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

[Peter Smith via llvm-dev]

On Thu, 15 Nov 2018 at 14:18, Sjoerd Meijer <Sjoerd.Meijer at arm.com> wrote:
>
> Ahhh, typo in my previous mail:
>
>
> > when I noticed that -Os gives me Thumb on Cortex-A{8,9,17}
>
>
> I wanted to say:
>
>
> when I noticed that "GCC -Os" gives me Thumb on Cortex-A{8,9,17}
>
>

Yes. Just to clarify my response. That particular linaro toolchain
(I've got 5.3) defaults to -mthumb. It will also give you thumb code
if you compile with -O3 though. It is a toolchain default and not an
implication of -Os or -Oz.

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.

Peter
 ________________________________
> From: Sjoerd Meijer
> Sent: 15 November 2018 14:15:26
> To: Peter Smith; Bruce Hoult
> Cc: llvm-dev; nd
> Subject: Re: [cfe-dev] [RFC][ARM] -Oz implies -mthumb
>
>
>
> Yes, exactly this:
>
> > Sure, none of the cortex-m cores support ARM mode. Try cortex-a{5,7,8,9,15,53} etc and you'll see it works.
>
> Sorry for being a bit vague and unclear here: yes, I should have said cortex-a{5,7,8,9,15,53}.
> I was just having a play with this native compiler:
>
> gcc-5 (Ubuntu/Linaro 5.4.0-6ubuntu1~16.04.10) 5.4.0 20160609
>
> when I noticed that -Os gives me Thumb on Cortex-A{8,9,17}, which is what I would expect, but with Clang -Oz I get A32.
>
> I haven't thought about the implementation yet, but I hope passing -mthumb in the driver is not that difficult when we i) target an A-core and AArch32 state, and ii) we optimise for minimum size. And if I have to update a lot of tests, then that's what it is I am afraid, if we agree this is a sensible change that is.
>
> Cheers,
> Sjoerd.
>
>
>
>
>
>
> ________________________________
> From: cfe-dev <cfe-dev-bounces at lists.llvm.org> on behalf of Bruce Hoult via cfe-dev <cfe-dev at lists.llvm.org>
> Sent: 15 November 2018 13:31
> To: Peter Smith
> Cc: llvm-dev; Clang Dev
> Subject: Re: [cfe-dev] [RFC][ARM] -Oz implies -mthumb
>
> Sure, none of the cortex-m cores support ARM mode. Try cortex-a{5,7,8,9,15,53} etc and you'll see it works.
>
> On Thu, Nov 15, 2018 at 5:10 AM, Peter Smith <peter.smith at linaro.org> wrote:
>
> My understanding is that whether a gcc toolchain defaults to ARM or
> Thumb is a configuration time decision by whomever builds the
> toolchain. The linaro arm-linux-gnueabihf toolchain I have defaults to
> -mthumb and that doesn't vary for -mcpu or any other command line
> option. I haven't got a gcc to hand that defaults to -marm so that I
> can test whether -mcpu=cortex-m3 will change that to Thumb. If I try
> -marm -mcpu=cortex-m3 I get "error: target CPU does not support ARM
> mode".
>
> Can you give us a more concrete example about where GCC is inconsistent?
>
> For clang I'm not particularly fond of -Oz implying a change of
> instruction set state. I think that it would be difficult to document
> properly, especially how to tell clang that I really did mean -Oz on
> ARM. As well as being a bit messy to implement.
>
> I think that most users of clang would prefer to use Thumb(2) than
> ARM, although how to make change globally and give people enough
> warning could be challenging and we'd need to get a consensus from the
> community. I'd also not want to be the person updating all the tests
> with -marm.
>
> The reset vectors on some old architectures did need to be ARM state,
> although I think they had to be written in assembly. It is possible
> though that someone is using clang as the assembler driver and we'd
> pass through -mthumb when they weren't expecting it.
>
> Peter
> On Thu, 15 Nov 2018 at 12:39, Tim Northover via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
> >
> > On Thu, 15 Nov 2018 at 12:25, Bruce Hoult <brucehoult at sifive.com> wrote:
> > > OK, I just checked, and -mcpu=cortex-{m3,m4,m7,a7,a9,a15,a53} gives Thumb at -O1, -O1, -Os on the following gcc:
> >
> > If anything I'd be inclined to just default to Thumb always. I haven't
> > checked myself, but rumour has it the icache benefits make it faster
> > than ARM code as well as smaller in most cases. My one worry there is
> > with reset vectors, which I believe must be implemented in ARM in some
> > cases; but since GCC itself appears to be inconsistent here, hopefully
> > those people are already explicit about their needs.
> >
> > Cheers.
> >
> > Tim.
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