[PATCH] D20867: [PCH] Fix timestamp check on windows hosts.

[Richard Smith via cfe-commits]

On 7 Jun 2016 7:52 a.m., "pierre gousseau" <pierregousseau14 at gmail.com>
wrote:
>
> pgousseau added a comment.
>
> > > On Linux, if the timestamp of a header file, included in the pch, is
modified, then including the pch without regenerating it causes a fatal
error, which is reasonable.
>
> >
>
> > >  On Windows the check is ifdefed out, allowing the compilation to
continue in a broken state.
>
> >
>
> > >  The root of the broken state is that, if timestamps dont match, the
preprocessor will reparse a header without discarding the pch data.
>
> >
>
> > >  This leads to "#pragma once" header to be included twice.
>
> >
>
> > >  The reason behind the ifdefing of the check lacks documentation, and
was done 6 years ago.
>
> >
>
>
>
>
> > It's documented in the comment you deleted:
>
> >
>
> >   // In our regression testing, the Windows file system seems to
>
> >   // have inconsistent modification times that sometimes
>
> >   // erroneously trigger this error-handling path.
>
> >
>
> >
>
> > We should confirm whether this is in fact still the case. Maybe this is
caused by building on a networked file system, where a locally-changed file
might have a different mtime locally and remotely (the local mtime may be
precise where the remote one has been rounded to a multiple of 2 seconds by
an underlying FAT filesystem)? If it's something like that, we could
perhaps work around this by rounding the mtime to a multiple of 2 seconds
ourselves.
>
>
> I am not sure how to reproduce this build scenario, would you be able to
provide some more stepped details?
> I have tried emitting and including a PCH on a networked FAT32 drive, but
no false warnings observed so far.
>
> Are you asking for the 2 seconds tolerance to be part of this patch?
> Or, as it seems the main problem here is the lack of a reproducible, are
you ok with the idea of committing this patch first, to see if the
inconsistent time stamps is still an issue?

Yes, let's commit a patch to remove the #if first, and see if anything
actually breaks (and if so, what). There's no need to add an option for
this we have no uses for it.

> http://reviews.llvm.org/D20867
>
>
>
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