Introduce a typing refinenement on tagged data

using the 'volatile' qualifier. This should not have any operational consequences
on code, because tags should always be stripped off (giving a non-volatile pointer)
before dereferencing. The new qualification is there to catch some attempts to use
tagged pointers in a context where an untagged pointer is appropriate.

Notably this approach does not catch dereferencing of tagged pointers, but helps
in separating the two concepts a bit.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57641 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 8 insertions, 10 deletions

diff --git a/include/llvm/Use.h b/include/llvm/Use.h
index d966703..f88d435 100644
--- a/include/llvm/Use.h
+++ b/include/llvm/Use.h
@@ -34,27 +34,27 @@ enum Tag { noTag, tagOne, tagTwo, tagThree };
 
 /// addTag - insert tag bits into an (untagged) pointer
 template <typename T, typename TAG>
-inline T *addTag(const T *P, TAG Tag) {
+inline volatile T *addTag(const T *P, TAG Tag) {
     return reinterpret_cast<T*>(ptrdiff_t(P) | Tag);
 }
 
 /// stripTag - remove tag bits from a pointer,
 /// making it dereferencable
 template <ptrdiff_t MASK, typename T>
-inline T *stripTag(const T *P) {
+inline T *stripTag(const volatile T *P) {
   return reinterpret_cast<T*>(ptrdiff_t(P) & ~MASK);
 }
 
 /// extractTag - extract tag bits from a pointer
 template <typename TAG, TAG MASK, typename T>
-inline TAG extractTag(const T *P) {
+inline TAG extractTag(const volatile T *P) {
   return TAG(ptrdiff_t(P) & MASK);
 }
 
 /// transferTag - transfer tag bits from a pointer,
 /// to an untagged pointer
 template <ptrdiff_t MASK, typename T>
-inline T *transferTag(const T *From, const T *To) {
+inline volatile T *transferTag(const volatile T *From, const T *To) {
   return reinterpret_cast<T*>((ptrdiff_t(From) & MASK) | ptrdiff_t(To));
 }
 
@@ -126,7 +126,7 @@ private:
   static Use *initTags(Use *Start, Use *Stop, ptrdiff_t Done = 0);
   
   Value *Val;
-  Use *Next, **Prev;
+  Use *Next, *volatile*Prev;
 
   void setPrev(Use **NewPrev) {
     Prev = transferTag<fullStopTag>(Prev, NewPrev);
diff --git a/lib/VMCore/Use.cpp b/lib/VMCore/Use.cpp
index d96a0e5..5eadaad 100644
--- a/lib/VMCore/Use.cpp
+++ b/lib/VMCore/Use.cpp
@@ -127,7 +127,7 @@ void Use::zap(Use *Start, const Use *Stop, bool del) {
 //===----------------------------------------------------------------------===//
 
 struct AugmentedUse : Use {
-  User *ref;
+  volatile User *ref;
   AugmentedUse(); // not implemented
 };
 
@@ -138,12 +138,10 @@ struct AugmentedUse : Use {
 
 User *Use::getUser() const {
   const Use *End = getImpliedUser();
-  User *She = static_cast<const AugmentedUse*>(End - 1)->ref;
-  She = extractTag<Tag, tagOne>(She)
+  volatile User *She = static_cast<const AugmentedUse*>(End - 1)->ref;
+  return extractTag<Tag, tagOne>(She)
       ? llvm::stripTag<tagOne>(She)
       : reinterpret_cast<User*>(const_cast<Use*>(End));
-
-  return She;
 }
 
 //===----------------------------------------------------------------------===//