manmanren added a comment.
I forgot to upload the testing case, I will add it now.
Manman
================
Comment at: include/clang/Basic/FileManager.h:176
+ /// Manage memory buffers associated with pcm files.
+ std::unique_ptr<PCMCache> BufferMgr;
+
----------------
benlangmuir wrote:
> Why is this inside the FileManager? It isn't used by the FileManager.
I initially had another patch that puts PCMCache object at the top level (i.e
whenever we create a FileManager, we create a PCMCache object). The initial
patch is much bigger than this patch. PCMCache is cacheing the content of a PCM
File, and is shared across threads that we spawned to build modules implicitly,
just like FileManager. I had some discussions with Bruno and Adrian, we felt
FileManager is a better place. But if you have other suggestions, please let me
know!
================
Comment at: include/clang/Basic/FileManager.h:308
+ /// a thread, we pop a ThreadContext.
+ struct ThreadContext {
+ /// Keep track of all module files that have been validated in this thread
----------------
benlangmuir wrote:
> Can we call this something like "ModuleLoadContext" or maybe
> "ModuleCompilationContext"? The word thread is misleading, since the threads
> are not run concurrently, and are only an implementation detail of our crash
> recovery.
>
> Also, can you explain why it is only necessary to keep information about the
> the current stack of contexts? In a situation like
>
> A imports B imports C
> A imports D imports C
>
> We would no longer have information about C being validated, right? What
> happens if there's a mismatch there? Can this never happen?
Sure, we can call it ModuleCompilationContext because we are spawning threads
to build modules implicitly.
The issue we are trying to detect is use-after-free, i.e we don't want a thread
to hold on to an invalid FileEntry. We error out before trying to delete the
FileEntry that is live in another thread.
In a situation like
A imports B imports C
A imports D imports C
A will start a thread to build B, and B will start a thread to build C, C will
be validated in B's compilation thread and A's compilation thread. Later on, if
D tries to invalidate C, D knows that an ancestor thread A has validated C, so
the compiler will error out. And it is okay for A to invalidate C in A's
compilation thread, because we can clean up the references to C's FileEntry.
https://reviews.llvm.org/D28299
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