On 07/07/2012 04:54 AM, Mark Roos wrote:

yes, that the basic idea.

It's not a bytecode trick, it's a JIT trick. The VM profiles jump instruction like 'if' to know which branch is taken, and doesn't generate code but a deoptimization code if a branch is never taken. So the generator of a backend will generate a code like this for a + 1:

int r1 = ... Object o1 = ... if (o1 == null) { r2 = r1 + 1 o2 = null } else { r2 = 0 o2 = invokedynamic + o1, 1 }

if (o2 == null) { // next instruction ... } else { ... ...

and because o1 is never null, the JIT will generate

jne a_deoptimization_code inc r2

also because the JIT propagates null value aggressively, the jump can also disappear because there is perhaps already another check before, by example, the check o2 == null will be removed here.

So the idea is to consider that if a variable can store an int, you should use two variables in the bytecode, so result of an operation will be spilled into two variables instead of using the stack. I call this idea, split-path and this is really effective, the main drawback is that the generated bytecode size is big compared to the code of the generated assembler so the JIT may decide to not inline something that it should.

You have also to figure out how to get two return values from a method call, but exceptions are your best friend here.

rgds, Rémi