Standard way to supply hints to branches

[Walter Bright]

On 9/10/2024 1:48 PM, Manu wrote:
> On Sat, 7 Sept 2024 at 21:16, Walter Bright via Digitalmars-d 
>     Since CPU branch prediction for forward branches assumes they are not taken, it
>     seems your style is statistically less likely. (As I assume the CPU designers
>     did collect statistics on this.)
> 
> 
> The statistical majority that you allude to is overwhelmingly dominated by the 
> branch UP and the end of every loop cycle.
> Statistically speaking, almost all branches encountered during program flow are 
> up-branches at the end of loop cycles, which infers the rule that an up branch 
> should be predicted true, and therefore given a 
> sign-bit-branch-preiction strategy, down-branch must therefore predict false. No 
> other statistical realities past the loop continuation branch are relevant.

I think that is what I wrote.

> I'd also suggest that, other than maybe the statistical probability of a loop 
> continuation being a good choice to predict in the true case, the strategy of 
> sign-bit based prediction is more a useful tool for a compiler than an inherent 
> architectural optimising feature. It gives the compiler agency that it might not 
> otherwise have, and in the cases where the compiler can't infer meaningful 
> predictions, we should be able to supply them...

I did write that forward branches were considered not likely, and backward 
branches likely. We are in agreement on that.