[cfe-dev] [ARM][AArch64] Default to -fno-common

[James Y Knight via cfe-dev]

IMO, it would be less confusing to just change the default to false across
the board.

If anything should still use -fcommon by default, I'd want that to be based
on vendor/OS, not CPU.

On Mon, Feb 24, 2020 at 11:38 AM Sjoerd Meijer via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> >  Is there any reason to limit this to just the ARM targets?
>
> No good reason other than that I don't know how beneficial this is for
> other targets.
> But I definitely don't mind giving other targets the same treatment though
> while I am at it.
>
> Cheers,
> Sjoerd.
>
> ------------------------------
> *From:* Tom Stellard <tstellar at redhat.com>
> *Sent:* 24 February 2020 16:31
> *To:* Sjoerd Meijer <Sjoerd.Meijer at arm.com>; cfe-dev at lists.llvm.org
> Developers <cfe-dev at lists.llvm.org>
> *Subject:* Re: [cfe-dev] [ARM][AArch64] Default to -fno-common
>
> On 02/24/2020 08:28 AM, Sjoerd Meijer via cfe-dev wrote:
> > Hello,
> >
> > Subject says most of it: I would like to propose that the Arm targets
> default to -fno-common for performance and code-size reasons. In addition,
> GCC has also flipped the switch recently, see [1], so we would get the same
> behaviour as GCC which would also be nice. I've learned that the switch for
> GCC was relatively straightforward (for users), and there is some GNU
> documentation with Q&As to help toolchain users to transition if they find
> problems. I have put a draft patch up for review here:
> https://reviews.llvm.org/D75056. This definitely needs some doc changes
> (and I need to look at the tests), but first I would like to see how we
> feel about flipping the switch: any ideas, comments, objections?
> >
>
> Is there any reason to limit this to just the ARM targets?
>
> -Tom
>
> > Cheers,
> > Sjoerd.
> >
> >
> > [1] The GCC commit:
> >
> > commit 6271dd984d7f920d4fb17ad37af6a1f8e6b796dc
> > Author: Wilco Dijkstra <wdijkstr at arm.com>
> > Date:   Wed Nov 20 16:29:23 2019 +0000
> >
> >     PR85678: Change default to -fno-common
> >     GCC currently defaults to -fcommon.  As discussed in the PR, this is
> an ancient
> >     C feature which is not conforming with the latest C standards.  On
> many targets
> >     this means global variable accesses have a codesize and performance
> penalty.
> >     This applies to C code only, C++ code is not affected by -fcommon.
> It is about
> >     time to change the default.
> >
> >
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> >
>
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