On 03/22/2015 08:26 PM, Jan Hubicka wrote:
> Hi,
> this is my honest attempt to explain how command line options works with LTO
> in current compiler and how to deal with these sanely.
>
> It is bit of mess, but an improvement over past releases. We finished the
> transition to per-function attributes. In GCC 6 I plan to add per-variable
> counterpart. I wonder if we want to have something in the texinfo too?
>
> Honza
>
> Index: changes.html
> ===================================================================
> RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-5/changes.html,v
> retrieving revision 1.90
> diff -u -r1.90 changes.html
> --- changes.html 23 Mar 2015 02:00:12 -0000 1.90
> +++ changes.html 23 Mar 2015 02:22:51 -0000
> @@ -78,6 +78,25 @@
> Streaming extra information needed to merge types adds about 2-6% of
> memory size and object size increase. This can be controlled by
> <code>-flto-odr-type-merging</code>.</li>
> + <li><p>Command line optimization and target options are now streamed on
> + per-function basis and honored by the link-time optimizer.
> + This change makes the link-time optimization more tranpsarent
> + replacement of the per-file optimization.
> + It is now possible to build projects that require different
> optimization
> + settings for different translation units (such as
> + <code>-ffast-math</code>, <code>-mavx</code>, or
> <code>-finline</code>).
> + Contrary to the earlier GCC releases, the optimization and target
> + options passed to the command line invoking linker are ignored.</p>
> + <p>Note that this apply only to those command
> + line options that can be passed to <code>optimize</code> and
> + <code>target</code> attributes. Command line options affecting global
> + code generation (such as <code>-fpic</code>), warnings
> + (such as <code>-Wodr</code>), optimization affecting way static
> variables
> + are optimized (such as <code>-fcommon</code>), debug output (such as
> + <code>-g</code>) and <code>--param</code> parameters can be applied
> only
> + to the whole link-time optimization unit.
> + It these cases it is recommended to consistently use the same setting
> + both at compile-time and link-time.</p>
> <li>GCC bootstrap now uses slim LTO object files.</li>
> <li>Memory usage and link times were improved. Tree merging was sped
> up,
> memory usage of GIMPLE declarations and types was reduced, and,
>
How about this for a copy-edited version of the new text?
<li><p>Command-line optimization and target options are now streamed on
a per-function basis and honored by the link-time optimizer.
This change makes the link-time optimization a more transparent
replacement of per-file optimizations.
It is now possible to build projects that require
different optimization
settings for different translation units (such as
<code>-ffast-math</code>, <code>-mavx</code>, or
<code>-finline</code>).
Contrary to the earlier GCC releases, the optimization and target
options passed on the link command line are ignored.</p>
<p>Note that this applies only to those command-line options
that can be passed to <code>optimize</code> and
<code>target</code> attributes.
Command-line options affecting global code generation
(such as <code>-fpic</code>), warnings
(such as <code>-Wodr</code>),
optimizations affecting the way static variables
are optimized (such as <code>-fcommon</code>), debug output (such as
<code>-g</code>),
and <code>--param</code> parameters can be applied only
to the whole link-time optimization unit.
In these cases, it is recommended to consistently use the same
options at both compile time and link time.</p></li>
-Sandra
