Check-in new alias analysis infrastructure

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3465 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/AliasAnalysis.cpp

@@ -0,0 +1,155 @@
+//===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
+//
+// This file implements the generic AliasAnalysis interface which is used as the
+// common interface used by all clients and implementations of alias analysis.
+//
+// This file also implements the default version of the AliasAnalysis interface
+// that is to be used when no other implementation is specified.  This does some
+// simple tests that detect obvious cases: two different global pointers cannot
+// alias, a global cannot alias a malloc, two different mallocs cannot alias,
+// etc.
+//
+// This alias analysis implementation really isn't very good for anything, but
+// it is very fast, and makes a nice clean default implementation.  Because it
+// handles lots of little corner cases, other, more complex, alias analysis
+// implementations may choose to rely on this pass to resolve these simple and
+// easy cases.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/BasicAliasAnalysis.h"
+#include "llvm/BasicBlock.h"
+#include "llvm/Support/InstVisitor.h"
+#include "llvm/iMemory.h"
+#include "llvm/Constants.h"
+#include "llvm/GlobalValue.h"
+#include "llvm/Pass.h"
+
+// Register the AliasAnalysis interface, providing a nice name to refer to.
+static RegisterAnalysisGroup<AliasAnalysis> X("Alias Analysis");
+
+// CanModify - Define a little visitor class that is used to check to see if
+// arbitrary chunks of code can modify a specified pointer.
+//
+namespace {
+  struct CanModify : public InstVisitor<CanModify, bool> {
+    const AliasAnalysis &AA;
+    const Value *Ptr;
+
+    CanModify(const AliasAnalysis *aa, const Value *ptr)
+      : AA(*aa), Ptr(ptr) {}
+
+    bool visitInvokeInst(InvokeInst &II) {
+      return AA.canInvokeModify(II, Ptr);
+    }
+    bool visitCallInst(CallInst &CI) {
+      return AA.canCallModify(CI, Ptr);
+    }
+    bool visitStoreInst(StoreInst &SI) {
+      assert(!SI.hasIndices() && "Only support stores without indexing!");
+      return AA.alias(Ptr, SI.getOperand(1));
+    }
+
+    // Other instructions do not alias anything.
+    bool visitInstruction(Instruction &I) { return false; }
+  };
+}
+
+// AliasAnalysis destructor: DO NOT move this to the header file for
+// AliasAnalysis or else clients of the AliasAnalysis class may not depend on
+// the AliasAnalysis.o file in the current .a file, causing alias analysis
+// support to not be included in the tool correctly!
+//
+AliasAnalysis::~AliasAnalysis() {}
+
+// canBasicBlockModify - Return true if it is possible for execution of the
+// specified basic block to modify the value pointed to by Ptr.
+//
+bool AliasAnalysis::canBasicBlockModify(const BasicBlock &bb,
+                                        const Value *Ptr) const {
+  CanModify CM(this, Ptr);
+  BasicBlock &BB = const_cast<BasicBlock&>(bb);
+
+  for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I)
+    if (CM.visit(I))        // Check every instruction in the basic block...
+      return true;
+
+  return false;
+}
+
+// canInstructionRangeModify - Return true if it is possible for the execution
+// of the specified instructions to modify the value pointed to by Ptr.  The
+// instructions to consider are all of the instructions in the range of [I1,I2]
+// INCLUSIVE.  I1 and I2 must be in the same basic block.
+//
+bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
+                                              const Instruction &I2,
+                                              const Value *Ptr) const {
+  assert(I1.getParent() == I2.getParent() &&
+         "Instructions not in same basic block!");
+  CanModify CM(this, Ptr);
+  BasicBlock::iterator I = const_cast<Instruction*>(&I1);
+  BasicBlock::iterator E = const_cast<Instruction*>(&I2);
+  ++E;  // Convert from inclusive to exclusive range.
+
+  for (; I != E; ++I)
+    if (CM.visit(I))        // Check every instruction in the basic block...
+      return true;
+
+  return false;
+}
+
+//===----------------------------------------------------------------------===//
+// BasicAliasAnalysis Pass Implementation
+//===----------------------------------------------------------------------===//
+//
+// Because of the way .a files work, the implementation of the
+// BasicAliasAnalysis class MUST be in the AliasAnalysis file itself, or else we
+// run the risk of AliasAnalysis being used, but the default implementation not
+// being linked into the tool that uses it.  As such, we register and implement
+// the class here.
+//
+namespace {
+  // Register this pass...
+  RegisterOpt<BasicAliasAnalysis>
+  X("basicaa", "Basic Alias Analysis (default AA impl)");
+
+  // Declare that we implement the AliasAnalysis interface
+  RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y;
+}  // End of anonymous namespace
+
+
+
+// hasUniqueAddress - Return true if the 
+static inline bool hasUniqueAddress(const Value *V) {
+  return isa<GlobalValue>(V) || isa<MallocInst>(V) || isa<AllocaInst>(V);
+}
+
+AliasAnalysis::Result BasicAliasAnalysis::alias(const Value *V1,
+                                                const Value *V2) const {
+  // Strip off constant pointer refs if they exist
+  if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
+    V1 = CPR->getValue();
+  if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
+    V2 = CPR->getValue();
+
+  // Are we checking for alias of the same value?
+  if (V1 == V2) return MustAlias;
+
+  if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
+    return NoAlias;  // Scalars cannot alias each other
+
+  bool V1Unique = hasUniqueAddress(V1);
+  bool V2Unique = hasUniqueAddress(V2);
+
+  if (V1Unique && V2Unique)
+    return NoAlias;         // Can't alias if they are different unique values
+
+  if ((V1Unique && isa<ConstantPointerNull>(V2)) ||
+      (V2Unique && isa<ConstantPointerNull>(V1)))
+    return NoAlias;         // Unique values don't alias null
+
+  // TODO: Handle getelementptr with nonzero offset
+
+  return MayAlias;
+}