[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, 6 insertions, 0 deletions

diff --git a/llvm/test/Bitcode/attributes.ll b/llvm/test/Bitcode/attributes.ll
index c75ee80dc66..9fbdfeb0ef0 100644
--- a/llvm/test/Bitcode/attributes.ll
+++ b/llvm/test/Bitcode/attributes.ll
@@ -245,6 +245,12 @@ define void @f41(i8* align 32, double* align 64) {
         ret void
 }
 
+; CHECK: define dereferenceable_or_null(8) i8* @f42(i8* dereferenceable_or_null(8) %foo)
+define dereferenceable_or_null(8) i8* @f42(i8* dereferenceable_or_null(8) %foo) {
+ entry:
+  ret i8* %foo
+}
+
 ; CHECK: attributes #0 = { noreturn }
 ; CHECK: attributes #1 = { nounwind }
 ; CHECK: attributes #2 = { readnone }