It could be that integer powers are done with binary shifts in software and
the floating point powers are computed in the fpu.
On Thursday, September 24, 2015 at 3:47:22 PM UTC-4, Kristoffer Carlsson
wrote:
>
> I think Tom iir right here. These lines call the pow function
>
> movabsq $pow, %rax
> callq *%rax
>
> but the actual pow functions that is being called is different. I am
> surprised it is that much of a difference in performance between the two
> pow functions... That seems odd.
>
> What Mauro says is also interesting that the speed difference is there
> (and is as large) even without the fastmath macro.
>
> My question now is, what does IEEE say about x^double vs x^int. Is there
> any reason these should have different performance? If not, it seems to
> make sense to always convert the exponent to a double and call the libm
> version? All doubles should be able to exactly represent the integers that
> the power function take?
>
>
> On Thursday, September 24, 2015 at 9:18:45 PM UTC+2, Mauro wrote:
>>
>> I dissected the bench-method into two, just to be sure (on 0.4-RC2).
>>
>> julia> function bench(N)
>> for i = 1:N
>> f(π/4)
>> end
>> end
>> bench (generic function with 1 method)
>>
>> julia> function bench_f(N)
>> for i = 1:N
>> f_float(π/4)
>> end
>> end
>> bench_f (generic function with 1 method)
>>
>> They also have identical native code but run differently:
>>
>> julia> @time bench_f(10^7)
>> 0.190613 seconds (5 allocations: 176 bytes)
>>
>> julia> @time bench(10^7)
>> 0.780212 seconds (5 allocations: 176 bytes)
>>
>> I thought that @code_native shows the code which is actually run, so why
>> different speeds?
>>
>> If I define the f* functions without the @fastmath macro, then I get
>> the same performance as above:
>>
>> julia> @time bench_f(10^7)
>> 0.203071 seconds (5 allocations: 176 bytes)
>>
>> julia> @time bench(10^7)
>> 0.787696 seconds (5 allocations: 176 bytes)
>>
>> but with different native-codes.
>>
>> > I can reproduce... I think the 2 versions will call these methods
>> > respectively... I guess there's a performance difference?
>> >
>> > pow_fast{T<:FloatTypes}(x::T, y::Integer) =
>> >> box(T, Base.powi_llvm(unbox(T,x), unbox(Int32,Int32(y))))
>> >>
>> >
>> >
>> >> pow_fast(x::Float64, y::Float64) =
>> >> ccall(("pow",libm), Float64, (Float64,Float64), x, y)
>> >
>>
>> Tom, or are those two functions called within the native-code? I'm no
>> good assembler reader.
>>
>
