On 26 July 2017 at 22:32, Hans Wennborg <h...@chromium.org> wrote: > On Wed, Jul 26, 2017 at 11:27 AM, Hans Wennborg <h...@chromium.org> wrote: > > On Wed, Jul 26, 2017 at 5:20 AM, Alex Lorenz via cfe-commits > > <cfe-commits@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. > Wouldn't cases 1) and 3) catch most situations where this happens > unintentionally? E.g. when one #includes a file that forgets to > #pragma pop, and then includes a new file afterwards? > > I've reverted in r309186 in the meantime. >
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