r234202 - MSan told me that we actually dump the entire scratch buffer into PCH files, initialize it.

[Benjamin Kramer]

On Mon, Apr 6, 2015 at 10:09 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
> On Mon, Apr 6, 2015 at 1:01 PM, Benjamin Kramer <benny.kra at googlemail.com>
> wrote:
>>
>> Author: d0k
>> Date: Mon Apr  6 15:01:49 2015
>> New Revision: 234202
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=234202&view=rev
>> Log:
>> MSan told me that we actually dump the entire scratch buffer into PCH
>> files, initialize it.
>>
>> Writing 4k of zeros is preferrable to 4k of random memory. Document that.
>> While
>> there remove the initialization of the first byte of the buffer and start
>> at
>> index zero. It was writing a literal '0' instead of a null byte at the
>> beginning anyways, which didn't matter since we never read it.
>
>
> Wonder if we could just get smarter about writing it out - only writing the
> needed bytes (ones that have been initialized) - should be relatively easy,
> unless we leave holes in it? (& if we need the rest of the zeros, either
> just resizing the file, or writing zeros right to the file without writing
> them to this buffer first might be do-able)

I was wondering the same but this buffer looks in no way special to
the AST writer (except the name) and we don't know where it ends
without scanning the buffer. Implementing a general zero tail
compression scheme doesn't feel like it's worth it.

- Ben

>
>>
>>
>> Modified:
>>     cfe/trunk/lib/Lex/ScratchBuffer.cpp
>>
>> Modified: cfe/trunk/lib/Lex/ScratchBuffer.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Lex/ScratchBuffer.cpp?rev=234202&r1=234201&r2=234202&view=diff
>>
>> ==============================================================================
>> --- cfe/trunk/lib/Lex/ScratchBuffer.cpp (original)
>> +++ cfe/trunk/lib/Lex/ScratchBuffer.cpp Mon Apr  6 15:01:49 2015
>> @@ -64,12 +64,13 @@ void ScratchBuffer::AllocScratchBuffer(u
>>    if (RequestLen < ScratchBufSize)
>>      RequestLen = ScratchBufSize;
>>
>> +  // Get scratch buffer. Zero-initialize it so it can be dumped into a
>> PCH file
>> +  // deterministically.
>>    std::unique_ptr<llvm::MemoryBuffer> OwnBuf =
>> -      llvm::MemoryBuffer::getNewUninitMemBuffer(RequestLen, "<scratch
>> space>");
>> +      llvm::MemoryBuffer::getNewMemBuffer(RequestLen, "<scratch space>");
>>    llvm::MemoryBuffer &Buf = *OwnBuf;
>>    FileID FID = SourceMgr.createFileID(std::move(OwnBuf));
>>    BufferStartLoc = SourceMgr.getLocForStartOfFile(FID);
>>    CurBuffer = const_cast<char*>(Buf.getBufferStart());
>> -  BytesUsed = 1;
>> -  CurBuffer[0] = '0';  // Start out with a \0 for cleanliness.
>> +  BytesUsed = 0;
>>  }
>>
>>
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>
>