r192494 - Adds Microsoft compatiable C++ record layout code to clang.

Warren Hunt whunt at google.com
Wed Feb 19 15:15:36 PST 2014

I've talked to Richard (Smith).  He's not certain as to why PCH causes
there to be an external source for layouts.  It seems that PCH would take
one of two strategies, either don't store layouts in the PCH and recompute
them at compile time (which I don't see why would impose an external
layout) or serialize the layout in the PCH, in which case there's no need
to call layout at all.  To my understanding ExternalSource is related to
DWARF.  It's entirely possible that we shouldn't get getting the
ExternalSource flag during PCH.  Further investigation is required to
figure out exactly what's wrong.  Both Richard and I are reasonably busy,
so I wouldn't expect to get to figuring it out for a while.  If you know
more about how PCH works (you've been using it) I'm more than happy to have
a discussion about why ExternalSource is firing and if it should and if we
should ignore it.


On Tue, Feb 11, 2014 at 11:28 AM, Warren Hunt <whunt at google.com> wrote:

> I have concerns about why PCH uses external layout and how PCH and pragmas
> (such as pack) interact.  Richard Smith is our local expert on clang's PCH
> but it out at a C++ committee meeting this week so he's not around to
> question.  It may well be the case that PCH should not use external layouts
> and that should be fixed.  If it's working for you w/o the guard you can
> continue doing so but I don't guarantee anything until I've had a detailed
> discussion about how external layouts and PCH are supposed to work.  I'll
> get back to you as soon as I have a plan.
> thanks and good luck,
> -Warren
> On Tue, Feb 11, 2014 at 12:34 AM, Will Wilson <will at indefiant.com> wrote:
>> Hi Warren,
>> I've been using PCH with clang on Windows quite heavily (although for
>> custom AST analysis only) and have a bit of experience with clang
>> serialization code. As far as I can tell everything you rely on should
>> already be serialized - at least local tests on large Windows codebases
>> don't reveal any issues with the !D->getASTContext().getExternalSource()
>> conditional removed. Probably the best bet would be to add a few basic
>> tests using a PCH and try removing the conditional. YMMV but I think you
>> might be pleasantly surprised and find you have no work to do.
>> Thanks for the great work on it!
>> Will.
>> On 11 February 2014 09:03, Warren Hunt <whunt at google.com> wrote:
>>> There is no code in the MS Record Layout builder to handle external
>>> sources so the behavior is unlikely to be correct in the case of an
>>> external source (hence the guard).  I don't know anything about how PCH
>>> works or interacts with clang on windows.  I'll try to get a better answer
>>> for you but in the time being I'd suggest not using PCH and clang on
>>> windows in combination.
>>> -Warren
>>> On Thu, Feb 6, 2014 at 4:23 PM, Will Wilson <will at indefiant.com> wrote:
>>>> Hi Warren,
>>>> I've just been hunting a bug in MS mode where records were being given
>>>> different layouts depending on if the TU was built with a PCH or not. In
>>>> the end I tracked it down to ASTContext::getASTRecordLayout()
>>>> introduced in r192494:
>>>>   if (isMsLayout(D) && !D->getASTContext().getExternalSource()) {
>>>>     NewEntry = BuildMicrosoftASTRecordLayout(D);
>>>> Removing the "&& !D->getASTContext().getExternalSource()" restored the
>>>> expected behaviour. Is this check still needed?
>>>> Cheers,
>>>> Will.
>> --
>> *Indefiant Ltd.*
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