More hash optimization

[oscardssmith]

This is two separate optimizations.

The first is speeding up hash(::Union{Float16, Float32}) using the fact that they always are within the range of Int64 so you don't need to check that (or check whether they are within the UInt64 range.

The second is that AbstractFloats and Integers will never have a denominator that isn't 1, so we can change the rules of hashing to not hash the denominator if it is 1.

[sjkelly]

Do you have benchmarks?

[oscardssmith]

#Before
julia> @btime hash(x, UInt(1)) setup=x=rand(Int128)
  16.148 ns (0 allocations: 0 bytes)

julia> @btime hash(x, UInt(1)) setup=x=big(rand(Int128))
  18.352 ns (0 allocations: 0 bytes)

#After
julia> @btime hash(x, UInt(1)) setup=x=rand(Int128)
  11.877 ns (0 allocations: 0 bytes)

julia> @btime hash(x, UInt(1)) setup=x=big(rand(Int128))
  18.248 ns (0 allocations: 0 bytes)

[oscardssmith]

Now that we've successfully unbroken hashing on master, I think this is ready to go.

[LilithHafner]

For optimization 1, the OP is out of date, only Float16s are changed, not Float32s as well? I also don't see the speedup:

julia> @btime sum(hash, reinterpret(Float16, typemin(UInt16):typemax(UInt16)));
  224.000 μs (0 allocations: 0 bytes)

shell> git checkout more-hash-optimization -- base/float.jl base/gmp.jl base/rational.jl

julia> @btime sum(hash, reinterpret(Float16, typemin(UInt16):typemax(UInt16)));
  223.333 μs (0 allocations: 0 bytes)

[oscardssmith]

Updated benchmarks. I agree that I'm not seeing much improvement for Float16. Not fully sure why.

[LilithHafner]

If this doesn't provide measurable performance benefits, we should probably remove it:

julia/base/float.jl

Lines 658 to 669 in 0a2d6fc

	function hash(x::Float16, h::UInt)
	# see comments on trunc and hash(Real, UInt)
	if isfinite(x) # all finite Float16 fit in Int64
	xi = fptosi(Int64, x)
	if isequal(xi, x)
	return hash(xi, h)
	end
	elseif isnan(x)
	return hx_NaN ⊻ h # NaN does not have a stable bit pattern
	end
	return hash_uint64(bitcast(UInt64, Float64(x))) - 3h
	end