Standard way to supply hints to branches

[Walter Bright]

On 9/13/2024 4:56 AM, Timon Gehr wrote:
> Well, it is the attribute being associated with the program path being 
> ill-defined that is being criticized in that blog post. The difference is that 
> for path-associated, you are saying that a specific statement is likely or 
> unlikely to be executed, for branch-associated, you are saying in which 
> direction a specific branch is likely to go.

Ok, thanks for the explanation. The branch predictor on CPUs defaults to a 
forward branch being considered unlikely, and a backwards branch being 
considered likely.

I'm sure the CPU designers collected statistics on this before burning this into 
the branch predictor hardware.

It's a simple and easily remembered rule, and so why dmd behaves as it does.

As for an attribute, I cannot see it being viable for anything other than the 
`if`, as the undocumented menace of it being applied to anything else is 
apparent in the blog post.

Hence, if D were to support something along those lines, it would be a keyword 
such as:

```
ifrarely (i) return 0;
```

as the least ugly approach. But I've been unable to come up with an obviously 
good keyword for it. And we would need buyin from Iain and Martin.