//
// 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.
+// a type system of a higher level language. This can be used to implement
+// typical C/C++ TBAA, but it can also be used to implement custom alias
+// analysis behavior for other languages.
//
-// 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.
+// The current metadata format is very simple. TBAA MDNodes have up to
+// three fields, e.g.:
+// !0 = metadata !{ metadata !"an example type tree" }
+// !1 = metadata !{ metadata !"int", metadata !0 }
+// !2 = metadata !{ metadata !"float", metadata !0 }
+// !3 = metadata !{ metadata !"const float", metadata !2, i64 1 }
//
-// This is a work-in-progress. It doesn't work yet, and the metadata
-// format isn't stable.
+// The first field is an identity field. It can be any value, usually
+// an MDString, which uniquely identifies the type. The most important
+// name in the tree is the name of the root node. Two trees with
+// different root node names are entirely disjoint, even if they
+// have leaves with common names.
//
-// TODO: getModRefBehavior. The AliasAnalysis infrastructure will need to
-// be extended.
-// TODO: struct fields
+// The second field identifies the type's parent node in the tree, or
+// is null or omitted for a root node. A type is considered to alias
+// all of its decendents and all of its ancestors in the tree. Also,
+// a type is considered to alias all types in other trees, so that
+// bitcode produced from multiple front-ends is handled conservatively.
+//
+// If the third field is present, it's an integer which if equal to 1
+// indicates that the type is "constant" (meaning pointsToConstantMemory
+// should return true; see
+// http://llvm.org/docs/AliasAnalysis.html#OtherItfs).
+//
+// TODO: The current metadata format doesn't support struct
+// fields. For example:
+// struct X {
+// double d;
+// int i;
+// };
+// void foo(struct X *x, struct X *y, double *p) {
+// *x = *y;
+// *p = 0.0;
+// }
+// Struct X has a double member, so the store to *x can alias the store to *p.
+// Currently it's not possible to precisely describe all the things struct X
+// aliases, so struct assignments must use conservative TBAA nodes. There's
+// no scheme for attaching metadata to @llvm.memcpy yet either.
//
//===----------------------------------------------------------------------===//
#include "llvm/Module.h"
#include "llvm/Metadata.h"
#include "llvm/Pass.h"
+#include "llvm/Support/CommandLine.h"
using namespace llvm;
+// For testing purposes, enable TBAA only via a special option.
+static cl::opt<bool> EnableTBAA("enable-tbaa");
+
namespace {
/// TBAANode - This is a simple wrapper around an MDNode which provides a
/// higher-level interface by hiding the details of how alias analysis
/// getNode - Get the MDNode for this TBAANode.
const MDNode *getNode() const { return Node; }
- /// getParent - Get this TBAANode's Alias DAG parent.
+ /// getParent - Get this TBAANode's Alias tree parent.
TBAANode getParent() const {
if (Node->getNumOperands() < 2)
return TBAANode();
ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(2));
if (!CI)
return false;
- // TODO: Think about the encoding.
- return CI->isOne();
+ return CI->getValue()[0];
}
};
}
public AliasAnalysis {
public:
static char ID; // Class identification, replacement for typeinfo
- TypeBasedAliasAnalysis() : ImmutablePass(ID) {}
+ TypeBasedAliasAnalysis() : ImmutablePass(ID) {
+ initializeTypeBasedAliasAnalysisPass(*PassRegistry::getPassRegistry());
+ }
virtual void initializePass() {
InitializeAliasAnalysis(this);
return this;
}
+ bool Aliases(const MDNode *A, const MDNode *B) const;
+
private:
virtual void getAnalysisUsage(AnalysisUsage &AU) const;
virtual AliasResult alias(const Location &LocA, const Location &LocB);
AliasAnalysis::getAnalysisUsage(AU);
}
-AliasAnalysis::AliasResult
-TypeBasedAliasAnalysis::alias(const Location &LocA,
- const Location &LocB) {
- // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
- // be conservative.
- const MDNode *AM = LocA.TBAATag;
- if (!AM) return AliasAnalysis::alias(LocA, LocB);
- const MDNode *BM = LocB.TBAATag;
- if (!BM) return AliasAnalysis::alias(LocA, LocB);
-
+/// Aliases - Test whether the type represented by A may alias the
+/// type represented by B.
+bool
+TypeBasedAliasAnalysis::Aliases(const MDNode *A,
+ const MDNode *B) const {
// 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)
+ // Climb the tree from A to see if we reach B.
+ for (TBAANode T(A); ; ) {
+ if (T.getNode() == B)
// B is an ancestor of A.
- return AliasAnalysis::alias(LocA, LocB);
+ return true;
RootA = T;
T = T.getParent();
break;
}
- // Climb the DAG from B to see if we reach A.
- for (TBAANode T(BM); ; ) {
- if (T.getNode() == AM)
+ // Climb the tree from B to see if we reach A.
+ for (TBAANode T(B); ; ) {
+ if (T.getNode() == A)
// A is an ancestor of B.
- return AliasAnalysis::alias(LocA, LocB);
+ return true;
RootB = T;
T = T.getParent();
// 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 AliasAnalysis::alias(LocA, LocB);
+ if (RootA.getNode() != RootB.getNode())
+ return true;
+
+ // If they have the same root, then we've proved there's no alias.
+ return false;
+}
+
+AliasAnalysis::AliasResult
+TypeBasedAliasAnalysis::alias(const Location &LocA,
+ const Location &LocB) {
+ if (!EnableTBAA)
+ return AliasAnalysis::alias(LocA, LocB);
+
+ // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
+ // be conservative.
+ const MDNode *AM = LocA.TBAATag;
+ if (!AM) return AliasAnalysis::alias(LocA, LocB);
+ const MDNode *BM = LocB.TBAATag;
+ if (!BM) return AliasAnalysis::alias(LocA, LocB);
+
+ // If they may alias, chain to the next AliasAnalysis.
+ if (Aliases(AM, BM))
+ return AliasAnalysis::alias(LocA, LocB);
+
+ // Otherwise return a definitive result.
+ return NoAlias;
}
bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
+ if (!EnableTBAA)
+ return AliasAnalysis::pointsToConstantMemory(Loc);
+
const MDNode *M = Loc.TBAATag;
if (!M) return false;
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