On Nov 6, 2013, at 11:30 AM, Peter Levart <[email protected]> wrote:
> Well, indexOf(char) or lastIndexOf(char) searches for a single char. We can
> do better searching backwards for two chars at the same time.
>
> If the "name" of VM-anonymous class is always ending with pattern: "slash
> followed by some decimal digits" then the following would be even faster:
Although this reasoning is plausible, it is not a reliable conclusion, and
should not drive edits of Java code without careful measurement. The reasoning
assumes a performance model based on the interpreter and bytecode count. But
performance depends on native code produced by the JIT.
An optimizing JIT will usually transform the code deeply. For string scanning,
for example, HotSpot has an intrinsic for String.indexOf(String) that uses
totally different code from a user-coded loop. (It is not currently so for
String.indexOf(int), but String.indexOf("/") is potentially very fast.)
Also, with your example code, the combined loop may often be faster than two
back-to-back loops, but simpler loops can sometimes be vectorized more
robustly, to the point where back-to-back simple loops, if vectorized, may be
competitive with a hand-fused loop.
So loop complexity and method intrinsics can create surprises for those who
rely on simple performance modesl
Some day we will get to a world where loops are specified stream-wise, and
robustly optimized without this sort of manual intervention. In the mean time,
be careful about advising hand-optimizations of Java code. They can backfire,
by confusing the JIT and preventing optimizations that would apply to simpler
code.
— John
