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

[David Blaikie]

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)


>
> 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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