Performance of tables slower than built in?

[Basilez B.]

On Thursday, 23 May 2019 at 10:16:42 UTC, Alex wrote:
> On Wednesday, 22 May 2019 at 08:25:58 UTC, Basile B. wrote:
>> On Wednesday, 22 May 2019 at 00:22:09 UTC, JS wrote:
>>> I am trying to create some fast sin, sinc, and exponential 
>>> routines to speed up some code by using tables... but it 
>>> seems it's slower than the function itself?!?
>>>
>>> [...]
>>
>> Hi, lookup tables ARE faster but the problem you have here, 
>> and I'm surprised that nobody noticed it so far, is that YOUR 
>> SWITCH LEADS TO A RUNTIME STRING COMPARISON AT RUNTIME. Just 
>> replace it with a static if (Method = "Linear") { /*...*/} 
>> else { /*...*/}
>>
>> Also takes care to the type used. With DMD the implicit 
>> coercion of float and double can lead to extra conversions.
>>
>> You'll directly see a 15% gain after refactoring the switch.
>
> Surely not?!?! Surely the compiler can optimize that switch 
> since the value passed is CT? I thought the whole point of not 
> having static switch(analogous to static if) was because it 
> would go ahead and optimize these cases for us... and it's just 
> a switch, just a jmp table.

Try by yourself but to be clear note that I don't like your 
attitude, which I find disrespectful. I'm just here to help, I 
explained you the big problem you have in your code and you start 
discussing something that's not to be discussed AT ALL. Look at 
this https://d.godbolt.org/z/vtzVdp, in sinTab you'll be able to 
see

   call    pure nothrow @nogc @safe int 
object.__switch!(immutable(char), "Linear", 
"Constant").__switch(scope const(immutable(char)[]))@PLT

and this even with LDC2 -O3. That's why your LUT is so slow. 
refactor the switch with "static if", the branch will be 
eliminated and you'll to see the perf improvement.