[Attr] Add "willreturn" function attribute

This patch introduces a new function attribute, willreturn, to indicate
that a call of this function will either exhibit undefined behavior or
comes back and continues execution at a point in the existing call stack
that includes the current invocation.

This attribute guarantees that the function does not have any endless
loops, endless recursion, or terminating functions like abort or exit.

Patch by Hideto Ueno (@uenoku)

Reviewers: jdoerfert

Subscribers: mehdi_amini, hiraditya, steven_wu, dexonsmith, lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62801

llvm-svn: 364555

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1272,6 +1272,8 @@ static uint64_t getRawAttributeMask(Attribute::AttrKind Val) {
     return 1ULL << 60;
   case Attribute::ImmArg:
     return 1ULL << 61;
+  case Attribute::WillReturn:
+    return 1ULL << 62;
   case Attribute::Dereferenceable:
     llvm_unreachable("dereferenceable attribute not supported in raw format");
     break;
@@ -1510,6 +1512,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::SwiftSelf;
   case bitc::ATTR_KIND_UW_TABLE:
     return Attribute::UWTable;
+  case bitc::ATTR_KIND_WILLRETURN:
+    return Attribute::WillReturn;
   case bitc::ATTR_KIND_WRITEONLY:
     return Attribute::WriteOnly;
   case bitc::ATTR_KIND_Z_EXT: