//
// 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 descendants 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/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Passes.h"
+#include "llvm/Constants.h"
+#include "llvm/LLVMContext.h"
#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");
+// A handy option for disabling TBAA functionality. The same effect can also be
+// achieved by stripping the !tbaa tags from IR, but this option is sometimes
+// more convenient.
+static cl::opt<bool> EnableTBAA("enable-tbaa", cl::init(true));
namespace {
/// TBAANode - This is a simple wrapper around an MDNode which provides a
ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(2));
if (!CI)
return false;
- // TODO: Think about the encoding.
- return CI->isOne();
+ return CI->getValue()[0];
}
};
}
private:
virtual void getAnalysisUsage(AnalysisUsage &AU) const;
virtual AliasResult alias(const Location &LocA, const Location &LocB);
- virtual bool pointsToConstantMemory(const Location &Loc);
+ virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
+ virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
+ virtual ModRefBehavior getModRefBehavior(const Function *F);
+ virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
+ const Location &Loc);
+ virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
+ ImmutableCallSite CS2);
};
} // End of anonymous namespace
return NoAlias;
}
-bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
+bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc,
+ bool OrLocal) {
if (!EnableTBAA)
- return AliasAnalysis::pointsToConstantMemory(Loc);
+ return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
const MDNode *M = Loc.TBAATag;
- if (!M) return false;
+ if (!M) return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
// If this is an "immutable" type, we can assume the pointer is pointing
// to constant memory.
if (TBAANode(M).TypeIsImmutable())
return true;
- return AliasAnalysis::pointsToConstantMemory(Loc);
+ return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
+}
+
+AliasAnalysis::ModRefBehavior
+TypeBasedAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
+ if (!EnableTBAA)
+ return AliasAnalysis::getModRefBehavior(CS);
+
+ ModRefBehavior Min = UnknownModRefBehavior;
+
+ // If this is an "immutable" type, we can assume the call doesn't write
+ // to memory.
+ if (const MDNode *M = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
+ if (TBAANode(M).TypeIsImmutable())
+ Min = OnlyReadsMemory;
+
+ return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
+}
+
+AliasAnalysis::ModRefBehavior
+TypeBasedAliasAnalysis::getModRefBehavior(const Function *F) {
+ // Functions don't have metadata. Just chain to the next implementation.
+ return AliasAnalysis::getModRefBehavior(F);
+}
+
+AliasAnalysis::ModRefResult
+TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
+ const Location &Loc) {
+ if (!EnableTBAA)
+ return AliasAnalysis::getModRefInfo(CS, Loc);
+
+ if (const MDNode *L = Loc.TBAATag)
+ if (const MDNode *M =
+ CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
+ if (!Aliases(L, M))
+ return NoModRef;
+
+ return AliasAnalysis::getModRefInfo(CS, Loc);
+}
+
+AliasAnalysis::ModRefResult
+TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
+ ImmutableCallSite CS2) {
+ if (!EnableTBAA)
+ return AliasAnalysis::getModRefInfo(CS1, CS2);
+
+ if (const MDNode *M1 =
+ CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
+ if (const MDNode *M2 =
+ CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
+ if (!Aliases(M1, M2))
+ return NoModRef;
+
+ return AliasAnalysis::getModRefInfo(CS1, CS2);
}
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