MC: Implement COFF .linkonce directive

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185753 91177308-0d34-0410-b5e6-96231b3b80d8

docs/Extensions.rst

@@ -4,7 +4,6 @@ LLVM Extensions
 
 .. contents::
    :local:
-   :depth: 1
 
 .. toctree::
    :hidden:
@@ -21,6 +20,9 @@ Machine-specific Assembly Syntax
 X86/COFF-Dependent
 ------------------
 
+Relocations
+^^^^^^^^^^^
+
 The following additional relocation type is supported:
 
 **@IMGREL** (AT&T syntax only) generates an image-relative relocation that
@@ -37,3 +39,55 @@ corresponds to the COFF relocation types ``IMAGE_REL_I386_DIR32NB`` (32-bit) or
     .long fun@IMGREL
     .long (fun@imgrel + 0x3F)
     .long $unwind$fun@imgrel
+
+
+``.linkonce`` Directive
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+
+   ``.linkonce [ comdat type [ section identifier ] ]``
+
+Supported COMDAT types:
+
+``discard``
+   Discards duplicate sections with the same COMDAT symbol. This is the default
+   if no type is specified.
+
+``one_only``
+   If the symbol is defined multiple times, the linker issues an error.
+
+``same_size``
+   Duplicates are discarded, but the linker issues an error if any have
+   different sizes.
+
+``same_contents``
+   Duplicates are discarded, but the linker issues an error if any duplicates
+   do not have exactly the same content.
+
+``associative``
+   Links the section if a certain other COMDAT section is linked. This other
+   section is indicated by its section identifier following the comdat type.
+   The following restrictions apply to the associated section:
+
+   1. It must be the name of a section already defined.
+   2. It must differ from the current section.
+   3. It must be a COMDAT section.
+   4. It cannot be another associative COMDAT section.
+
+``largest``
+   Links the largest section from among the duplicates.
+
+``newest``
+   Links the newest section from among the duplicates.
+
+
+.. code-block:: gas
+
+  .section .text$foo
+  .linkonce
+    ...
+
+  .section .xdata$foo
+  .linkonce associative .text$foo
+    ...