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Re: [wwwdocs] IPA and LTO updates
On Sun, 21 Nov 2010, Jan Hubicka wrote:
> here is updated version.
Thanks, Jan. I threw in a couple of linguistic changes and markup
fixes (<code> instead of <tt> and some real one) and committed the
thusly updated version.
Pleaes find the patch as committed below.
Good stuff!
Gerald
Index: changes.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-4.6/changes.html,v
retrieving revision 1.63
retrieving revision 1.66
diff -u -3 -p -r1.63 -r1.66
--- changes.html 20 Nov 2010 20:00:13 -0000 1.63
+++ changes.html 21 Nov 2010 23:08:45 -0000 1.66
@@ -51,21 +51,57 @@
<h2>General Optimizer Improvements</h2>
<ul>
- <li>A new general optimization level, <code>-Ofast</code> has been
+ <li>A new general optimization level, <code>-Ofast</code> has been
introduced. It combines the existing optimization level <code>-O3</code>
with options that can affect standards compliance but result in
better optimized code. For example <code>-Ofast</code> enables
<code>-ffast-math</code>.</li>
+ <li>Link-time optimization improvements:
+ <ul>
+ <li>The <a href="../projects/lto/whopr.pdf">Scalable Whole
+ Program Optimizer (WHOPR)</a> project has stabilized to the
+ point of being usable. It has become the default mode when
+ using the LTO optimization model. Link time optimization can
+ now split itself into multiple parallel compilations. Parallelism
+ is controlled with <code>-flto=<em>n</em></code> (where
+ <em>n</em> specifies the number of compilations to execute in
+ parallel. GCC can also cooperate with a GNU make job server
+ by specifying the <code>-flto=jobserver</code> option and
+ adding <code>+</code> to the to the beginning of the of
+ the Makefile rule executing the linker.</li>
+ <li>A large number of bugs were fixed. GCC itself, Mozilla
+ Firefox and other other large applications can be built with
+ LTO enabled.</li>
+ <li>Resolution information from the linker plugin is used to drive
+ whole program assumptions. Use of linker plugin results in
+ more aggressive optimization on binaries and on shared libraries
+ that use the hidden visibility attribute.</li>
+ <li>Hidden symbols used from non-link time objects now have to be
+ explicitly annotated with <code>externally_visible</code> when
+ the linker plugin is not used.</li>
+ <li>C++ inline functions and virtual tables are now privatized more
+ aggressively, leading to better inter-procedural optimization
+ and faster dynamic linking.</li>
+ <li>Memory usage and intermediate language streaming performance
+ has been improved.</li>
+ <li>Static constructors and destructors from individual units are
+ inlined into a single function.
+ This can significantly improve startup times of large C++
+ applications where static constructors are very common. For
+ example, static contructors are used when including the
+ <code>iostream</code> header.</li>
+ </ul>
+ </li>
<li>Interprocedural optimization improvements
<ul>
<li>The interprocedural framework was re-tuned for link time
- optimization.</li>
+ optimization. Several scalability issues were solved.</li>
<li>Improved auto-detection of <code>const</code> and <code>pure</code>
functions. Newly, <code>noreturn</code> functions are auto-detected.
<p>The <a href="http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#Warning-Options";><code>-Wsuggest-attribute=[const|pure|noreturn]</code></a>
flag is available that informs users when adding
attributes to headers might improve code generation.</p></li>
- <li>Inlining heuristics were improved:
+ <li>A number of inlining heuristic improvements. In particular:
<ul>
<li>Partial inlining is now supported and enabled by default
at <code>-O2</code> and greater. The feature can be
@@ -79,12 +115,27 @@
</p>
</li>
<li>Scalability for large compilation units was improved
- significantly.</li>
+ significantly.</li>
<li>Inlining of callbacks is now more aggressive.</li>
<li>Virtual methods considered for inlining when caller is
inlined and devirtualization is possible then.</li>
+ <li>Inlining when optimizing for size (either in cold
+ regions of a program or when compiling with
+ <code>-Os</code>) was improved to better handle C++
+ programs with larger abstraction penalty, leading
+ to smaller and faster code.</li>
</ul>
</li>
+ <li>The IPA reference optimization pass detecting global
+ variables used or modified by functions was strengthened
+ and sped up.</li>
+ <li>Functions whose address was taken are now optimized out
+ when all references to them are dead.</li>
+ <li>A new inter-procedural static profile estimation pass detects
+ functions that are executed once or unlikely to be executed.
+ Unlikely executed functions are optimized for size. Functions
+ executed once are optimized for size except for the inner
+ loops.</li>
</ul>
</li>
<li>A new switch <code>-fstack-usage</code> has been added. It makes
@@ -124,6 +175,11 @@
<code>float</code> is implicitly promoted to <code>double</code>.
This is especially helpful for CPUs that handle the former in
hardware, but emulate the latter in software.</li>
+ <li>A new function attribute <code>leaf</code> was introduced.
+ This attribute allows better inter-procedural optimization across
+ calls to functions that return to the current unit only via returning
+ or exception handling. This is the case for most library functions
+ that have no callbacks.</li>
</ul>
<h3 id="c">C</h3>