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 <ztur...@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@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) >> >> _______________________________________________ >> lldb-dev mailing list >> lldb-dev@lists.llvm.org >> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev >> >
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