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Re: [wwwdocs] IPA and LTO updates


Hello Jan,

if I may point out a few nits:

* Jan Hubicka wrote on Sun, Nov 21, 2010 at 12:25:03AM CET:
> *** changes.html	20 Nov 2010 20:00:13 -0000	1.63
> --- changes.html	20 Nov 2010 23:24:41 -0000
> ***************
> *** 56,71 ****
>         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
> --- 56,103 ----
>         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 complications. Parallelism

s/complications/compilations/

> + 	    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 <tt>-flto=jobserver</tt> option and adding <tt>+</tt> in the

s/specifying/& the/

> + 	    beginning of <tt>Makefile</tt> rule executing the linker.

to the beginning of the

> + 	<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 has to be

s/has/have/

> + 	    explicitly annotated with <tt> externally_visible</tt> when the
> + 	    linker plugin is not used</li>

period after user.

> + 	<li>C++ inline functions and virtual tables are now more aggressively privatized

comma at the end

> + 	    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. Static constructor is implied by, for example,

I'd write:
  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>

s/was/have been/

>   	  <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>Number of various inlining heuristic improvements. In particular:

A number of inlining ...

>   	      <ul>
>   		 <li>Partial inlining is now supported and enabled by default
>   		     at <code>-O2</code> and greater.  The feature can be
> ***************
> *** 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
> --- 111,135 ----
>   		     </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 program

of a program

> + 		     or when compilig with <tt>-Os</tt>) was improved to handle

compiling

> + 		     better C++ program with larger abstraction penalty leading

s/\(better \)\(.*penalty \)/\2\1,/

> + 		     to smaller code size and better performance.</li>
>   	      </ul>
>   	  </li>
> + 	  <li>IPA reference optimization pass detecting global variables used

The IPA ...

> + 	      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
> ***************
> *** 124,129 ****
> --- 169,179 ----
>         <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 tu the current unit only via returning

s/tu/to/

> +       or exception handling.  This is the case of most of the library functions
> +       that have no callbacks.</tt>
>     </ul>
>   
>   <h3 id="c">C</h3>

Cheers,
Ralf


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