Decouple dllexport/dllimport from linkage

Representing dllexport/dllimport as distinct linkage types prevents using
these attributes on templates and inline functions.

Instead of introducing further mixed linkage types to include linkonce and
weak ODR, the old import/export linkage types are replaced with a new
separate visibility-like specifier:

  define available_externally dllimport void @f() {}
  @Var = dllexport global i32 1, align 4

Linkage for dllexported globals and functions is now equal to their linkage
without dllexport. Imported globals and functions must be either
declarations with external linkage, or definitions with
AvailableExternallyLinkage.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199204 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 53 insertions, 29 deletions

diff --git a/docs/BitCodeFormat.rst b/docs/BitCodeFormat.rst
index 89531dcfea..9363a62d08 100644
--- a/docs/BitCodeFormat.rst
+++ b/docs/BitCodeFormat.rst
@@ -658,7 +658,7 @@ for each library name referenced.
 MODULE_CODE_GLOBALVAR Record
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-``[GLOBALVAR, pointer type, isconst, initid, linkage, alignment, section, visibility, threadlocal, unnamed_addr]``
+``[GLOBALVAR, pointer type, isconst, initid, linkage, alignment, section, visibility, threadlocal, unnamed_addr, dllstorageclass]``
 
 The ``GLOBALVAR`` record (code 7) marks the declaration or definition of a
 global variable. The operand fields are:
@@ -713,12 +713,20 @@ global variable. The operand fields are:
 * *unnamed_addr*: If present and non-zero, indicates that the variable has
   ``unnamed_addr``
 
+.. _dllstorageclass:
+
+* *dllstorageclass*: If present, an encoding of the DLL storage class of this variable:
+
+  * ``default``: code 0
+  * ``dllimport``: code 1
+  * ``dllexport``: code 2
+
 .. _FUNCTION:
 
 MODULE_CODE_FUNCTION Record
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-``[FUNCTION, type, callingconv, isproto, linkage, paramattr, alignment, section, visibility, gc, prefix]``
+``[FUNCTION, type, callingconv, isproto, linkage, paramattr, alignment, section, visibility, gc, prefix, dllstorageclass]``
 
 The ``FUNCTION`` record (code 8) marks the declaration or definition of a
 function. The operand fields are:
@@ -762,10 +770,12 @@ function. The operand fields are:
 * *prefix*: If non-zero, the value index of the prefix data for this function,
   plus 1.
 
+* *dllstorageclass*: An encoding of the `dllstorageclass`_ of this function
+
 MODULE_CODE_ALIAS Record
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
-``[ALIAS, alias type, aliasee val#, linkage, visibility]``
+``[ALIAS, alias type, aliasee val#, linkage, visibility, dllstorageclass]``
 
 The ``ALIAS`` record (code 9) marks the definition of an alias. The operand
 fields are
@@ -778,6 +788,8 @@ fields are
 
 * *visibility*: If present, an encoding of the `visibility`_ of the alias
 
+* *dllstorageclass*: If present, an encoding of the `dllstorageclass`_ of the alias
+
 MODULE_CODE_PURGEVALS Record
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index ffe3f09d83..3540bdec8a 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -274,27 +274,8 @@ linkage:
     visible, meaning that it participates in linkage and can be used to
     resolve external symbol references.
 
-The next two types of linkage are targeted for Microsoft Windows
-platform only. They are designed to support importing (exporting)
-symbols from (to) DLLs (Dynamic Link Libraries).
-
-``dllimport``
-    "``dllimport``" linkage causes the compiler to reference a function
-    or variable via a global pointer to a pointer that is set up by the
-    DLL exporting the symbol. On Microsoft Windows targets, the pointer
-    name is formed by combining ``__imp_`` and the function or variable
-    name.
-``dllexport``
-    "``dllexport``" linkage causes the compiler to provide a global
-    pointer to a pointer in a DLL, so that it can be referenced with the
-    ``dllimport`` attribute. On Microsoft Windows targets, the pointer
-    name is formed by combining ``__imp_`` and the function or variable
-    name. Since this linkage exists for defining a dll interface, the
-    compiler, assembler and linker know it is externally referenced and
-    must refrain from deleting the symbol.
-
 It is illegal for a function *declaration* to have any linkage type
-other than ``external``, ``dllimport`` or ``extern_weak``.
+other than ``external`` or ``extern_weak``.
 
