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add an explicit section on static constructors.

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llvm-svn: 150037
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Chris Lattner 2012-02-08 01:44:00 +00:00
parent d0b4cd4c46
commit 1c0f1d4d1f
1 changed files with 50 additions and 16 deletions
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@ -31,6 +31,7 @@
Errors</a></li>
<li><a href="#ci_portable_code">Write Portable Code</a></li>
<li><a href="#ci_rtti_exceptions">Do not use RTTI or Exceptions</a></li>
<li><a href="#ci_static_ctors">Do not use Static Constructors</a></li>
<li><a href="#ci_class_struct">Use of <tt>class</tt>/<tt>struct</tt> Keywords</a></li>
</ol></li>
</ol></li>
@ -447,6 +448,51 @@ than <tt>dynamic_cast&lt;&gt;</tt>.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ci_static_ctors">Do not use Static Constructors</a>
</h4>
<div>
<p>Static constructors and destructors (e.g. global variables whose types have
a constructor or destructor) should not be added to the code base, and should be
removed wherever possible. Besides <a
href="http://yosefk.com/c++fqa/ctors.html#fqa-10.12">well known problems</a>
where the order of initialization is undefined between globals in different
source files, the entire concept of static constructors is at odds with the
common use case of LLVM as a library linked into a larger application.</p>
<p>Consider the use of LLVM as a JIT linked into another application (perhaps
for <a href="http://llvm.org/Users.html">OpenGL, custom languages</a>,
<a href="http://llvm.org/devmtg/2010-11/Gritz-OpenShadingLang.pdf">shaders in
movies</a>, etc</a>). Due to the design of static constructors, they must be
executed at startup time of the entire application, regardless of whether or
how LLVM is used in that larger application. There are two problems with
this:</p>
<ol>
<li>The time to run the static constructors impacts startup time of
applications &mdash; a critical time for GUI apps, among others.</li>
<li>The static constructors cause the app to pull many extra pages of memory
off the disk: both the code for the constructor in each <tt>.o</tt> file and
the small amount of data that gets touched. In addition, touched/dirty pages
put more pressure on the VM system on low-memory machines.</li>
</ol>
<p>We would really like for there to be zero cost for linking in an additional
LLVM target or other library into an application, but static constructors
violate this goal.</p>
<p>That said, LLVM unfortunately does contain static constructors. It would be
a <a href="http://llvm.org/PR11944">great project</a> for someone to purge all
static constructors from LLVM, and then enable the
<tt>-Wglobal-constructors</tt> warning flag (when building with Clang) to ensure
we do not regress in the future.
</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
@ -1151,22 +1197,10 @@ prefer it.</p>
<div>
<p>The use of <tt>#include &lt;iostream&gt;</tt> in library files is
hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
support clients using LLVM libraries as part of larger systems. In particular,
we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
the static constructors are run whenever an application starts up that uses the
dynamic library. There are two problems with this:</p>
<ol>
<li>The time to run the static c'tors impacts startup time of applications
&mdash; a critical time for GUI apps.</li>
<li>The static c'tors cause the app to pull many extra pages of memory off the
disk: both the code for the static c'tors in each <tt>.o</tt> file and the
small amount of data that gets touched. In addition, touched/dirty pages
put more pressure on the VM system on low-memory machines.</li>
</ol>
hereby <b><em>forbidden</em></b>, because many common implementations
transparently inject a <a href="#ci_static_ctors">static constructor</a> into
every translation unit that includes it.</p>
<p>Note that using the other stream headers (<tt>&lt;sstream&gt;</tt> for
example) is not problematic in this regard &mdash;
just <tt>&lt;iostream&gt;</tt>. However, <tt>raw_ostream</tt> provides various