change mixins

[grauzone]

retard wrote:
> Tue, 16 Feb 2010 21:11:32 +0100, grauzone wrote:
> 
>> retard wrote:
>>>> Granted, even if logging is disabled, the compiler will push the
>>>> arguments on the stack and call logInfo().
>>> This is a case where a JIT compiler wins. The code runs equally fast
>>> without the call at all and with logging turned off (dynamically). In
>>> languages like D, the like has to be statically turned off unless
>>> compilers have any new dynamic optimizations.
>> Can the JIT really do this optimization? It sounds as if it'd detect
>> that the variable never changes, and generate code with that fixed
>> value.
> 
> It sure can recompile the code assuming that the logging policy doesn't 
> change that often - in case it does change often switching the code might 
> have larger overhead than testing the condition. In real life you would 
> probably start the program with either logging on or off. With JIT 
> compiled languages, the decision can be postponed to runtime, in D it 

We're talking about a globally accessible runtime variable here, right? 
Not just a local variable that was detected to be constant. What I don't 
understand is: how does the VM efficiently detect, that the value never 
changes, and that generating code for that fixed value may be worthwhile?

It is well possible in theory: the VM could just set up write barriers 
to detect changes to a variable, and then recompile speed critical code 
that uses the variable.

But it had to collect statistics about all accessed variables, and then 
decide to recompile the code. Both collecting and analyzing statistics 
and recompiling code takes time, and it has to be fast enough to not 
slow down the program even if there are "misses" (e.g. unexpected 
variable writes).

Other typical JIT optimizations sound easy compared to this case. I'd be 
really interested how this works out.

> probably has to be done on compile time and provide two binaries, if both 
> logging and efficiency are required options. I can post a rather short 
> test case in Java/Scala here, if you insist.