Standard way to supply hints to branches

[Manu]

On Sat, 31 Aug 2024 at 17:16, Dom DiSc via Digitalmars-d <
digitalmars-d at puremagic.com> wrote:

> On Friday, 30 August 2024 at 02:54:28 UTC, Manu wrote:
> > On Fri, 30 Aug 2024 at 04:32, Walter Bright via Digitalmars-d <
> > digitalmars-d at puremagic.com> wrote:
> >
> >> On 8/28/2024 2:45 AM, Manu wrote:
> >> > Here's one that I just wrote a short while ago:
> >> > https://gist.github.com/TurkeyMan/0e49da245cc0086f852ac18deed21a9c
> >>
> >>
> >> ```
> >> if (data.length < 4) // unlikely
> >>      return 0;
> >> ```
> >>
> >> replace with:
> >>
> >> ```
> >> if (data.length < 4)
> >>      goto Lreturn0;
> >> ```
> >>
> >
> > How is that any different? The branch prediction hasn't changed.
>
> I already asked this question.
> He said a single break or goto is NOT considered the hot branch
> by the compiler.
>

The goto is not a branch at all. The if() is the branch... and it still
predicts the pessimistic case incorrectly.
Replacing the return with a goto hasn't changed the control flow in any
way; goto and return are equivalent; they are not branches.


> But I don't like this, because it's an implementation detail that
> every compiler may implement or not, and it's not documented
> anywhere.
> Maybe if the documentation would clearly state at a prominent
> point that a single break or goto has to be considered the cold
> path by the compiler, but a function call is to be considered the
> hot path (and all related queries about branch priorization link
> there), then I would consider this a solution. But is it likely
> this will happen?
>
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