[LangRef] Clarify comdat

* ELF supports `nodeduplicate`.
* ELF calls the concept "section group". `GRP_COMDAT` emulates the PE COMDAT deduplication feature.
* "COMDAT group" is an ELF term. Avoid it for PE/COFF.
* WebAssembly supports comdat but only supports the `any` selection kind. https://bugs.llvm.org/show_bug.cgi?id=50531
* A comdat must be included or omitted as a unit. Both the compiler and the linker must obey this rule.
* A global object can be a member of at most one comdat.
* COFF requires a non-local linkage for non-`nodeduplicate` selection kinds.
* llvm.global_ctors/.llvm.global_dtors: if the third field is used on ELF, it must reference a global variable or function in a comdat

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D106300

llvm/docs/LangRef.rst

@@ -915,21 +915,24 @@ Syntax::
 Comdats
 -------
 
-Comdat IR provides access to COFF and ELF object file COMDAT functionality.
+Comdat IR provides access to object file COMDAT/section group functionality
+which represents interrelated sections.
 
-Comdats have a name which represents the COMDAT key. All global objects that
-specify this key will only end up in the final object file if the linker chooses
-that key over some other key. Aliases are placed in the same COMDAT that their
-aliasee computes to, if any.
+Comdats have a name which represents the COMDAT key and a selection kind to
+provide input on how the linker deduplicates comdats with the same key in two
+different object files. A comdat must be included or omitted as a unit.
+Discarding the whole comdat is allowed but discarding a subset is not.
 
-Comdats have a selection kind to provide input on how the linker should
-choose between keys in two different object files.
+A global object may be a member of at most one comdat. Aliases are placed in the
+same COMDAT that their aliasee computes to, if any.
 
 Syntax::
 
     $<Name> = comdat SelectionKind
 
-The selection kind must be one of the following:
+For selection kinds other than ``nodeduplicate``, only one of the duplicate
+comdats may be retained by the linker and the members of the remaining comdats
+must be discarded. The following selection kinds are supported:
 
 ``any``
     The linker may choose any COMDAT key, the choice is arbitrary.
@@ -944,10 +947,13 @@ The selection kind must be one of the following:
     The linker may choose any COMDAT key but the sections must contain the
     same amount of data.
 
-Note that XCOFF and the Mach-O platform don't support COMDATs, and ELF and
-WebAssembly only support ``any`` as a selection kind.
+- XCOFF and Mach-O don't support COMDATs.
+- COFF supports all selection kinds. Non-``nodeduplicate`` selection kinds need
+  a non-local linkage COMDAT symbol.
+- ELF supports ``any`` and ``nodeduplicate``.
+- WebAssembly only supports ``any``.
 
-Here is an example of a COMDAT group where a function will only be selected if
+Here is an example of a COFF COMDAT where a function will only be selected if
 the COMDAT key's section is the largest:
 
 .. code-block:: text
@@ -966,7 +972,7 @@ the global name:
 
   $foo = comdat any
   @foo = global i32 2, comdat
-
+  @bar = global i32 3, comdat($foo)
 
 In a COFF object file, this will create a COMDAT section with selection kind
 ``IMAGE_COMDAT_SELECT_LARGEST`` containing the contents of the ``@foo`` symbol
@@ -7737,6 +7743,7 @@ functions with the same priority is not defined.
 If the third field is non-null, and points to a global variable
 or function, the initializer function will only run if the associated
 data from the current module is not discarded.
+On ELF the referenced global variable or function must be in a comdat.
 
 .. _llvmglobaldtors:
 
@@ -7757,6 +7764,7 @@ order of functions with the same priority is not defined.
 If the third field is non-null, and points to a global variable
 or function, the destructor function will only run if the associated
 data from the current module is not discarded.
+On ELF the referenced global variable or function must be in a comdat.
 
 Instruction Reference
 =====================