llvm dev - Nov 2018 - [RFC][ARM] -Oz implies -mthumb

Hello,


I would like to address an issue/inconsistency related to command line options
and compiling for minimum code size, and wanted to check if there would be any
problems or objections to my change.


The problem is that compiling for minimum code size like this:


    -Oz --target=arm-arm--eabi -mcpu=cortex-xyz


does not really give minimum code size because -mthumb is not enabled. This
looks like a sub-optimal user experience to me, and also, it is inconsistent
with GCC's behaviour.


In other words: for AArch32, optimisation level -Oz targets A32, but I would
like to change that to T32, and so I would like to propose that -Oz implies
-mthumb.


Cheers,

Sjoerd.

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Hi Sjoerd,

On Thu, 15 Nov 2018 at 11:49, Sjoerd Meijer via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> does not really give minimum code size because -mthumb is not enabled. This
looks like a sub-optimal user experience to me, and also, it is inconsistent
with GCC's behaviour.
Which version? None of the GCCs on Godbolt exhibit the kind of
switching you're describing as far as I can tell.

Cheers.

Tim.

I've never tried -mcpu=cortex-xyz but I know -march=armv7 defaults to Thumb

OK, I just checked, and -mcpu=cortex-{m3,m4,m7,a7,a9,a15,a53} gives Thumb
at -O1, -O1, -Os on the following gcc:

arm-linux-gnueabihf-gcc (Ubuntu/Linaro 7.3.0-27ubuntu1~18.04) 7.3.0

cortex-m0 fails because it doesn't do hard float. I don't have an eabi
compiler around.


On Thu, Nov 15, 2018 at 4:14 AM, Tim Northover via cfe-dev <
cfe-dev at lists.llvm.org> wrote:
> Hi Sjoerd,
>
> On Thu, 15 Nov 2018 at 11:49, Sjoerd Meijer via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
> > does not really give minimum code size because -mthumb is not enabled.
> This looks like a sub-optimal user experience to me, and also, it is
> inconsistent with GCC's behaviour.
>
> Which version? None of the GCCs on Godbolt exhibit the kind of
> switching you're describing as far as I can tell.
>
> Cheers.
>
> Tim.
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
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