4 ms·

Assuming that vector instructions are available, shouldn't it be much faster to actually compare the buffer contents against a vector register initialized to al

Assuming that vector instructions are available, shouldn't it be much faster to actually compare the buffer contents against a vector register initialized to all-zeros rather than comparing against some other memory? Or would memcmp automatically optimize that away because of the precondition that the first 16 bytes are already known to be 0?

BeeOnRope 5y ago
It is probably faster, yes (half the number of reads) – but the point of this truck is that you can re-use the (hopefully) vectorized memcmp on every platform with portable code rather than getting on the SIMD ISA treadmill yourself.
- rwmj 5y ago
  The qemu implementation does indeed do it the hard way. It's a lot of code: https://gitlab.com/qemu-project/qemu/-/blob/master/util/bufferiszero.c https://gitlab.com/qemu-project/qemu/-/blob/master/util/buff...
  - pm215 5y ago
    Interestingly, we used to have a special-case aarch64 version, but we dropped it because the C version was faster: https://gitlab.com/qemu-project/qemu/-/commit/2250d3a293d36ed9d8143d4c3d3e94086c429af4 https://gitlab.com/qemu-project/qemu/-/commit/2250d3a293d36e... (Might or might not still be true on more modern aarch64 hardware...)
  - pm215 5y ago
    Does your memset version beat QEMU's plain-old-C fallback version ?