[PATCH v7 2/2] Optimize bsearch() implementation for performance

[Kuan-Wei Chiu]

On Wed, Sep 11, 2024 at 06:16:32PM -0400, DJ Delorie wrote:
> Kuan-Wei Chiu <visitorckw@gmail.com> writes:
> > Optimize the bsearch() function to improve binary search performance.
> > Although the code size grew by 8 bytes, the new implementation achieves
> > a 15% reduction in execution time on my x86 machine, according to the
> > bench-bsearch benchmark results.
> 
> LGTM as-is but I noted something to try that might speed it up more.
> Reviewed-by: DJ Delorie <dj@redhat.com>

Thank you for your review and suggestions!
> 
> > diff --git a/bits/stdlib-bsearch.h b/bits/stdlib-bsearch.h
> 
> >  bsearch (const void *__key, const void *__base, size_t __nmemb, size_t __size,
> >  	 __compar_fn_t __compar)
> >  {
> > -  size_t __l, __u, __idx;
> >    const void *__p;
> >    int __comparison;
> >  
> > -  __l = 0;
> > -  __u = __nmemb;
> > -  while (__l < __u)
> > +  while (__nmemb)
> >      {
> > -      __idx = (__l + __u) / 2;
> > -      __p = (const void *) (((const char *) __base) + (__idx * __size));
> > +      __p = (const void *) (((const char *) __base) + ((__nmemb >> 1) * __size));
> 
> We're replacing an add/shift with just a shift, but using the existing
> __base and __nmemb instead of the new __l and __u.  Ok.
> 
> After this, __p points to base[__nmemb>>1]
> 
> >        __comparison = (*__compar) (__key, __p);
> > -      if (__comparison < 0)
> > -	__u = __idx;
> > -      else if (__comparison > 0)
> > -	__l = __idx + 1;
> > -      else
> > +      if (__comparison == 0)
> 
> It seems to me performance would be better if the more likely cases went
> first - checking "__comparison > 0" would, approximately 50% of the
> time, skip the "__comparison == 0" check, but not the other way around.
> Have you tried this with the ">0" test before the "==0" test, or with
> marking the "==0" test as unlikely?
> 
> Keeping the >0 and <0 paths close to the top of the loop might help with
> cache hits too, depending on link alignment.

I tried adding unlikely(), but it seems that gcc generates the exact
same code (Note: I only checked on x86). As for moving the > 0 test
first, I also gave it a try, but it actually worsened the benchmark
results. I haven’t looked closely at the generated code to understand
why, but here are the benchmark results:

For the current patch:
{
 "timing_type": "hp_timing",
 "functions": {
  "bsearch": {
   "bench-variant": "default",
   "results": [
    {
     "array-size": 100000,
     "element-size": 4,
     "key-pattern": "descending",
     "contained": "no",
     "simple": "yes",
     "timing": 110.092
    },
    {
     "array-size": 100000,
     "element-size": 4,
     "key-pattern": "descending",
     "contained": "yes",
     "simple": "yes",
     "timing": 99.0945
    },
    {
     "array-size": 100000,
     "element-size": 4,
     "key-pattern": "ascending",
     "contained": "no",
     "simple": "yes",
     "timing": 112.395
    },
    {
     "array-size": 100000,
     "element-size": 4,
     "key-pattern": "ascending",
     "contained": "yes",
     "simple": "yes",
     "timing": 98.5953
    }]
  }
 }
}

With the > 0 test moved to the front:
{
 "timing_type": "hp_timing",
 "functions": {
  "bsearch": {
   "bench-variant": "default",
   "results": [
    {
     "array-size": 100000,
     "element-size": 4,
     "key-pattern": "descending",
     "contained": "no",
     "simple": "yes",
     "timing": 113.875
    },
    {
     "array-size": 100000,
     "element-size": 4,
     "key-pattern": "descending",
     "contained": "yes",
     "simple": "yes",
     "timing": 101.111
    },
    {
     "array-size": 100000,
     "element-size": 4,
     "key-pattern": "ascending",
     "contained": "no",
     "simple": "yes",
     "timing": 113.713
    },
    {
     "array-size": 100000,
     "element-size": 4,
     "key-pattern": "ascending",
     "contained": "yes",
     "simple": "yes",
     "timing": 105.429
    }]
  }
 }
}
> 
> >  	{
> >  #if __GNUC_PREREQ(4, 6)
> >  # pragma GCC diagnostic push
> > @@ -46,6 +38,12 @@ bsearch (const void *__key, const void *__base, size_t __nmemb, size_t __size,
> >  # pragma GCC diagnostic pop
> >  #endif
> >  	}
> > +      if (__comparison > 0)
> > +	{
> > +	  __base = ((const char *) __p) + __size;
> > +	  --__nmemb;
> > +	}
> > +      __nmemb >>= 1;
> 
> So if comparison < 0 (key is before probe):
>   __nmemb is set to the number of elements before __p which is base[__nmemb>>1]
>   __base is unchanged
>     - range is 0..__nmemb-1
> 
> if comparison is > 0 (key is after probe):
>   __base is set to the element after the probe
>   If (__nmemb is odd)
>     probe point is at (__nmemb-1) >> 1
>     same numbers before and after probe
>     subtracting one is unneeded but has no effect
> 
> 0 1 2 3 4 5 6 7 8  (n == 9)  (n>>1 == 4)
>         P
> 0 1 2 3 4 5 6  (n == 7)  (n>>1 == 3)
>       P
> 
>   If (__nmemb is even)
>     probe point is at (__nmemb) >> 1
>     even numbers before but odd number after
>     subtracting one is needed
> 
> 0 1 2 3 4 5 6 7  (n == 8)  (n>>1 == 4)
>         P
> 0 1 2 3 4 5  (n == 6)  (n>>1 == 3)
>       P
> 
> Ok.
> 
> >      }
> >  
> >    return NULL;
>