[llvm-dev] [cfe-dev] [RFC][ARM] -Oz implies -mthumb

[Peter Smith via llvm-dev]

On Thu, 15 Nov 2018 at 19:56, Sjoerd Meijer <Sjoerd.Meijer at arm.com> wrote:
>
> Hi Tim and Peter,
>
>
> Thanks for your comments! Yes, I now see now  that with GCC, the ARM/Thumb state does not depends on optimisation levels, but it is a toolchain default (I've learned something today! :-)).
>
>
> But I guess it doesn't change much about my observations:
>
> - we have an inconsistency/discrepancy between GCC's and Clang/LLVM's behaviour,
>
> - and most likely Clang's default behaviour, for a native toolchain users which I am now, gives surprising and perhaps undesired results. I.e., the equivalent of GCC's "gcc -Os", which is "clang -Oz", doesn't really do what I want/expect. And being a native toolchain user is quite important here, because that means I don't expect it would be necessary to provide --target and then provide some triple to get me Thumb when I've already specified -Oz (but perhaps I am totally wrong here).
>
>
> I agree now that changing Thumb/ARM state depending on optimisation levels might be confusing and not worth the effort, but that leaves me wondering what our options are (not a rhetorical question). I guess that is:
>
> 1) keep it as it is (I haven't looked at the docs yet, but perhaps document this better if necessary), or
>
> 2) adopt GCC's behaviour and flip the default?
>
My current thought is that 2 has too many problems associated with it.
In particularly I can't think of a good solution to the inline
assembly/intrinsics problem that Eli mentioned.

For documentation it looks like there is a CPU Architecture Features
and Limitations section
(http://clang.llvm.org/docs/UsersManual.html#id79) that looks like it
is in need of an update. Perhaps that would be a good place to mention
advice on whether to use Arm or Thumb state and how to do it?
Alternatively maybe a separate LLVM documentation page on how to
choose Arm command line options. I suspect most people will arrive
there via their search engine of choice so the location probably
doesn't matter too much.

Peter

>
> Cheers,
>
> Sjoerd.
>
> ________________________________
> From: Tim Northover <t.p.northover at gmail.com>
> Sent: 15 November 2018 16:48:38
> To: Peter Smith
> Cc: Sjoerd Meijer; LLVM Developers Mailing List; nd
> Subject: Re: [llvm-dev] [cfe-dev] [RFC][ARM] -Oz implies -mthumb
>
> On Thu, 15 Nov 2018 at 14:27, Peter Smith via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > My vote is not imply ARM/Thumb state changes with optimization
> > options. We've already got two ways to do it --target=thumb-none-eabi,
> > --target=arm-none-eabi and -mthumb/-marm I think the potential
> > confusion outweighs the potential benefit. I'm just one voice though.
>
> I agree with that.
>
> Tim.