[wwwdocs] IPA and LTO updates

[Jan Hubicka <hubicka at ucw dot cz>]

Hi,
here are some updates to changes.html for new LTO stuff I can think of.

Honza

Index: changes.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-4.6/changes.html,v
retrieving revision 1.52
diff -c -p -r1.52 changes.html
*** changes.html	21 Oct 2010 16:02:11 -0000	1.52
--- changes.html	14 Nov 2010 16:58:44 -0000
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*** 48,63 ****
        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 optimimization 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
--- 48,93 ----
        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><A HREF="http://gcc.gnu.org/projects/lto/whopr.pdf";>Scalable Whole
! 	    Program Optimizer (WHOPR)</A> project got into useable shape and replaced
! 	    by default the serial LTO optimization model.  Link time optimization
! 	    can now split itself into multiple parallel complications. The parallelizm
! 	    is controlled using using <tt>-flto=</tt><em>n</em> (where
! 	    <em>n</em> specify number of parallel compilations to execute.  GCC
! 	    can also cooperate with GNU make job server
! 	    by specifying <tt>-flto=jobserv</tt> option and adding <tt>+</tt> in the
! 	    beginning of <tt>Makefile</tt> rule executing the linker.
! 	<li>Large number of bugs was fixed.  GCC now bootstrap with the link
! 	    time optimization and builds working Mozilla Firefox as well as other
! 	    large applications.</li>
! 	<li>Resolution information from the linker plugin is now used to drive
! 	    whole program assumptions.  Use of linker plugin results in a lot stronger
! 	    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
! 	    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
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*** 71,82 ****
  		     </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
--- 101,125 ----
  		     </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
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*** 116,121 ****
--- 159,169 ----
        <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>