Re: Java puzzler

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 12 May 2011 18:00:41 -0700

Message-ID:

<u_adndVeLeVeH1HQnZ2dnUVZ_qadnZ2d@earthlink.com>

On 5/12/2011 5:47 PM, Nancy 3 wrote:

On 12/05/2011 5:32 PM, markspace wrote:

On 5/12/2011 1:56 PM, Nancy 3 wrote:

There's also the minor little matter of number crunching performance
going straight to hell if every little add or multiply suddenly has
some test-and-branch instructions added near it.

I believe arithmetic overflow could be handled efficiently at the
machine level, without the need for an explicit test-and-branch. I
haven't looked at any datasheets recently, but I recall most CPUs have
an option to trap, automatically, on overflow detection.

I'm sorry. I assumed you wanted an ArithmeticException or some other
Java exception on overflow, rather than an application fault ala SIGSEGV
that brings down the whole JVM. :)

There are two separate issues:

1. How to detect. That needs to be done as efficiently as possible,
presumably using hardware features where available.

2. How to report. Typically, the operating system would turn something
like that into a signal. The JVM would set its own handler for the
signal. That handler could e.g. force a Java throw into the thread that
trapped. One hopes that the overflows will be rare, so the reporting is
not performance critical.

Patricia