Standard way to supply hints to branches

[Manu]

On Fri, 23 Aug 2024, 12:26 Nicholas Wilson via Digitalmars-d, <
digitalmars-d at puremagic.com> wrote:

> On Friday, 23 August 2024 at 01:47:37 UTC, Manu wrote:
> > I'm working on microcontrollers, and many don't have a branch
> > predictor. They do "static" prediction, that is, they just
> > predict branch-not-taken and it's on you to write your code as
> > such, and that's not always possible. (like the loop condition
> > in a for loop)
> >
> > This leads to a lot of awkward looking if's written in
> > unnatural 'backwards' terms, and also, leads to undesirable
> > depth of nested scopes.
> >
> > The compiler backend can and should re-jig the comparisons, but
> > it needs to receive a hint which way is 'likely' from the
> > programmer.
> >
> > How can we add an attribute to the branch condition that the
> > backend can take advantage of? I think it needs to be in the
> > language spec...
>
> For LDC see,
>
> https://github.com/ldc-developers/ldc/blob/master/runtime/druntime/src/ldc/intrinsics.di#L619
> and see also `llvm_assume` just beneath it.
>
> GDC probably has something similar too.
>

Yes exactly, this is my point, we need something in the spec. I think there
might be an opportunity to express branch prediction hints in a more
user-friendly way than assume statements... But a language assume() would
also be very useful!

>
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