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

[Johan Engelen]

Hi Alp,
   The patch solves the problem, thanks!

One comment: the format function returns 'false' when no errors 
occurred. So I believe it should read
       if (Rewrite.overwriteChangedFiles()) {
         return false;
       }

Cheers,
   Johan


On 2-11-2013 21:05, Alp Toker wrote:
> Hello Johan,
>
> I've attached a patch I'm using locally to reduce flicker in my IDE
> while running clang-format using atomic file save.
>
> I suspect it resolves you issue too, could you try it and report back?
>
> 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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