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NNTP-Posting-Date: Mon, 06 Feb 2012 18:10:38 -0600
Message-ID: <4F306BFB.2561@mindspring.com>
Date: Mon, 06 Feb 2012 19:10:35 -0500
From: pete <pfiland@mindspring.com>
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Subject: Re: Optimisation questions
References: <jgkbiu$432$1@speranza.aioe.org> <jgkfso$1bh$2@dont-email.me> <jgkgra$886$1@dont-email.me> <jgkou6$g96$1@dont-email.me> <20120205034114.797@kylheku.com> <jgpnon$qm7$1@dont-email.me>
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.c:16785

Stephen Sprunk wrote:
> 
> On 04-Feb-12 21:04, Kaz Kylheku wrote:
> > On 2012-02-05, James Kuyper <jameskuyper@verizon.net> wrote:
> >> On 02/04/2012 07:00 PM, Stephen Sprunk wrote:
> >>> On 04-Feb-12 17:44, James Kuyper wrote:
> >>>> As a general rule,
> >>>> your compiler should generate exactly the same code
> >>>> for x/8 and x>>3,
> >>>> because they're required to have exactly the same behavior.
> >>>
> >>> That only holds if x is non-negative.
> >>
> >> You're right
> >> - the result is implementation-defined if x is negative.
> >
> > Yes and the point is that this is what helps to optimize it better.
> 
> You missed the point.  If x is negative,
> they are not guaranteed to have the same behavior,
> so the compiler is not allowed to make such an optimization.  

I think Kaz was talking about a different optimization.

The value of (x>>3) being implementation defined 
for negative values of x,
suggests that the resulting opcodes for (x>>3)
should be simpler than they would be
if (x>>3) was defined as always being equal to (x/8).


-- 
pete