IR: Introduce local_unnamed_addr attribute.

If a local_unnamed_addr attribute is attached to a global, the address
is known to be insignificant within the module. It is distinct from the
existing unnamed_addr attribute in that it only describes a local property
of the module rather than a global property of the symbol.

This attribute is intended to be used by the code generator and LTO to allow
the linker to decide whether the global needs to be in the symbol table. It is
possible to exclude a global from the symbol table if three things are true:
- This attribute is present on every instance of the global (which means that
  the normal rule that the global must have a unique address can be broken without
  being observable by the program by performing comparisons against the global's
  address)
- The global has linkonce_odr linkage (which means that each linkage unit must have
  its own copy of the global if it requires one, and the copy in each linkage unit
  must be the same)
- It is a constant or a function (which means that the program cannot observe that
  the unique-address rule has been broken by writing to the global)

Although this attribute could in principle be computed from the module
contents, LTO clients (i.e. linkers) will normally need to be able to compute
this property as part of symbol resolution, and it would be inefficient to
materialize every module just to compute it.

See:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160509/356401.html
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160516/356738.html
for earlier discussion.

Part of the fix for PR27553.

Differential Revision: http://reviews.llvm.org/D20348

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272709 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 12 insertions, 6 deletions

diff --git a/docs/BitCodeFormat.rst b/docs/BitCodeFormat.rst
index ac051c49bc8..f9989936a1f 100644
--- a/docs/BitCodeFormat.rst
+++ b/docs/BitCodeFormat.rst
@@ -731,8 +731,14 @@ global variable. The operand fields are:
   * ``initialexec``: code 3
   * ``localexec``: code 4
 
-* *unnamed_addr*: If present and non-zero, indicates that the variable has
-  ``unnamed_addr``
+.. _bcunnamedaddr:
+
+* *unnamed_addr*: If present, an encoding of the ``unnamed_addr`` attribute of this
+  variable:
+
+  * not ``unnamed_addr``: code 0
+  * ``unnamed_addr``: code 1
+  * ``local_unnamed_addr``: code 2
 
 .. _bcdllstorageclass:
 
@@ -791,8 +797,8 @@ function. The operand fields are:
 * *gc*: If present and nonzero, the 1-based garbage collector index in the table
   of `MODULE_CODE_GCNAME`_ entries.
 
-* *unnamed_addr*: If present and non-zero, indicates that the function has
-  ``unnamed_addr``
+* *unnamed_addr*: If present, an encoding of the
+  :ref:`unnamed_addr<bcunnamedaddr>` attribute of this function
 
 * *prologuedata*: If non-zero, the value index of the prologue data for this function,
   plus 1.
@@ -830,8 +836,8 @@ fields are
 * *threadlocal*: If present, an encoding of the
   :ref:`thread local property<bcthreadlocal>` of the alias
 
-* *unnamed_addr*: If present and non-zero, indicates that the alias has
-  ``unnamed_addr``
+* *unnamed_addr*: If present, an encoding of the
+  :ref:`unnamed_addr<bcunnamedaddr>` attribute of this alias
 
 MODULE_CODE_PURGEVALS Record
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^