[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, 28 insertions, 6 deletions

diff --git a/llvm/lib/AsmParser/LLParser.cpp b/llvm/lib/AsmParser/LLParser.cpp
index 64ed2a2b7e3..2b306dbd642 100644
--- a/llvm/lib/AsmParser/LLParser.cpp
+++ b/llvm/lib/AsmParser/LLParser.cpp
@@ -976,6 +976,7 @@ bool LLParser::ParseFnAttributeValuePairs(AttrBuilder &B,
       break;
     case lltok::kw_byval:
     case lltok::kw_dereferenceable:
+    case lltok::kw_dereferenceable_or_null:
     case lltok::kw_inalloca:
     case lltok::kw_nest:
     case lltok::kw_noalias:
@@ -1220,11 +1221,18 @@ bool LLParser::ParseOptionalParamAttrs(AttrBuilder &B) {
     case lltok::kw_byval:           B.addAttribute(Attribute::ByVal); break;
     case lltok::kw_dereferenceable: {
       uint64_t Bytes;
-      if (ParseOptionalDereferenceableBytes(Bytes))
+      if (ParseOptionalDerefAttrBytes(lltok::kw_dereferenceable, Bytes))
         return true;
       B.addDereferenceableAttr(Bytes);
       continue;
     }
+    case lltok::kw_dereferenceable_or_null: {
+      uint64_t Bytes;
+      if (ParseOptionalDerefAttrBytes(lltok::kw_dereferenceable_or_null, Bytes))
+        return true;
+      B.addDereferenceableOrNullAttr(Bytes);
+      continue;
+    }
     case lltok::kw_inalloca:        B.addAttribute(Attribute::InAlloca); break;
     case lltok::kw_inreg:           B.addAttribute(Attribute::InReg); break;
     case lltok::kw_nest:            B.addAttribute(Attribute::Nest); break;
@@ -1284,11 +1292,18 @@ bool LLParser::ParseOptionalReturnAttrs(AttrBuilder &B) {
       return HaveError;
     case lltok::kw_dereferenceable: {
       uint64_t Bytes;
-      if (ParseOptionalDereferenceableBytes(Bytes))
+      if (ParseOptionalDerefAttrBytes(lltok::kw_dereferenceable, Bytes))
         return true;
       B.addDereferenceableAttr(Bytes);
       continue;
     }
+    case lltok::kw_dereferenceable_or_null: {
+      uint64_t Bytes;
+      if (ParseOptionalDerefAttrBytes(lltok::kw_dereferenceable_or_null, Bytes))
+        return true;
+      B.addDereferenceableOrNullAttr(Bytes);
+      continue;
+    }
     case lltok::kw_inreg:           B.addAttribute(Attribute::InReg); break;
     case lltok::kw_noalias:         B.addAttribute(Attribute::NoAlias); break;
     case lltok::kw_nonnull:         B.addAttribute(Attribute::NonNull); break;
@@ -1516,12 +1531,19 @@ bool LLParser::ParseOptionalAlignment(unsigned &Alignment) {
   return false;
 }
 
-/// ParseOptionalDereferenceableBytes
+/// ParseOptionalDerefAttrBytes
 ///   ::= /* empty */
-///   ::= 'dereferenceable' '(' 4 ')'
-bool LLParser::ParseOptionalDereferenceableBytes(uint64_t &Bytes) {
+///   ::= AttrKind '(' 4 ')'
+///
+/// where AttrKind is either 'dereferenceable' or 'dereferenceable_or_null'.
+bool LLParser::ParseOptionalDerefAttrBytes(lltok::Kind AttrKind,
+                                           uint64_t &Bytes) {
+  assert((AttrKind == lltok::kw_dereferenceable ||
+          AttrKind == lltok::kw_dereferenceable_or_null) &&
+         "contract!");
+
   Bytes = 0;
-  if (!EatIfPresent(lltok::kw_dereferenceable))
+  if (!EatIfPresent(AttrKind))
     return false;
   LocTy ParenLoc = Lex.getLoc();
   if (!EatIfPresent(lltok::lparen))