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Re: [wwwdocs] IPA and LTO updates


> On Sun, 14 Nov 2010, Diego Novillo wrote:
> > Thanks for doing this!  I'll defer to Gerald for final approval.  He
> > may want other grammar fixes.
> 
> Me?  Never! ;-)  Jan, there has been some discussion going on, could you 
> send a new patch incorporating those (and ideally avoiding some of the 
> longer lines which wrap oddly in the patch)?  We should be able to quickly 
> get this applied, then; assuming I understand things well enough to make 
> changes I volunteer to do so and commit the result.
Hi,
here is updated patch.
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
*** 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
+ 	    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
+ 	    beginning 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 has 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. Static constructor is implied by, for example,
+ 	    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>Number of various 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
***************
*** 79,90 ****
  		     </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
+ 		     or when compilig with <tt>-Os</tt>) was improved to handle
+ 		     better C++ program with larger abstraction penalty leading
+ 		     to smaller code size and better performance.</li>
  	      </ul>
  	  </li>
+ 	  <li>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
***************
*** 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
+       or exception handling.  This is the case of most of the library functions
+       that have no callbacks.</tt>
    </ul>
  
  <h3 id="c">C</h3>


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