[llvm-dev] Overlapping memcpy

Joerg Sonnenberger via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 7 08:39:52 PST 2015

On Mon, Dec 07, 2015 at 07:33:29AM +0100, Maciej Adamczyk via llvm-dev wrote:
> Hello.
> My friend's data compressor has a problem. During decompression stage, on
> some corrupted files, it may issue an overlapping memcpy.
> He has two easy solutions for that:
>  * switch to memmove
>  * add a branch to detect such case
> However, he's not happy with either of them as they slow the decompression
> down to handle a case that will never happen to almost everyone.

While I don't think any of this is really LLVM specific, the second is
certainly the correct approach if the file format explicitly disallows
such overlapping ranges. LZMA streams for example are perfectly well
defined for that case and it even make sense for certain overlapping
pattern to say "copy 256 Bytes starting from offsets -8". If you
know that this condition is invalid for well formed input, mark the
condition as predicted false and the compiler will try to turn it into a branch
statement that the branch prediction of the CPU can understand.


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