On 7 Jun 2016 7:52 a.m., "pierre gousseau" <pierregoussea...@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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