11–20 of 26 posts
This reminded me of PyPy, where (I think) you write an interpreter, than add what they call JIT hints to the interpreter to help it do a good job of JITting the interpreted code.
The authors of programs that are being interpreted by that interpreter are unable to provide any information to the JIT.
I am looking forward to see Graal eventually replace Hotspot, but so far it is only a possible Java 9 feature.
Earlier quoted context omitted.
I'd be interested to see how much of a difference cooperating with the compiler makes. I think the usual advice that until you profile you don't know what the bottlenecks in your code are applies here. Similarly, I think that it is unlikely that the places where humans could add anything of value to the compiler have much of an impact on actual performance (with the exception of types which I'll discuss below). And i…
Aliasing information is one place the programmer can help the compiler. The 'restrict' keyword in C/C++ is a great example of this. Also information provided by 'const' can be very helpful for the compiler. While both of these aren't strictly hints because they limit what you can do and still get correct results, they do tell the compiler facts that would be otherwise difficult or impossible to prove.
I am looking forward to see Graal eventually replace Hotspot, but so far it is only a possible Java 9 feature.
Is it a replacement for HotSpot or an add-on?
Maxime is/was a meta-circular JVM done at Sun research labs.
Since Oracle bought Sun, Maxime project changed into Graal and there have been a few presentations where the goal to replace Hotspot has been hinted a few times.
There are a few Oracle presentations where it is listed as possible Java Next (8+) feature, but without any kind of commitment when.
Earlier quoted context omitted.
I'd be interested to see how much of a difference cooperating with the compiler makes. I think the usual advice that until you profile you don't know what the bottlenecks in your code are applies here. Similarly, I think that it is unlikely that the places where humans could add anything of value to the compiler have much of an impact on actual performance (with the exception of types which I'll discuss below). And i…
Regarding the "register" keyword, I'm pretty sure I was reading books in the 90s which described it as a historical curiosity that was ignored by all modern, right-thinking compilers.
I don't know if the authors would agree, but Dalvik's JIT compiler appears to be a "precision" JIT. The goals for Dalvik's JIT compiler were to speed up apps without reducing battery life by focusing on compiling only the parts of code that will deliver the most performance increase. One can also suppose that, with more memory and more power-efficient processors, Google has calculated that in real-world cases, ART's…
It is my great sorrow that I have no time to read these excellent things. The rest of you, please do it justice.
At one point the programmer was expected to regularly override the compiler, replacing sections with hand-tuned assembly for maximum throughput, based on their knowledge of the CPU architecture, hand-profiling, & expecting the calling pattern. Then the programmer was expected mostly to trust the compiler, which either applies massive, unwieldy (for a human) optimizations at compile time, or develops new optimizations…
I'd be interested to see how much of a difference cooperating with the compiler makes. I think the usual advice that until you profile you don't know what the bottlenecks in your code are applies here. Similarly, I think that it is unlikely that the places where humans could add anything of value to the compiler have much of an impact on actual performance (with the exception of types which I'll discuss below). And i…
And that can cross libraries: if you #include a standard header, a C++ compiler may assume that calling, say, malloc() or exit() does what the standard says it does. That way, even a function that calls malloc or gettimeofday can be optimized away before it is even linked.
And of course, in many compilers, programmers can use #pragma's or attributes to instruct the compiler "you can assume this function is pure/doesn't return/returns newly allocated memory/etc (http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html)
If you do that meticulously, the compiler has way more room for optimizations.