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

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

Hmm.  I implied there were other regressions, but I just finished scanning them.  Shootout-C++/hash2 is the only major one.  The others were small, and only at -O0.

> On 2016-Jul-13, at 17:38, Duncan P. N. Exon Smith via cfe-commits <cfe-commits at lists.llvm.org> wrote:
> 
> 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?

^ Still interested in an answer, though ;).

> 
>> 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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