Re: Announce: The Ultimate SI Units Cracker

[Mark Biggar <mark.a.biggar@attbi.com>]

Dmitry A.Kazakov wrote:
> Wes Groleau wrote:
> 
> 
>>"Dmitry A.Kazakov" wrote:
>>
>>>Then any discriminant-based solution faces the following problems:
>>>
>>>1. There is no way to force the compiler to remove statically known
>>>discriminants and calculate expressions on them at compile-time.
>>
>>I prefer to look at it as
>>"There's no rule to prevent a compiler vendor from ....
>> and no way to prevent a user from selecting that compiler."
> 
> 
> Is it absolutely true? I mean, ARM precisely defines what is "static" and 
> what is not. So an expression involving static discriminants could be 
> *formally* non-static, even if its outcome were known at compile-time. Thus 
> such an outcome would be not allowed to appear where a static value is 
> expected.

Yes, the ARM precisely defines which expressions must be static, but
it also goes on the define a "duck test"* for all expressions that
says that even though the expression is not in a static context
if it only contains static values then it should be statically
evaulated at compile time.  In addition, the optimization "as if" rule
allows a compiler to do all sorts of things like Wes wants.

* Duck Test: if it looks like a duck, has a bill, feathers, quacks, has 
webbed feet, etc., then it's a duck even if it appears where no duck
has ever been seen before.


-- 
Mark Biggar
mark.a.biggar@attbi.com