[libcxx] r275114 - Don't compute modulus of hash if it is smaller than the bucket count.

[Duncan P. N. Exon Smith via cfe-commits]

We saw mixed results from this on LNT, including some major regressions.  For example, on x86_64, SingleSource/Benchmarks/Shootout-C++/hash2 regressed 18.5% at -O3 and over 20% at -Os.

Is this expected?

> On 2016-Jul-11, at 15:02, Eric Fiselier via cfe-commits <cfe-commits at lists.llvm.org> wrote:
> 
> Author: ericwf
> Date: Mon Jul 11 17:02:02 2016
> New Revision: 275114
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=275114&view=rev
> Log:
> Don't compute modulus of hash if it is smaller than the bucket count.
> 
> This cleans up a previous optimization attempt in hash, and results in
> additional performance improvements over that previous attempt. Additionally
> this new optimization does not hinder the power of 2 bucket count optimization.
> 
> Modified:
>    libcxx/trunk/include/__hash_table
> 
> Modified: libcxx/trunk/include/__hash_table
> URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/__hash_table?rev=275114&r1=275113&r2=275114&view=diff
> ==============================================================================
> --- libcxx/trunk/include/__hash_table (original)
> +++ libcxx/trunk/include/__hash_table Mon Jul 11 17:02:02 2016
> @@ -90,7 +90,8 @@ inline _LIBCPP_INLINE_VISIBILITY
> size_t
> __constrain_hash(size_t __h, size_t __bc)
> {
> -    return !(__bc & (__bc - 1)) ? __h & (__bc - 1) : __h % __bc;
> +    return !(__bc & (__bc - 1)) ? __h & (__bc - 1) :
> +        (__h < __bc ? __h : __h % __bc);
> }
> 
> inline _LIBCPP_INLINE_VISIBILITY
> @@ -2201,8 +2202,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>
>         if (__nd != nullptr)
>         {
>             for (__nd = __nd->__next_; __nd != nullptr &&
> -                (__hash == __nd->__hash_
> -                    || __constrain_hash(__nd->__hash_, __bc) == __chash);
> +                __constrain_hash(__nd->__hash_, __bc) == __chash;
>                                                            __nd = __nd->__next_)
>             {
>                 if ((__nd->__hash_ == __hash) && key_eq()(__nd->__value_, __k))
> @@ -2231,8 +2231,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>
>         if (__nd != nullptr)
>         {
>             for (__nd = __nd->__next_; __nd != nullptr &&
> -                (__hash == __nd->__hash_
> -                    || __constrain_hash(__nd->__hash_, __bc) == __chash);
> +                  __constrain_hash(__nd->__hash_, __bc) == __chash;
>                                                            __nd = __nd->__next_)
>             {
>                 if ((__nd->__hash_ == __hash) && key_eq()(__nd->__value_, __k))
> 
> 
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