[lldb-dev] Improve performance of crc32 calculation

Zachary Turner via lldb-dev lldb-dev at lists.llvm.org
Wed Apr 12 12:42:43 PDT 2017

It would be nice if we could simply update LLVM's implementation to be
faster.  Having multiple implementations of the same thing seems
undesirable, especially if one (fast) implementation is always superior to
some other reason.  i.e. there's no reason anyone would ever want to use a
slow implementation if a fast one is available.

Can we change the JamCRC implementation in LLVM to use 4-byte slicing and
parallelize it ourselves?  This way there's no dependency on zlib, so even
people who have non-zlib enabled builds of LLDB get the benefits of the
fast algorithm.

On Wed, Apr 12, 2017 at 12:36 PM Scott Smith <scott.smith at purestorage.com>

> I didn't realize that existed; I just checked and it looks like there's
> JamCRC which uses the same polynomial.  I don't know what "Jam" means in
> this context, unless it identifies the polynomial some how?  The code is
> also byte-at-a-time.
> Would you prefer I use JamCRC support code instead, and then change JamCRC
> to optionally use zlib if it's available?
> On Wed, Apr 12, 2017 at 12:23 PM, Zachary Turner <zturner at google.com>
> wrote:
>> Zlib is definitely optional and we cannot make it required.
>> Did you check to see if llvm has a crc32 function somewhere in Support?
>> On Wed, Apr 12, 2017 at 12:15 PM Scott Smith via lldb-dev <
>> lldb-dev at lists.llvm.org> wrote:
>>> The algorithm included in ObjectFileELF.cpp performs a byte at a time
>>> computation, which causes long pipeline stalls in modern processors.
>>> Unfortunately, the polynomial used is not the same one used by the SSE 4.2
>>> instruction set, but there are two ways to make it faster:
>>> 1. Work on multiple bytes at a time, using multiple lookup tables. (see
>>> http://create.stephan-brumme.com/crc32/#slicing-by-8-overview)
>>> 2. Compute crcs over separate regions in parallel, then combine the
>>> results.  (see
>>> http://stackoverflow.com/questions/23122312/crc-calculation-of-a-mostly-static-data-stream
>>> )
>>> As it happens, zlib provides functions for both:
>>> 1. The zlib crc32 function uses the same polynomial as
>>> ObjectFileELF.cpp, and uses slicing-by-4 along with loop unrolling.
>>> 2. The zlib library provides crc32_combine.
>>> I decided to just call out to the zlib library, since I see my version
>>> of lldb already links with zlib; however, the llvm CMakeLists.txt declares
>>> it optional.
>>> I'm including my patch that assumes zlib is always linked in.  Let me
>>> know if you prefer:
>>> 1. I make the change conditional on having zlib (i.e. fall back to the
>>> old code if zlib is not present)
>>> 2. I copy all the code from zlib and put it in ObjectFileELF.cpp.
>>> However, I'm going to guess that requires updating some documentation to
>>> include zlib's copyright notice.
>>> This brings startup time on my machine / my binary from 50 seconds down
>>> to 32.
>>> (time ~/llvm/build/bin/lldb -b -o 'b main' -o 'run' MY_PROGRAM)
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