[PATCH] [IR] Introduce a dereferenceable_or_null(N) attribute.

[hfinkel at anl.gov]

================
Comment at: docs/LangRef.rst:1017
@@ +1016,3 @@
+    This indicates that the parameter or return pointer is either
+    ``dereferenceable(<n>)`` or ``null`` or both.  This attribute may
+    only be applied to pointer typed parameters.
----------------
Please explicitly note that it can only be both when not in addrspace(0). When not in addrspace(0), does this attribute imply that null is not dereferenceable (even if it would be otherwise)?

On the other hand, if dereferenceable_or_null(n) is equivalent to dereferenceable(n) for non-addrspace(0) pointers, maybe we should disallow it to avoid confusion.


================
Comment at: lib/AsmParser/LLParser.cpp:1557
@@ +1556,3 @@
+///   ::= 'dereferenceable_or_null' '(' 4 ')'
+bool LLParser::ParseOptionalDereferenceableOrNullBytes(uint64_t &Bytes) {
+  Bytes = 0;
----------------
Please refactor this so it shares common logic with dereferenceable(n).

================
Comment at: lib/IR/Attributes.cpp:307
@@ -293,1 +306,3 @@
 
+  if (hasAttribute(Attribute::DereferenceableOrNull)) {
+    std::string Result;
----------------
We now have three essentially-identical blocks of code here. Please add some lambda function and combine them instead.

================
Comment at: test/Assembler/deref-or-null.ll:1
@@ +1,2 @@
+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
----------------
Don't add another separate test for this. Just add this to test/Bitcode/attributes.ll (along with the tests for dereferenceable).

http://reviews.llvm.org/D8650

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