Re: [PATCH][RFC] Add -Og

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Sep 3, 2012 at 6:28 AM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Fri, Aug 10, 2012 at 1:30 PM, Richard Guenther <rguenther@suse.de> wrote:
>>
>> This adds a new optimization level, -Og, as previously discussed.
>> It aims at providing fast compilation, a superior debugging
>> experience and reasonable runtime performance.  Instead of making
>> -O1 this optimization level this adds a new -Og.
>>
>> It's a first cut, highlighting that our fixed pass pipeline and
>> simply enabling/disabling individual passes (but not pass copies
>> for example) doesn't scale to properly differentiate between
>> -Og and -O[23].  -O1 should get similar treatment, eventually
>> just building on -Og but not focusing on debugging experience.
>> That is, I expect that in the end we will at least have two post-IPA
>> optimization pipelines.  It also means that you cannot enable
>> PRE or VRP with -Og at the moment because these passes are not
>> anywhere scheduled (similar to the situation with -O0).
>>
>> It has some funny effect on dump-file naming of the pass copies
>> though, which hints at that the current setup is too static.
>> For that reason the new queue comes after the old, to not confuse
>> too many testcases.
>>
>> It also does not yet disable any of the early optimizations that
>> make debugging harder (SRA comes to my mind here, as does
>> switch-conversion and partial inlining).
>>
>> The question arises if we want to support in any reasonable
>> way using profile-feedback or LTO for -O[01g], thus if we
>> rather want to delay some of the early opts to after IPA
>> optimizations.
>>
>> Not bootstrapped or fully tested, but it works for the compile
>> torture.
>>
>> Comments welcome,
>
> No comments?  Then I'll drop this idea for 4.8.
>

When I debug binutils, I have to use -O0 -g to get precise
line and variable info.  Also glibc has to be compiled with
-O, which makes debug a challenge.  Will -Og help bintils
and glibc debug?


H.J.