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[LangRef] Describe linkage types, allocation size of declarations for global variables 

Linkage type was only referenced for functions, not for global
variables.

Clarify that LLVM doesn't make assumption about the allocation size when
no definitive initializer for a global variable is known.

Differential Revision: https://reviews.llvm.org/D78952
This commit is contained in:
Matthias Schiffer 2020-05-08 16:20:43 -07:00 committed by Eli Friedman
parent e1815eb2e1
commit a2247d42e4
1 changed files with 11 additions and 2 deletions
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13
llvm/docs/LangRef.rst
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@ -272,8 +272,8 @@ linkage:
visible, meaning that it participates in linkage and can be used to
resolve external symbol references.
It is illegal for a function *declaration* to have any linkage type
other than ``external`` or ``extern_weak``.
It is illegal for a global variable or function *declaration* to have any
linkage type other than ``external`` or ``extern_weak``.
.. _callingconv:
@ -615,6 +615,8 @@ Global variable definitions must be initialized.
Global variables in other translation units can also be declared, in which
case they don't have an initializer.
Global variables can optionally specify a :ref:`linkage type <linkage>`.
Either global variable definitions or declarations may have an explicit section
to be placed in and may have an optional explicit alignment specified. If there
is a mismatch between the explicit or inferred section information for the
@ -686,6 +688,13 @@ 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. The maximum alignment is ``1 << 29``.
For global variables declarations, as well as definitions that may be
replaced at link time (``linkonce``, ``weak``, ``extern_weak`` and ``common``
linkage types), LLVM makes no assumptions about the allocation size of the
variables, except that they may not overlap. The alignment of a global variable
declaration or replaceable definition must not be greater than the alignment of
the definition it resolves to.
Globals can also have a :ref:`DLL storage class <dllstorageclass>`,
an optional :ref:`runtime preemption specifier <runtime_preemption_model>`,
an optional :ref:`global attributes <glattrs>` and