From: Julian Seward <jseward@acm.org>
To: qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH] Fix overflow conditions for MIPS add / subtract
Date: Fri, 28 Apr 2006 16:52:07 +0100 [thread overview]
Message-ID: <200604281652.07359.jseward@acm.org> (raw)
In-Reply-To: <20060428132818.GA27384@nevyn.them.org>
> > - if ((T0 >> 31) ^ (T1 >> 31) ^ (tmp >> 31)) {
> > + if (((tmp ^ T1 ^ (-1)) & (T0 ^ T1)) >> 31) {
> > + /* operands of same sign, result different sign */
> > CALL_FROM_TB1(do_raise_exception_direct, EXCP_OVERFLOW);
> > }
>
> I see this went in, but - huh? The math doesn't make sense.
>
> T0 ^ T1 -> operands of different sign
> tmp ^ T1 ^ (-1) -> result has same sign as T1
The definitive reference for all this bit twiddling magic and
much more besides is an excellent book, "Hacker's Delight", by
Hank Warren. It has loads of stuff about integer overflow and
whatnot.
J
prev parent reply other threads:[~2006-04-28 15:52 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-13 18:49 [Qemu-devel] [PATCH] Fix overflow conditions for MIPS add / subtract Stefan Weil
2006-04-28 13:28 ` Daniel Jacobowitz
2006-04-28 14:51 ` Dirk Behme
2006-04-28 15:47 ` Daniel Jacobowitz
2006-04-28 15:52 ` Julian Seward [this message]
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