This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Overwhelmed by GCC frustration


On 8/1/17, Richard Biener <richard.guenther@gmail.com> wrote:
> On Mon, Jul 31, 2017 at 7:08 PM, Andrew Haley <aph@redhat.com> wrote:
>> On 31/07/17 17:12, Oleg Endo wrote:
>>> On Mon, 2017-07-31 at 15:25 +0200, Georg-Johann Lay wrote:
>>>> Around 2010, someone who used a code snipped that I published in
>>>> a wiki, reported that the code didn't work and hang in an
>>>> endless loop.  Soon I found out that it was due to some GCC
>>>> problem, and I got interested in fixing the compiler so that
>>>> it worked with my code.
>>>>
>>>> 1 1/2 years later, in 2011, [...]
>>>
>>> I could probably write a similar rant.  This is the life of a
>>> "minority target programmer".  Most development efforts are being
>>> done with primary targets in mind.  And as a result, most changes
>>> are being tested only on such targets.
>>>
>>> To improve the situation, we'd need a lot more target specific tests
>>> which test for those regressions that you have mentioned.  Then of
>>> course somebody has to run all those tests on all those various
>>> targets.  I think that's the biggest problem.  But still, with a
>>> test case at hand, it's much easier to talk to people who have
>>> silently introduced a regression on some "other" targets.  Most of
>>> the time they just don't know.
>>
>> It's a fundamental problem for compilers, in general: every
>> optimization pass wants to be the last one, and (almost?) no-one who
>> writes a pass knows all the details of all the subsequent passes.  The
>> more sophisticated and subtle an optimization, the more possibility
>> there is of messing something up or confusing someone's back end or a
>> later pass.  We've seen this multiple times, with apparently
>> straightforward control flow at the source level turning into a mess
>> of spaghetti in the resulting assembly.  But we know that the
>> optimization makes sense for some kinds of program, or at least that
>> it did at the time the optimization was written.  However, it is
>> inevitable that some programs will be made worse by some
>> optimizations.  We hope that they will be few in number, but it
>> really can't be helped.
>>
>> So what is to be done?  We could abandon the eternal drive for more
>> and more optimizations, back off, and concentrate on simplicity and
>> robustness at the expens of ultimate code quality.  Should we?  It
>> would take courage, and there will be an eternal pressume to improve
>> code.  And, of course, we'd risk someone forking GCC and creating the
>> "superoptimized GCC" project, starving FSF GCC of developers.  That's
>> happened before, so it's not an imaginary risk.
>
> Heh.  I suspect -Os would benefit from a separate compilation pipeline
> such as -Og.  Nowadays the early optimization pipeline is what you
> want (mostly simple CSE & jump optimizations, focused on code
> size improvements).  That doesn't get you any loop optimizations but
> loop optimizations always have the chance to increase code size
> or register pressure.
>

Maybe in addition to the -Os optimization level, GCC mainline could
also add the -Oz optimization level like Apple's GCC had, and clang
still has? Basically -Os is -O2 with additional code size focus,
whereas -Oz is -O0 with the same code size focus. Adding it to the
FSF's GCC, too, could help reduce code size even further than -Os
currently does.

> But yes, targeting an architecture like AVR which is neither primary
> nor secondary (so very low priority) _plus_ being quite special in
> target abilities (it seems to be very easy to mess up things) is hard.
>
> SUSE does have some testers doing (also) code size monitoring
> but as much data we have somebody needs to monitor it, further
> bisect and report regressions deemed worthwhile.  It's hard to
> avoid slow creep -- compile-time and memory use are a similar
> issue here.
>
> Richard.
>
>> --
>> Andrew Haley
>> Java Platform Lead Engineer
>> Red Hat UK Ltd. <https://www.redhat.com>
>> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
>


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]