Switch codegen.

[Johan]

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).

-Johan


On Monday, 7 August 2023 at 20:22:06 UTC, claptrap wrote:
> Given code like this...
>
> ```d
> const int x;
> while(...)
> {
>     switch (x)
>     {
>         case 0: whatever; break;
>         case 1: whatever; break;
>         case 2: whatever; break;
>         case 3: whatever; break;
>         case 4: whatever; break;
>     }
> }
> ```
>
> It generates a jump table if there's at least 4 or 5 cases, but 
> at the start of the switch it generates 6 instructions (x64), 
> check the bounds, lookup / calc the destination, and the actual 
> jump. Like this...
>
> ```d
>         lea     r13, [rip + .LJTI0_0]
>         cmp     r14d, 7
>         ja      .LBB0_11
>         movsxd  rax, dword ptr [r13 + 4*r12]
>         add     rax, r13
>         jmp     rax
> ```
>
> So the point is if the switch is in a loop and x is constant, 
> most of that could be done ahead, outside the loop. It could 
> actually be reduced to a single indirect jump inside the loop. 
> It seems there is a LLVM IR instruction that may be useful for 
> this, "indirectbr", it's a sort of computed goto.
>
> https://llvm.org/docs/LangRef.html#indirectbr-instruction
>
> Is this at all feasible? If so how would this be done? In the 
> actual IR codegen or as an optimization pass? I willing to do 
> the work if it's possible.