JVM Flag ergonomics and constraints

[David Holmes]

On 3/02/2023 5:12 pm, Ioi Lam wrote:
> 
> 
> On 2/1/2023 10:37 PM, David Holmes wrote:
>> <trimming>
>>
>> On 2/02/2023 3:25 pm, Thomas Stüfe wrote:
>>> On Thu, Feb 2, 2023 at 5:30 AM David Holmes <david.holmes at oracle.com 
>>> <mailto:david.holmes at oracle.com>> wrote:
>>>
>>>     That would have to be determined case-by-case. For each ergo flag we
>>>     would have to decide what value it is calculating and based on what,
>>>     and
>>>     determine whether an out-of-range value is an error or something 
>>> to be
>>>     wary of.
>>>
>>> Sure, what I meant was how to control it? Two sets of FLAT_SET_... 
>>> macros, one that asserts on constraint violation, one that caps?
>>
>> No I expect to just see the places where FLAG_SET_ERGO is called do 
>> the right thing by checking the return value and proceeding accordingly.
> 
> David, are you proposing this?
> 
>     int value = calculate_ergo();
>     if (FLAG_SET_ERGO (TheFlag, value) == error) {
>       value = do_it_again()
>       if (FLAG_SET_ERGO (TheFlag, value) == error) {
>          huh?
>       }
>     }
> 
> In the above code, all FLAG_SET_ERGO does is to check the range for you. 
> It doesn't tell you why the range check fails. So the caller would have 
> to redo the calculation (with no more information, other than that the 
> last value was no good). What if the new value is also no good?

To me that indicates we don't know what the heck we are calculating! How 
can you calculate a setting for a flag if you don't even understand the 
valid range of that flag. Which may mean the max, for example, needs to 
be exposed programmatically.

> With this style, you almost have to wrap every FLAG_SET_ERGO  in a loop, 
> keep trying (in the dark) until you succeed.
> 
> ==================
> I think it's better to just assert inside FLAG_SET_ERGO if the value is 
> out of range. The caller should make sure it has calculated a correct 
> value.

If the calculation uses some externally settable value then an assert 
won't help if we're in a product build.

As I said it depends on exactly what we are using to do the calculation. 
If it is "closed world" then it is a fatal error to get it wrong and 
should be detected during testing. Otherwise the code has to deal with 
the possibility of error.

> 
> Maybe we can add a new FLAG_SET_ERGO_CAPPED() that would cap the value, 
> but that should be an opt-in. FLAG_SET_ERGO() should not automatically 
> cap the value.

That might be suitable for some flags/calculations.

I don't like trying to talk about this in generalities. Each specific 
situation needs to be examined to understand what is being calculated 
and how. But it is the callers of FLAG_SET_ERGO that need to know what 
they are doing.

Cheers,
David

> 
> Thanks
> - Ioi
>