r309106 - Recommit r308327 2nd time: Add a warning for missing

[Alex L via cfe-commits]

Thanks, I recommitted it in r309386.

On 27 July 2017 at 15:53, Hans Wennborg <hans at chromium.org> wrote:

> On Thu, Jul 27, 2017 at 3:41 AM, Alex L <arphaman at gmail.com> wrote:
> >
> >
> > On 26 July 2017 at 22:32, Hans Wennborg <hans at chromium.org> wrote:
> >>
> >> On Wed, Jul 26, 2017 at 11:27 AM, Hans Wennborg <hans at chromium.org>
> wrote:
> >> > On Wed, Jul 26, 2017 at 5:20 AM, Alex Lorenz via cfe-commits
> >> > <cfe-commits at lists.llvm.org> wrote:
> >> >> Author: arphaman
> >> >> Date: Wed Jul 26 05:20:57 2017
> >> >> New Revision: 309106
> >> >>
> >> >> URL: http://llvm.org/viewvc/llvm-project?rev=309106&view=rev
> >> >> Log:
> >> >> Recommit r308327 2nd time: Add a warning for missing
> >> >> '#pragma pack (pop)' and suspicious uses of '#pragma pack' in
> included
> >> >> files
> >> >>
> >> >> The first recommit (r308441) caused a "non-default #pragma pack value
> >> >> might
> >> >> change the alignment of struct or union members in the included file"
> >> >> warning
> >> >> in LLVM itself. This recommit tweaks the added warning to avoid
> >> >> warnings for
> >> >> #includes that don't have any records that are affected by the
> >> >> non-default
> >> >> alignment. This tweak avoids the previously emitted warning in LLVM.
> >> >>
> >> >> Original message:
> >> >>
> >> >> This commit adds a new -Wpragma-pack warning. It warns in the
> following
> >> >> cases:
> >> >>
> >> >> - When a translation unit is missing terminating #pragma pack (pop)
> >> >> directives.
> >> >> - When entering an included file if the current alignment value as
> >> >> determined
> >> >>   by '#pragma pack' directives is different from the default
> alignment
> >> >> value.
> >> >> - When leaving an included file that changed the state of the current
> >> >> alignment
> >> >>   value.
> >> >>
> >> >> rdar://10184173
> >> >>
> >> >> Differential Revision: https://reviews.llvm.org/D35484
> >> >
> >> > We have code in Chromium that does exactly this:
> >> >
> >> > gles2_cmd_format.h does #pragma pack(push, 4) and then #includes a
> >> > file with some generated structs, with the intention that the pragma
> >> > applies to them.
> >> >
> >> > What's the best way to pacify the warning in this case?
> >> >
> >> > (We're tracking this in
> >> > https://bugs.chromium.org/p/chromium/issues/detail?id=749197)
> >>
> >> I agree that cases 1) and 3) from your patch description make sense to
> >> warn for, but I'm not sure that's the case for 2). Do you have
> >> examples where this catches any bugs? In our case #pragma packing an
> >> included file is intentional, and I suspect it might be a bit of a
> >> pattern.
> >
> >
> > I see, thanks for your input.
> >
> > 2) is generally designed for times when #pragma pack pop was accidentally
> > used too late (after some #includes that unintentionally receive the
> > alignment). I can see how some projects use this pattern heavily, and I
> > don't think there's a good way to pacify this warning in that case.
> >
> > I think that for us it would be reasonable to turn 2) off by default, and
> > allow users to enable it explicitly using a stronger flag (something like
> > -Wpragma-pack-suspicious-include?). I think that I will leave 2) out of
> this
> > commit, recommit it without 2) and then commit 2) as a non-default
> warning
> > that uses a separate flag.
>
> That sounds reasonable. You can probably still do it with the same
> commit, just moving 2) behind a separate flag.
>
> Thanks,
> Hans
>
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