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Re: [wwwdocs] IPA and LTO updates
Hi,
thanks,
here is updated version.
Honza
Index: changes.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-4.6/changes.html,v
retrieving revision 1.63
diff -c -p -r1.63 changes.html
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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>Interprocedural optimization improvements
<ul>
<li>The interprocedural framework was re-tuned for link time
! optimization.</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:
<ul>
<li>Partial inlining is now supported and enabled by default
at <code>-O2</code> and greater. The feature can be
--- 56,104 ----
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="http://gcc.gnu.org/projects/lto/whopr.pdf";>Scalable Whole
+ Program Optimizer (WHOPR)</a> project has stabilized to the point of being
+ usable. It is now the new default mode when using the LTO optimization
+ model. Link time optimization
+ can now split itself into multiple parallel compilations. Parallelism
+ is controlled with <tt>-flto=</tt><em>n</em> (where
+ <em>n</em> specifies number of parallel compilations to execute. GCC
+ can also cooperate with GNU make job server
+ by specifying the <tt>-flto=jobserver</tt> option and adding <tt>+</tt> in the
+ to the beginning of the of <tt>Makefile</tt> rule executing the linker.
+ <li>A large number of bugs were fixed. GCC can now bootstrap with LTO
+ enabled. Additionally, Mozilla Firefox and other large applications can
+ be built with LTO enabled.</li>
+ <li>Resolution information from the linker plugin is now 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 <tt> externally_visible</tt> when the
+ linker plugin is not used.</li>
+ <li>C++ inline functions and virtual tables are now more aggressively privatized,
+ leading to better inter-procedural optimization and faster dynamic linking.</li>
+ <li>Memory usage and intermediate language streaming performance was 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 including
+ <tt>iostream</tt> header.</li>
+ </ul>
+ </li>
<li>Interprocedural optimization improvements
<ul>
<li>The interprocedural framework was re-tuned for link time
! optimization. Several scalability issues was 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>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
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*** 79,90 ****
</p>
</li>
<li>Scalability for large compilation units was improved
! 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>
</ul>
</li>
</ul>
</li>
<li>A new switch <code>-fstack-usage</code> has been added. It makes
--- 112,136 ----
</p>
</li>
<li>Scalability for large compilation units was improved
! 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 compilig with <tt>-Os</tt>) was improved to handle
+ C++ program with larger abstraction penalty better, leading
+ to smaller code size and better performance.</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 can now be optimized out when
+ all references to them are dead.</li>
+ <li>A new inter-procedural static profile estimation pass detecting
+ functions that are executed once or unlikely 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
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*** 124,129 ****
--- 170,180 ----
<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 of most of the library functions
+ that have no callbacks.</tt>
</ul>
<h3 id="c">C</h3>