Aleksey Shipilev wrote:
Hi,
I had returned to idea to have @Inline and @NoBoundCheck annotation
support in classlib and Jitrino.
I will try to summarize the rationale for both these annotations:
1. @Inline. There are places where we're creating small methods to
get more consistent code, while we expect JIT should inline them to
reduce call penalty. Unfortunately, this is not always the case and
any JIT can miss the opportunities for inline. As classlib developers
we can dope our code with the hints saying "this method is really
should be inlined, as we know it would be the penalty leaving it not
inlined, just do it even when inline budget is exhausted". Jitrino
really have @Inline already as the part of vmmagic package, but I want
to see these annotations visible from the classlib part.
That's the case of new HashMap [1] for example:
/*
* Contract-related functionality
*/
static int computeHashCode(Object key) {
return key.hashCode();
}
static boolean areEqualKeys(Object key1, Object key2) {
return key1.equals(key2);
}
static boolean areEqualValues(Object value1, Object value2) {
return value1.equals(value2);
}
2. @NoBoundCheck. There are also cases in which we definitely know
that no bound check need to be performed. This is the case of HashMap
again:
...
int hash = computeHashCode(key);
index = hash & (elementData.length - 1);
entry = elementData[index];
...
Of course, good JIT compiler should also resolve such patterns and
eliminate bounds check here, but we can again hint the compiler they
are not necessary. There's a complication though that such pragma
could violate security if used in user code, but we could restrict its
usage to bootstrap classes only. ABCD gurus (Egor?) could shed more
light whether it's possible to implement on JIT side.
What do you think? I can elaborate with proof-of-concept patches to
see what advantage it would bring.
Thanks,
Aleksey.
[1] https://issues.apache.org/jira/browse/HARMONY-5791
Hi Aleksey,
an alternate approach may be to use a bytecode engineering library. For
example, Jikes RVM uses ASM to add annotations to the library during
bootstrap compilation [1]. A few of the other annotations we use are:
@Pure to indicate that a method can be called at compile time if its
arguments are constants. This allows us to turn
BigInteger.ONE.add(BigInteger.TEN) into a literal BigInteger holding the
value 11.
@NoEscapes indicates that if an aggregate (object or array) is passed as
an argument to a method, and this call is the sole reason an aggregate
escapes, the object can be replaced by scalars. We use this to avoid
stack trace generation when the stack trace is never read.
Regards,
Ian Rogers
[1]
http://jikesrvm.svn.sourceforge.net/viewvc/jikesrvm/rvmroot/trunk/tools/asm-tasks/src/org/jikesrvm/tools/asm/AnnotationAdder.java?revision=14102&view=markup
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