Re: [PATCH] clarify documentation of -Q --help=optimizers

[Martin Sebor <msebor@gmail.com>]

On 11/05/2015 04:30 PM, Segher Boessenkool wrote:
> On Thu, Nov 05, 2015 at 02:04:47PM -0700, Martin Sebor wrote:
>> On 11/05/2015 10:09 AM, Segher Boessenkool wrote:
>>> On Thu, Nov 05, 2015 at 08:58:25AM -0700, Martin Sebor wrote:
>>>> I don't think that reiterating in a condensed form what the manual
>>>> doesn't make clear in many more words will help. First, even users
>>>> who do find the relevant text in the manual often misunderstand it.
>>>> Others are misled by the "[enabled]" output into expecting the
>>>> optimizations to take place at -O0.
>>>
>>> The -Q --help* output is primarily aimed at GCC developers, just
>>> like plain -Q is.  The main problem here seems to be that the
>>> documentation pushes beginner GCC users to use this.
>>>
>>> Perhaps we should have a higher-level, more easy to use way to query
>>> what flags will do what, possibly integrated with the pass manager.
>>
>> I think that would be ideal (i.e., hiding each flag_xxx under
>> a get_flag_xxx() and set_flag_xxx() interface that would make
>> it possible to compute and dynamically query individual option
>> settings based on non-trivial dependencies). It strikes me as
>> quite a project.
>
> I was thinking more of showing at every flag whether it is relevant at
> all, or maybe not showing irrelevant ones except maybe when some
> --verbose or so option is given; and if you want to get really fancy
> (but also very helpful to both users and developers), have some option
> that shows all dependencies for a flag to take effect.
>
> This of course requires expressing some dependencies manually in the
> .opt files, but hopefully most can be done automatically with some
> magic.

Yes, the dependencies would have to be encoded somewhere, somehow
(once they were discovered, which seems like a challenge in and
if itself). Since they could be non-trivial (one option being
enabled by several others), to be sufficiently general, the data
structure itself would need to be able to express that any given
option is can be enabled or disabled by any combination of others.

>
>>>> Those who are in doubt and
>>>> ask on gcc-help are then told by GCC experts that -O0 disables
>>>> all optimizations without exception,
>>>
>>> And it does, for the plain meaning of "optimization"; anything else
>>> is arguably a bug.  But people have many different understandings
>>> of what a "compiler optimization" is, all the way to "anything the
>>> compiler does".
>>
>> Yes, that also sometimes causes confusion, though it's not something
>> I'm trying to solve with this patch.
>
> It's not something you _can_ solve as far as I see.  There is a
> considerable gap between what beginner and advanced users expect from
> the compiler, understand about the compiler, and worst of all, think
> they understand about the compiler.  GCC should be a useful compiler
> to that whole spectrum of audiences.  Currently -Q --help shows just
> the gory details, which is functionality we want to keep IMO, but not
> something we want to recommend everyone to use.
>
> Does that make sense?  I'm not sure I'm expressing myself well here,
> you cannot see my frantic handwaving :-)

It does make sense (even without the handwaving, though picturing
it in my mind was a big help :) I agree that solving this bigger
problem is well beyond what can be accomplished by changing the
compiler or its documentation.

Martin