It is well known that compilers can't optimise things as well if values may
alias. Because src and dest may overlap:
public static void multiply(int[] accum, int accumOffset, int[] src, int
srcOffset, int n){
for (var ii = 0; ii < n; ++ii) {
dest[ii] *= src[ii];
}
}
does not have the same semantics as possibly better optimised versions.
This is behind the difference between the memcpy and memmove functions in C
and why C99 has the restrict keyword and also why Fortran was historically
faster for a long time.
Now Java doesn't have the restrict keyword but you can create separate byte
arrays and then checking whether arrays can possibly overlap is just a
pointer comparison and then if you have to you can dispatch to manually
optimised versions for that case.
The thing is I am starting to wonder if aliasing is preventing one of my
programs from being optimised as much as it should.
Do modern JVMs use aliasing information? Is it possible to give them more
aliasing information?
Should I look into doing something like:
public static void multiply(int[] dest, int destStart, int[] src, int
srcStart, int n) {
if (src != dest) {
multiply_nonaliasing(dest, destStart, src, srcStart, n);
} else if (srcStart + n < destStart || destStart + n < srcStart) {
multiply_nonoverlapping(dest, destStart, src, srcStart, n);
} else {
// maybe throw an error instead
multiply_overlapping(dest, destStart, src, srcStart, n);
}
}
How would one code multiply_nonalaising and nonoverlapping give the JVM
such information?
I believe I have one of the worst case examples were I am emulating a big
long flat heap memory.
private static byte[] bigHeap = new byte[8 * 4096];
It seems silly to me but I am starting to wonder if an approach like:
private static byte[][] bigHeapOfPages = new byte[8][4096];
might theoretically be possible to make faster because I can make sure
things do not alias each other.
There seem to be some complicated scholarly papers online about alias
disambiguation which mention this would be useful for solving performance
difficulties on the JVM but I don't know what the current state of things
are right now and what I can do personally right now.
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