[cfe-dev] Option -mtune

[David Blaikie via cfe-dev]

On Mon, Apr 20, 2020 at 1:05 AM Sjoerd Meijer <Sjoerd.Meijer at arm.com> wrote:

> I understand the possible benefits of -mtune, but I just don't see how we
> benefit from a half baked implementation, or a no-op, and leaving the no-op
> in hoping that one day it will be implemented properly.
>

"I don't see how we benefit from <what's there today>" - I think we've
already covered the practical benefit of command line compatibility. That's
a fairly real/practical benefit & something that Clang does for lots of
options, not only this one.


> I also understand that removing the option could break some builds, but
> hey, compiler options and defaults change sometimes.
>

Pretty rarely are flags outright removed or semantics changed in ways that
would break builds, so far as I know.


> In my opinion the cost of migrating would be worth the benefits. But I
> understand the different angles here, and hey, at least I've tried now...
> :-)
> This is addressing the hard problem of setting optimal compiler options
> with -target, -march, -mcpu, and -mtune.  In this equation -mtune is just a
> minor annoyance, but if we could get rid of this part of the confusion then
> that would be a good change to me and avoid the regular question we get
> what the -mtune options should be.
>

It'd be interesting to understand the path your users go through that leads
to confusion - what documentation (command line --help, websites (ones
maintained/owned by your company, or Clang's open source/general website,
etc), etc) & perhaps where more clarity could be provided there.

- Daved


>
> Cheers.
> ------------------------------
> *From:* Peter Smith <Peter.Smith at arm.com>
> *Sent:* 20 April 2020 08:33
> *To:* David Blaikie <dblaikie at gmail.com>; Sjoerd Meijer <
> Sjoerd.Meijer at arm.com>
> *Cc:* cfe-dev at lists.llvm.org Developers <cfe-dev at lists.llvm.org>
> *Subject:* Re: [cfe-dev] Option -mtune
>
> I don't think that we could remove -mtune without breaking some builds
> that support both GCC and Clang with the same script, as -mtune is
> supported in GCC. As David points out -mtune isn't needed for correctness
> so ignoring it for at least these types of project seems to be reasonable.
> An opt-in warning for accepted but ignored options could be an option
> though.
>
> IIRC We did use to have some support for mtune in AArch64, unfortunately
> we had to take it out (mea culpa https://reviews.llvm.org/D39179) as it
> wasn't just affecting micro-architectural features it was including
> architecture features as well. I think that there is some scope for putting
> back support for mtune, it would need us to more cleanly separate out the
> micro-architectural features. The most recent post about this was
> http://lists.llvm.org/pipermail/llvm-dev/2017-October/118520.html
>
> Peter
>
> ________________________________________
> From: cfe-dev <cfe-dev-bounces at lists.llvm.org> on behalf of Sjoerd Meijer
> via cfe-dev <cfe-dev at lists.llvm.org>
> Sent: 19 April 2020 22:01
> To: David Blaikie
> Cc: cfe-dev at lists.llvm.org Developers
> Subject: Re: [cfe-dev] Option -mtune
>
> Thanks for your reply, and I can see that this is the only benefit of this
> no-op: if your move to Clang and if you still have -mtune set in your build
> environment, you don't get an "unknown option" error. But for me
> personally, silently ignoring things and giving the impression it works is
> worse than a simple Makefile fix, but maybe I am wrong, which is why I
> checked here on the list. I was guessing that a diagnostic in this case
> would only be value if it's enabled by default as I'm afraid many users
> won't enable it? And if you need to add a flag to get this diagnostic, you
> might as well get rid of -mtune?
>
> Cheers.
>
> ________________________________
> From: David Blaikie <dblaikie at gmail.com>
> Sent: 19 April 2020 20:58
> To: Sjoerd Meijer <Sjoerd.Meijer at arm.com>
> Cc: cfe-dev at lists.llvm.org Developers <cfe-dev at lists.llvm.org>
> Subject: Re: [cfe-dev] Option -mtune
>
> On Sun, Apr 19, 2020 at 12:55 PM Sjoerd Meijer via cfe-dev <
> cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
> Hello,
>
> Quick question what you think we should be doing with option -mtune.
> Problem is that it looks like we support it because it is documented, it
> can be supplied on the command line, but it is silently ignored:
>
>   // FIXME: Handle -mtune=.
>   (void)Args.hasArg(options::OPT_mtune_EQ);
>
> giving the false impression to users it is doing something is probably the
> worst of options we have
>
> Not /the worst/ as such, many options are added to Clang so it's command
> line compatible (in the sense that you'll get a running program that
> behaves correctly) with GCC - I imagine the commit history of this feature
> probably justifies the addition with that sort of reason.
>
> Seems quite reasonable for --help and web documentation to mention that
> it's a no-op/supported-for-compatibility flag.
>
> As for adding a warning for these sort of no-op flags, maybe? Probably
> opt-in, though.
>
> (we get regularly questions about this).
> We could simply remove it, or if this is too radical, issue a diagnostic
> that this is an unsupported option? Any thoughts/preferences?
>
> Cheers,
> Sjoerd.
>
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