[cfe-dev] Help solve bug 17216 for clang-format on Windows

[Alp Toker]

Hello Johan,

I've landed the fix by itself in r194250.

I've discovered that this is a problem that exists in various other bits
of LLVM and clang, and will be working to fix those uses as well.

You can see the commit message for the full background.

Daniel, let's start looking at the performance issues while we're at it,
can you review my RewriteBuffer optimisation posted to cfe-commits as a
starting point?

Alp.

On 01/11/2013 21:12, Johan Engelen wrote:
> Hello all,
>   I've been looking into why clang-format cannot write in-place for
> large files (> 16 kB) on Windows [1].
> I've found the culprit, but need help actually fixing it; the file
> handling code is somewhat complex for a new-comer like me. :)
>
> The difference between small and large files is the returned value by
> shouldUseMmap in /llvm/lib/Support/MemoryBuffer.cpp. For small files,
> shouldUseMmap returns false and the already created normal
> MemoryBuffer is kept and all is fine. But for larger files,
> shouldUseMmap returns true and the already created normal MemoryBuffer
> is destroyed and a new memory mapped one is created. This becomes the
> OwningPtr<MemoryBuffer> Code memorybuffer in ClangFormat.cpp.
> Now when the time comes to write the reformatted text in-place to the
> input file, llvm::raw_fd_ostream FileStream cannot be opened if the
> MemoryBuffer is a memory mapped one. Apparently Windows does not allow
> that. For small files, no Mmap buffer was created and all is fine.
>
> I do not know nearly enough about the file handling internals to
> propose a good fix.
> If shouldUseMmap is modified to always return false, clang-format
> works fine on large files. But that would be a very course "fix". I
> wanted to try releasing the Mmap interface to the file, right before
> writing back to it. But I was not able to figure out how to do that.
>
> Thanks for the help,
>     Johan
>
> [1]  http://llvm.org/bugs/show_bug.cgi?id=17216
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