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More GC documentation cleanup

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230402 91177308-0d34-0410-b5e6-96231b3b80d8
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Philip Reames 2015-02-24 23:51:37 +00:00
parent 8269a59b1c
commit 1e91e94aac
1 changed files with 24 additions and 21 deletions
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docs/GarbageCollection.rst
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@ -198,27 +198,24 @@ LLVM IR Features
This section describes the garbage collection facilities provided by the
:doc:`LLVM intermediate representation <LangRef>`. The exact behavior of these
IR features is specified by the binary interface implemented by a :ref:`code
generation plugin <plugin>`, not by this document.
These facilities are limited to those strictly necessary; they are not intended
to be a complete interface to any garbage collector. A program will need to
interface with the GC library using the facilities provided by that program.
IR features is specified by the selected :ref:`GC strategy description
<plugin>`.
Specifying GC code generation: ``gc "..."``
-------------------------------------------
.. code-block:: llvm
define ty @name(...) gc "name" { ...
define <returntype> @name(...) gc "name" { ... }
The ``gc`` function attribute is used to specify the desired GC style to the
The ``gc`` function attribute is used to specify the desired GC strategy to the
compiler. Its programmatic equivalent is the ``setGC`` method of ``Function``.
Setting ``gc "name"`` on a function triggers a search for a matching code
generation plugin "*name*"; it is that plugin which defines the exact nature of
the code generated to support GC. If none is found, the compiler will raise an
error.
Setting ``gc "name"`` on a function triggers a search for a matching subclass
of GCStrategy. Some collector strategies are built in. You can add others
using either the loadable plugin mechanism, or by patching your copy of LLVM.
It is the selected GC strategy which defines the exact nature of the code
generated to support GC. If none is found, the compiler will raise an error.
Specifying the GC style on a per-function basis allows LLVM to link together
programs that use different garbage collection algorithms (or none at all).
@ -324,12 +321,18 @@ pointer:
%derived = getelementptr %object, i32 0, i32 2, i32 %n
LLVM does not enforce this relationship between the object and derived pointer
(although a :ref:`plugin <plugin>` might). However, it would be an unusual
collector that violated it.
(although a particular :ref:`collector strategy <plugin>` might). However, it
would be an unusual collector that violated it.
The use of these intrinsics is naturally optional if the target GC does require
the corresponding barrier. Such a GC plugin will replace the intrinsic calls
with the corresponding ``load`` or ``store`` instruction if they are used.
The use of these intrinsics is naturally optional if the target GC does not
require the corresponding barrier. The GC strategy used with such a collector
should replace the intrinsic calls with the corresponding ``load`` or
``store`` instruction if they are used.
One known deficiency with the current design is that the barrier intrinsics do
not include the size or alignment of the underlying operation performed. It is
currently assumed that the operation is of pointer size and the alignment is
assumed to be the target machine's default alignment.
Write barrier: ``llvm.gcwrite``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@ -340,8 +343,8 @@ Write barrier: ``llvm.gcwrite``
For write barriers, LLVM provides the ``llvm.gcwrite`` intrinsic function. It
has exactly the same semantics as a non-volatile ``store`` to the derived
pointer (the third argument). The exact code generated is specified by a
compiler :ref:`plugin <plugin>`.
pointer (the third argument). The exact code generated is specified by the
Function's selected :ref:`GC strategy <plugin>`.
Many important algorithms require write barriers, including generational and
concurrent collectors. Additionally, write barriers could be used to implement
@ -356,8 +359,8 @@ Read barrier: ``llvm.gcread``
For read barriers, LLVM provides the ``llvm.gcread`` intrinsic function. It has
exactly the same semantics as a non-volatile ``load`` from the derived pointer
(the second argument). The exact code generated is specified by a
:ref:`compiler plugin <plugin>`.
(the second argument). The exact code generated is specified by the Function's
selected :ref:`GC strategy <plugin>`.
Read barriers are needed by fewer algorithms than write barriers, and may have a
greater performance impact since pointer reads are more frequent than writes.