• isharipo's avatar
    mime: do a pre-allocation in encodeWord · 3fb3ca08
    isharipo authored
    The preallocated memory size is comparable to bytes.Buffer bootstraping
    array length (bytes.Buffer was used before rewrite to strings.Builder).
    
    Without preallocation, encodeWord does more than one allocation for
    almost any possible input.
    
    The regression happens because bytes.Buffer did a 80-bytes allocation
    at the beginning of encodeWord while strings.Builder did several
    smaller allocations (started with cap=0).
    
    Comparison with reported regression:
    
      name           old time/op    new time/op    delta
      QEncodeWord-4     781ns ± 1%     593ns ± 1%  -24.08%  (p=0.008 n=5+5)
    
      name           old alloc/op   new alloc/op   delta
      QEncodeWord-4      152B ± 0%       80B ± 0%  -47.37%  (p=0.008 n=5+5)
    
      name           old allocs/op  new allocs/op  delta
      QEncodeWord-4      5.00 ± 0%      2.00 ± 0%  -60.00%  (p=0.008 n=5+5)
    
    Comparison with buffer solution (like before strings.Builder, but
    without sync pool for buffer re-using):
    
      name           old time/op    new time/op    delta
      QEncodeWord-4     595ns ± 1%     593ns ± 1%     ~     (p=0.460 n=5+5)
    
      name           old alloc/op   new alloc/op   delta
      QEncodeWord-4      160B ± 0%       80B ± 0%  -50.00%  (p=0.008 n=5+5)
    
      name           old allocs/op  new allocs/op  delta
      QEncodeWord-4      2.00 ± 0%      2.00 ± 0%     ~     (all equal)
    
    We avoid allocation in buf.String(), as expected.
    
    Fixes #25379
    
    Change-Id: I19763f0e593a27390c1a549b86ce6507b489046b
    Reviewed-on: https://go-review.googlesource.com/113235
    Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
    Reviewed-by: 's avatarBrad Fitzpatrick <bradfitz@golang.org>
    3fb3ca08
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