Re: RFC: Patch for gcc-5/changes.html

[Vladimir Makarov <vmakarov at redhat dot com>]

On 03/26/2015 04:38 PM, Sandra Loosemore wrote:
> On 03/26/2015 01:25 PM, Vladimir Makarov wrote:
> >    Hi, I neglected to write about RA changes for the previous releases
> > and people asked me to write about RA changes for GCC-5. So here is what
> > I'd like to add to gcc-5/changes.html.  I'll do it tomorrow.  So any
> > comments will be appreciated.
> >
> > Index: changes.html
> > ===================================================================
> > RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-5/changes.html,v
> > retrieving revision 1.91
> > diff -U 5 -r1.91 changes.html
> > --- changes.html    23 Mar 2015 10:12:23 -0000    1.91
> > +++ changes.html    26 Mar 2015 19:24:32 -0000
> > @@ -95,10 +95,40 @@
> >        <li>The new <code>gcov-tool</code> utility allows manipulating
> >            profiles.</li>
> >        <li>Profiles are now more tolerant to source file changes 
> > (this can be
> >        controlled by <code>--param 
> > profile-func-internal-id</code>).</li>
> >      </ul></li>
> > +    <li>Register allocation improvements:
> > +    <ul>
> > +      <li>A new local register allocator (LRA) sub-pass was added.
> > +          The sub-pass implements control-flow sensitive global
> > +          register rematerialization (controlled via
> > +      <code>-flra-remat</code>).  Instead of spilling and
>
> How about rewriting the first two sentences as:
>
> A new local register allocator (LRA) sub-pass, controlled by 
> <code>-flra-remat</code>, implements control-flow sensitive global 
> register materialization.
That is better.  Thanks.
> > +          restoring register value, it is recalculated if it is
>
> s/register value/a register value/
Fixed.
> > +          profitable.  The sub-pass improved SPEC2000 generated code
> > +          by 1% and 0.5% correspondingly on ARM and x86-64.</li>
> > +      <li>In GCC-4.9 and earlier releases PIC hard register was fixed
> > +          and was not used for other purposes when PIC code was
> > +          generated.  Reuse of PIC hard register was implemented in RA
> > +          for GCC-5.0.  It improves generated PIC code performance as
> > +          more hard registers can be used.  As an example, shared
> > +          libraries and OS Android would significantly benefit from
> > +          such optimization.  Currently it is switched on only for
> > +          x86/x86-64 targets.  As RA infrastructure is already
> > +          implemented for PIC register reuse, other targets might
> > +          follow this in the future.</li>
>
> How about making this less verbose and repetitive:
>
> Reuse of the PIC hard register, instead of using a fixed register, was 
> implemented on x86/x86-64 targets.  This improves generated PIC code 
> performance as more hard registers can be used.  Shared libraries can 
> significantly benefit from this optimization.
Fixed.  Thanks.
> > +      <li>A simple form of inter-procedural RA was implemented.  When
> > +          it is known that a called function does not use caller saved
>
> s/caller saved/caller-saved/
Done.
> > +          registers, save/restore code is not generated around the
> > +          call for such registers. This optimization can be controlled
> > +          by <code>-fipa-ra</code></li>
> > +      <li>On some architectures (e.g. modern Intel processors),
> > +          spilling general registers into vector registers can be more
> > +          profitable than spilling into memory.  LRA had already such
> > +          optimization.  It was significantly improved for GCC-5.0,
> > +          permitting more 85% such spills than in GCC-4.9.</li>
>
> I don't understand the last sentence.  How about just dropping that 
> 85% bit and making the whole thing less wordy and more focused on the 
> actual improvement:
>
> LRA is now much more effective at generating spills of general 
> registers into vector registers instead of memory on architectures 
> (e.g., modern Intel processors) where this is profitable.
Ok.  Fixed.

Thanks, Sandra.  That was very helpful as English is not my native language.
> > +    </ul></li>
> >      <li>UndefinedBehaviorSanitizer gained a few new sanitization 
> > options:
> >      <ul>
> > <li><code>-fsanitize=float-divide-by-zero</code>: detect floating-point
> >         division by zero;</li>
> > <li><code>-fsanitize=float-cast-overflow</code>: check that the result