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Given the recent thread by Manu, requesting a way to annotate likelihood for branches, I think I have found a way forward that is both in recognization of existing practices both in D and outside.
In this thread, Tejas provided a link that shows issues with the C/C++ design.
- The true path of a branch is assumed to be unlikely.
- The false path, or the fallthrough path if false path does not exist is assumed to be taken.
- D's if statement, is if-else, not if-elseif-else based, therefore it inherently has a priority and scope awareness and does not need a priority to be defined by the user.
- Switch statements have a priority for their cases by simply their order. The likely path is the default statement.
The unlikelihood is now defined against D using existing practices. No changes there.
What we want to define is a way to do a kind of swap, so to tell the compiler that no, the order in the code is the wrong way around.
switch(e) {
/*@likely*/
case E.unlikely1:
break;
default:
break;
}
if (e.unlikely1) /*@likely*/ {
} else {
}
@likely would be defined in core.attributes. It requires minimal changes, but instead it requires us to explicitly define this behavior of priorities and likelihood.
