Switch codegen.

[Johan]

On Sunday, 13 August 2023 at 18:57:42 UTC, claptrap wrote:
> On Sunday, 13 August 2023 at 13:13:30 UTC, Johan wrote:
>> I'm quite certain that if `x` is constant, the bounds check 
>> and table lookup will be hoisted out of the loop, indeed 
>> leading to just an indirect jump inside the loop.
>> Of course, this requires running the optimizer (-Ox, x>0).
>
> I've tried as many variations as I can figure and it always 
> generates this...
>
> ```D
>     cmp     r14d, 3
>     ja      .LBB0_9
>     movsxd  rax, dword ptr [r12 + 4*r15]
>     add     rax, r12
>     jmp     rax
> ```
>
> https://d.godbolt.org/z/jzoevxoG4
>
> That's with O2, with O3 I cant make head nor tail of the code 
> it generates, it's some pretty insane optimization.

It's easier to look at LLVM IR in this case.
https://d.godbolt.org/z/b6b3TTeMW

1)  By providing the implementation of `bar`, you are giving the 
optimizer extra information. It may not inline `bar`, but it 
_will_ make use of what it knows about the result of bar. That's 
why at -O3, there is insane optimization going on. Simply remove 
the implementation of `bar` for more insight (make it a function 
declaration, not a definition).

2) Indeed at -O2, the optimizer deems it non-profitable or does 
not see that it can hoist the load out of the for loop.  At -O3 
it does hoist it out, like you wanted.

cheers,
   Johan