[PING^4] Fix -fvar-tracking problems on big-endian systems

[PING^4] Fix -fvar-tracking problems on big-endian systems

[Richard Sandiford]

(One last try!)

http://gcc.gnu.org/ml/gcc-patches/2007-11/msg00835.html
Fix various problems with -fvar-tracking on big-endian systems.

http://gcc.gnu.org/ml/gcc-patches/2007-11/msg00836.html
Follow-on patch to fix problems with pass-by-reference parameters
(affects both big- and little-endian).

Richard

Re: [PING^4] Fix -fvar-tracking problems on big-endian systems

[Eric Botcazou]

> http://gcc.gnu.org/ml/gcc-patches/2007-11/msg00835.html
> Fix various problems with -fvar-tracking on big-endian systems.
>
> http://gcc.gnu.org/ml/gcc-patches/2007-11/msg00836.html
> Follow-on patch to fix problems with pass-by-reference parameters
> (affects both big- and little-endian).

I'm not sure we want this for 4.3, these are not trivial patches and the 
benefit doesn't look very high to me.

-- 
Eric Botcazou

Re: [PING^4] Fix -fvar-tracking problems on big-endian systems

[Richard Sandiford]

Eric Botcazou <ebotcazou@libertysurf.fr> writes:
>> http://gcc.gnu.org/ml/gcc-patches/2007-11/msg00835.html
>> Fix various problems with -fvar-tracking on big-endian systems.
>>
>> http://gcc.gnu.org/ml/gcc-patches/2007-11/msg00836.html
>> Follow-on patch to fix problems with pass-by-reference parameters
>> (affects both big- and little-endian).
>
> I'm not sure we want this for 4.3, these are not trivial patches and the 
> benefit doesn't look very high to me.

I think the benefit could be quite marked for big-endian systems
for -O2 -g (which is after all our library default).  The problem
is that the effect on -O2 -g itself is hard to quantify; there's no
off-the-shelf way of measuring it.  That's why I had to resort to
the two main measures I used:

  - GDB testing with an explicit -fvar-tracking.  This showed what
    I thought were pretty significant (though perhaps not massive)
    improvements

  - A comparison of assembly language output at -O2 -g, which showed
    many cases where bugs in the var-tracking output (and thus
    incorrect or misleading debug information).

This is also something that might have regressed from 4.2 in some cases,
because my 2007-10-12 patch created lowparts in cases where 4.2 didn't.
(And we needed to do that to avoid a bug which I think would otherwise
still exist after your patch.  I still believe that creating lowparts
in those situations is the right thing to do, but the problem is that
var-tracking generally mishandles lowparts on big-endian systems.)

I would understand your concern if the patch affected code generation,
but the patch only really touches code related to var-tracking on
big-endian systems, which is the problem area.

I wanted to push back a little because I think the benefit is higher
than you made it sound.  If you say no again, I'll accept it.

Richard

Re: [PING^4] Fix -fvar-tracking problems on big-endian systems

[Jakub Jelinek]

On Fri, Dec 14, 2007 at 12:18:19PM +0000, Richard Sandiford wrote:
> I wanted to push back a little because I think the benefit is higher
> than you made it sound.  If you say no again, I'll accept it.

I agree with Richard here that the benefit of this is quite big
and it would be good to have this in 4.3.

	Jakub

Re: [PING^4] Fix -fvar-tracking problems on big-endian systems

[Mark Mitchell]

Jakub Jelinek wrote:
> On Fri, Dec 14, 2007 at 12:18:19PM +0000, Richard Sandiford wrote:
>> I wanted to push back a little because I think the benefit is higher
>> than you made it sound.  If you say no again, I'll accept it.
> 
> I agree with Richard here that the benefit of this is quite big
> and it would be good to have this in 4.3.

Eric, would you be willing to review this from a technical perspective,
imagining that it will go into 4.4?  Then, report back with whatever
issues you might find and let us know if you're still concerned about it
going into 4.3.  If so, we can make a final 4.3/4.4 decision at that
time, but at least we'll have the patch reviewed for 4.4.

Thanks,

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713

Re: [PING^4] Fix -fvar-tracking problems on big-endian systems

[Richard Sandiford]

Mark Mitchell <mark@codesourcery.com> writes:
> Jakub Jelinek wrote:
>> On Fri, Dec 14, 2007 at 12:18:19PM +0000, Richard Sandiford wrote:
>>> I wanted to push back a little because I think the benefit is higher
>>> than you made it sound.  If you say no again, I'll accept it.
>> 
>> I agree with Richard here that the benefit of this is quite big
>> and it would be good to have this in 4.3.
>
> Eric, would you be willing to review this from a technical perspective,
> imagining that it will go into 4.4?  Then, report back with whatever
> issues you might find and let us know if you're still concerned about it
> going into 4.3.  If so, we can make a final 4.3/4.4 decision at that
> time, but at least we'll have the patch reviewed for 4.4.

Thanks for the reply.  For the record, Eric reviewed (and approved
a revised version of) the patch after Jakub sent his message, but the
discussion went back to the original thread rather than this one.
Sorry for the confusion!

Richard

Re: [PING^4] Fix -fvar-tracking problems on big-endian systems

[Mark Mitchell]

Richard Sandiford wrote:

>> Eric, would you be willing to review this from a technical perspective,
>> imagining that it will go into 4.4?  Then, report back with whatever
>> issues you might find and let us know if you're still concerned about it
>> going into 4.3.  If so, we can make a final 4.3/4.4 decision at that
>> time, but at least we'll have the patch reviewed for 4.4.
> 
> Thanks for the reply.  For the record, Eric reviewed (and approved
> a revised version of) the patch after Jakub sent his message, but the
> discussion went back to the original thread rather than this one.
> Sorry for the confusion!

OK, very good.  (I was catching up on email backlog, and reading by
thread, so I guess that's why I thought this was still open.)

Thanks,

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713