM)
+      // A is an ancestor of B.
+      return MayAlias;
+
+    RootB = T;
+    T = T.getParent();
+    if (!T.getNode())
+      break;
+  }
+
+  // Neither node is an ancestor of the other.
+  
+  // If they have the same root, then we've proved there's no alias.
+  if (RootA.getNode() == RootB.getNode())
+    return NoAlias;
+
+  // If they have different roots, they're part of different potentially
+  // unrelated type systems, so we must be conservative.
+  return MayAlias;
+}
+
+bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Value *P) {
+  // Currently, metadata can only be attached to Instructions.
+  const Instruction *I = dyn_cast<Instruction>(P);
+  if (!I) return false;
+
+  MDNode *M =
+    I->getMetadata(I->getParent()->getParent()->getParent()
+                    ->getMDKindID("tbaa"));
+  if (!M) return false;
+
+  // If this is an "immutable" type, we can assume the pointer is pointing
+  // to constant memory.
+  return TBAANode(M).TypeIsImmutable();
+}