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Replace other iter().position() instances with memchr #302

Merged
merged 3 commits into from
May 10, 2021
Merged

Replace other iter().position() instances with memchr #302

merged 3 commits into from
May 10, 2021

Commits on May 10, 2021

  1. Use memchr::memchr{,2} for archive parsing 

    This makes parsing of the archive headers significantly faster. The `ar`
example adjusted to parse the same archive 1 million times, when run
with the rlib of the `object` crate itself produces the following
metrics:

    788.19 msec     task-clock:u              #    0.998 CPUs utilized
 2,502,967,113      cycles:u                  #    3.176 GHz
 7,780,571,392      instructions:u            #    3.11  insn per cycle

In contrast to the following for the old code:

  1,061.09 msec     task-clock:u              #    0.998 CPUs utilized
 3,374,141,510      cycles:u                  #    3.180 GHz
12,012,570,139      instructions:u            #    3.56  insn per cycle

This results in a reduction of about 1B cycles, or 25% reduction in wall
clock time.

Originally `perf` would show a heavy hotspot (in the area of 50% of the
total runtime) in `parse_sysv_extended_name`.
    @nagisa
    nagisa committed May 10, 2021
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  2. Further reduce the runtime of parse_u64_digits 

    Here instead of figuring out the extents of the integer ahead of time we
check for the spaces while we compute the number itself. This further
reduces the runtime of the beforementioned case (see previous commit) to:

    580.57 msec     task-clock:u              #    0.997 CPUs utilized
 1,843,957,595      cycles:u                  #    3.176 GHz
 5,901,570,558      instructions:u            #    3.20  insn per cycle

`perf report` still shows that the most of the time is spent parsing
sysv archive names (which makes sense – its pretty much all the program
does after all!).
    @nagisa
    nagisa committed May 10, 2021
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  3. Replace other .position() patterns with memchr 

    This makes `memchr` a non-optional dependency, due to its use in
`read_string` method of `pod::Bytes`.
    @nagisa
    nagisa committed May 10, 2021
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