[IR] Introduce a dereferenceable_or_null(N) attribute.

Summary:
If a pointer is marked as dereferenceable_or_null(N), LLVM assumes it
is either `null` or `dereferenceable(N)` or both.  This change only
introduces the attribute and adds a token test case for the `llvm-as`
/ `llvm-dis`.  It does not hook up other parts of the optimizer to
actually exploit the attribute -- those changes will come later.

For pointers in address space 0, `dereferenceable(N)` is now exactly
equivalent to `dereferenceable_or_null(N)` && `nonnull`.  For other
address spaces, `dereferenceable(N)` is potentially weaker than
`dereferenceable_or_null(N)` && `nonnull` (since we could have a null
`dereferenceable(N)` pointer).

The motivating case for this change is Java (and other managed
languages), where pointers are either `null` or dereferenceable up to
some usually known-at-compile-time constant offset.

Reviewers: rafael, hfinkel

Reviewed By: hfinkel

Subscribers: nicholas, llvm-commits

Differential Revision: http://reviews.llvm.org/D8650

llvm-svn: 235132

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index 8e3ddaa4b1b..85b7521f316 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -352,6 +352,12 @@ void CallInst::addDereferenceableAttr(unsigned i, uint64_t Bytes) {
   setAttributes(PAL);
 }
 
+void CallInst::addDereferenceableOrNullAttr(unsigned i, uint64_t Bytes) {
+  AttributeSet PAL = getAttributes();
+  PAL = PAL.addDereferenceableOrNullAttr(getContext(), i, Bytes);
+  setAttributes(PAL);
+}
+
 bool CallInst::hasFnAttrImpl(Attribute::AttrKind A) const {
   if (AttributeList.hasAttribute(AttributeSet::FunctionIndex, A))
     return true;
@@ -617,6 +623,12 @@ void InvokeInst::addDereferenceableAttr(unsigned i, uint64_t Bytes) {
   setAttributes(PAL);
 }
 
+void InvokeInst::addDereferenceableOrNullAttr(unsigned i, uint64_t Bytes) {
+  AttributeSet PAL = getAttributes();
+  PAL = PAL.addDereferenceableOrNullAttr(getContext(), i, Bytes);
+  setAttributes(PAL);
+}
+
 LandingPadInst *InvokeInst::getLandingPadInst() const {
   return cast<LandingPadInst>(getUnwindDest()->getFirstNonPHI());
 }