IR: New representation for CFI and virtual call optimization pass metadata.

The bitset metadata currently used in LLVM has a few problems:

1. It has the wrong name. The name "bitset" refers to an implementation
   detail of one use of the metadata (i.e. its original use case, CFI).
   This makes it harder to understand, as the name makes no sense in the
   context of virtual call optimization.

2. It is represented using a global named metadata node, rather than
   being directly associated with a global. This makes it harder to
   manipulate the metadata when rebuilding global variables, summarise it
   as part of ThinLTO and drop unused metadata when associated globals are
   dropped. For this reason, CFI does not currently work correctly when
   both CFI and vcall opt are enabled, as vcall opt needs to rebuild vtable
   globals, and fails to associate metadata with the rebuilt globals. As I
   understand it, the same problem could also affect ASan, which rebuilds
   globals with a red zone.

This patch solves both of those problems in the following way:

1. Rename the metadata to "type metadata". This new name reflects how
   the metadata is currently being used (i.e. to represent type information
   for CFI and vtable opt). The new name is reflected in the name for the
   associated intrinsic (llvm.type.test) and pass (LowerTypeTests).

2. Attach metadata directly to the globals that it pertains to, rather
   than using the "llvm.bitsets" global metadata node as we are doing now.
   This is done using the newly introduced capability to attach
   metadata to global variables (r271348 and r271358).

See also: http://lists.llvm.org/pipermail/llvm-dev/2016-June/100462.html

Differential Revision: http://reviews.llvm.org/D21053

llvm-svn: 273729

1 files changed, 6 insertions, 12 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 42ee5bf7262..3cdddcb1eb6 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -4839,12 +4839,6 @@ the loop identifier metadata node directly:
    !1 = !{!1} ; an identifier for the inner loop
    !2 = !{!2} ; an identifier for the outer loop
 
-'``llvm.bitsets``'
-^^^^^^^^^^^^^^^^^^
-
-The ``llvm.bitsets`` global metadata is used to implement
-:doc:`bitsets <BitSets>`.
-
 '``invariant.group``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -12274,9 +12268,9 @@ sufficient overall improvement in code quality. For this reason,
 that the optimizer can otherwise deduce or facts that are of little use to the
 optimizer.
 
-.. _bitset.test:
+.. _type.test:
 
-'``llvm.bitset.test``' Intrinsic
+'``llvm.type.test``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
@@ -12284,20 +12278,20 @@ Syntax:
 
 ::
 
-      declare i1 @llvm.bitset.test(i8* %ptr, metadata %bitset) nounwind readnone
+      declare i1 @llvm.type.test(i8* %ptr, metadata %type) nounwind readnone
 
 
 Arguments:
 """"""""""
 
 The first argument is a pointer to be tested. The second argument is a
-metadata object representing an identifier for a :doc:`bitset <BitSets>`.
+metadata object representing a :doc:`type identifier <TypeMetadata>`.
 
 Overview:
 """""""""
 
-The ``llvm.bitset.test`` intrinsic tests whether the given pointer is a
-member of the given bitset.
+The ``llvm.type.test`` intrinsic tests whether the given pointer is associated
+with the given type identifier.
 
 '``llvm.donothing``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^