ZSH performance regression in 5.8.1.2-test

[Jordan Patterson <jordanp@gmail.com>]

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Hi:

This recent fix has led to a performance regression in zsh: 49792:
Non-interative shell input is line buffered.

I had noticed that my shell was loading slower and found this Gentoo
bug report (https://bugs.gentoo.org/839900).  They had recently picked
up this patch to apply to the current 5.8.1 version.

I've benchmarked loading zsh with the test release and a couple of
previous versions with my zsh configuration:

Benchmark 1: prefix/5.8/bin/zsh -i -c exit
  Time (mean ± σ):     262.6 ms ±  10.7 ms    [User: 184.7 ms, System:
66.4 ms]
  Range (min … max):   246.7 ms … 279.4 ms    11 runs

Benchmark 2: prefix/5.8.1/bin/zsh -i -c exit
  Time (mean ± σ):     226.5 ms ±   6.0 ms    [User: 155.7 ms, System:
57.4 ms]
  Range (min … max):   216.6 ms … 238.5 ms    13 runs

Benchmark 3: prefix/5.8.1.2-test/bin/zsh -i -c exit
  Time (mean ± σ):      2.088 s ±  0.027 s    [User: 0.503 s, System: 1.562 s]
  Range (min … max):    2.056 s …  2.149 s    10 runs

Summary
  'prefix/5.8.1/bin/zsh -i -c exit' ran
    1.16 ± 0.06 times faster than 'prefix/5.8/bin/zsh -i -c exit'
    9.22 ± 0.27 times faster than 'prefix/5.8.1.2-test/bin/zsh -i -c exit'

I've also generated some flamegraphs of the same commands, which I've
attached.  The test version now spends most of its time doing the
reads.

Jordan

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