3 ms·

Right at about the 20m mark: https://youtu.be/Z4oYSByyRak?t=20m2s https://youtu.be/Z4oYSByyRak?t=20m2s

• tom_mellior 8y ago
  Thanks! Grabbing the sources from https://www.nayuki.io/page/fast-sha2-hashes-in-x86-assembly https://www.nayuki.io/page/fast-sha2-hashes-in-x86-assembly and compiling them more or less as recommended (and unlike they are compiled in the talk): $ clang -O3 sha256-test.c sha256.c -o sha256-test ; for i in 1 2 3; do ./sha256-test ; done Self-check passed Speed: 197.2 MB/s Self-check passed Speed: 196.1 MB/s Self-check passed Speed: 196.1 MB/s $ gcc -O3 sha256-test.c sha256.c -o sha256-test ; for i in 1 2 3; do ./sha256-test ; done Self-check passed Speed: 209.7 MB/s Self-check passed Speed: 209.2 MB/s Self-check passed Speed: 209.1 MB/s This is a baseline for us. It has nothing to do with Zig and nothing to do with Andrew's machine (hardware or compiler versions). But wait, the page above suggests that adding -march=native might help. Indeed it does: $ clang -O3 -march=native sha256-test.c sha256.c -o sha256-test ; for i in 1 2 3; do ./sha256-test ; done Self-check passed Speed: 255.6 MB/s Self-check passed Speed: 259.0 MB/s Self-check passed Speed: 254.3 MB/s $ gcc -O3 -march=native sha256-test.c sha256.c -o sha256-test ; for i in 1 2 3; do ./sha256-test ; done Self-check passed Speed: 275.1 MB/s Self-check passed Speed: 268.4 MB/s Self-check passed Speed: 270.0 MB/s In the talk Andrew suggests that the difference might be due to using rorx instructions, which Zig might be able to do due to aggressive loop unrolling. Does -funroll-all-loops help GCC? It turns out that it doesn't, and that it cannot, on this program, because the C code for sha256_compress is already fully unrolled. But anyway, are we using rorx instructions at all? We are, but only with -march=native: $ clang -O3 -S sha256.c -o - | grep -c rorx 0 $ clang -O3 -march=native -S sha256.c -o - | grep -c rorx 542 And: $ gcc -O3 -S sha256.c -o - | grep -c rorx 0 $ gcc -O3 -march=native -S sha256.c -o - | grep -c rorx 576 [Edit: Changed the grep from "ror" to "rorx", which changed GCC's numbers a bit; it does generate "ror" without x without -march=native.] So. Testable theories: (a) On Andrew's machine, the same setup but with Clang using -march=native would outperform or at least match Zig. (b) Zig's compiler internally uses the equivalent of -march=native, possibly implicitly, at least in --release-fast mode. Nothing here is meant to imply that Andrew is dishonest. There are just lots of variables to take into account, and sometimes we don't. Also, "slower/faster than C" by a few percent is not very meaningful if even C compilers disagree by 6% or so, and the same C compiler with the right flags disagrees with itself by a lot more.