 .. _callingconv:
 
@@ -416,6 +397,25 @@ styles:
 
 .. _namedtypes:
 
+DLL Storage Classes
+-------------------
+
+All Global Variables, Functions and Aliases can have one of the following
+DLL storage class:
+
+``dllimport``
+    "``dllimport``" causes the compiler to reference a function or variable via
+    a global pointer to a pointer that is set up by the DLL exporting the
+    symbol. On Microsoft Windows targets, the pointer name is formed by
+    combining ``__imp_`` and the function or variable name.
+``dllexport``
+    "``dllexport``" causes the compiler to provide a global pointer to a pointer
+    in a DLL, so that it can be referenced with the ``dllimport`` attribute. On
+    Microsoft Windows targets, the pointer name is formed by combining
+    ``__imp_`` and the function or variable name. Since this storage class
+    exists for defining a dll interface, the compiler, assembler and linker know
+    it is externally referenced and must refrain from deleting the symbol.
+
 Named Types
 -----------
 
@@ -529,6 +529,15 @@ assume that the globals are densely packed in their section and try to
 iterate over them as an array, alignment padding would break this
 iteration.
 
+Globals can also have a :ref:`DLL storage class <dllstorageclass>`.
+
+Syntax::
+
+    [@<GlobalVarName> =] [Linkage] [Visibility] [DLLStorageClass] [ThreadLocal]
+                         [AddrSpace] [unnamed_addr] [ExternallyInitialized]
+                         <global | constant> <Type>
+                         [, section "name"] [, align <Alignment>]
+
 For example, the following defines a global in a numbered address space
 with an initializer, section, and alignment:
 
@@ -556,7 +565,8 @@ Functions
 
 LLVM function definitions consist of the "``define``" keyword, an
 optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
-style <visibility>`, an optional :ref:`calling convention <callingconv>`,
+style <visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`,
+an optional :ref:`calling convention <callingconv>`,
 an optional ``unnamed_addr`` attribute, a return type, an optional
 :ref:`parameter attribute <paramattrs>` for the return type, a function
 name, a (possibly empty) argument list (each with optional :ref:`parameter
@@ -567,7 +577,8 @@ curly brace, a list of basic blocks, and a closing curly brace.
 
 LLVM function declarations consist of the "``declare``" keyword, an
 optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
-style <visibility>`, an optional :ref:`calling convention <callingconv>`,
+style <visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`,
+an optional :ref:`calling convention <callingconv>`,
 an optional ``unnamed_addr`` attribute, a return type, an optional
 :ref:`parameter attribute <paramattrs>` for the return type, a function
 name, a possibly empty list of arguments, an optional alignment, an optional
@@ -603,7 +614,7 @@ be significant and two identical functions can be merged.
 
 Syntax::
 
-    define [linkage] [visibility]
+    define [linkage] [visibility] [DLLStorageClass]
            [cconv] [ret attrs]
            <ResultType> @<FunctionName> ([argument list])
            [fn Attrs] [section "name"] [align N]
@@ -616,12 +627,13 @@ Aliases
 
 Aliases act as "second name" for the aliasee value (which can be either
 function, global variable, another alias or bitcast of global value).
-Aliases may have an optional :ref:`linkage type <linkage>`, and an optional
-:ref:`visibility style <visibility>`.
+Aliases may have an optional :ref:`linkage type <linkage>`, an optional
+:ref:`visibility style <visibility>`, and an optional :ref:`DLL storage class
+<dllstorageclass>`.
 
 Syntax::
 
-    @<Name> = alias [Linkage] [Visibility] <AliaseeTy> @<Aliasee>
+    @<Name> = [Visibility] [DLLStorageClass] alias [Linkage] <AliaseeTy> @<Aliasee>
 
 The linkage must be one of ``private``, ``linker_private``,
 ``linker_private_weak``, ``internal``, ``linkonce``, ``weak``,