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From: earl@wright.mips.com (Earl Killian)
Newsgroups: comp.sys.mips
Subject: Re: Redefinition of left shift operator
Message-ID: <26036@wright.mips.COM>
Date: 23 Aug 89 02:00:02 GMT
References: <4724@portia.Stanford.EDU>
Sender: earl@mips.COM
Reply-To: earl@wright.mips.com (Earl Killian)
Distribution: usa
Organization: MIPS Computer Systems Inc.
Lines: 25
In-reply-to: edgar@portia.Stanford.EDU (Edgar Holmann)

In article <4724@portia.Stanford.EDU>, edgar@portia (Edgar Holmann) writes:

>Has the left shift operator been redefined in the MIPS systems from
>the typical shift to a modulo type shift?  By this I mean:
>   i << j
>redefined to
>   i << (j % 32)

The C language does not define shifts < 0 or >= the size of the type,
so any program where the above statements differ is illegal.  This
allows compilers to generate a raw shift instruction, which is in line
with C's efficiency goals.  All instruction set architectures (ISAs)
that I know take the shift count modulo some small power of two (e.g.
32-256).  The MIPS ISA in particular takes the shift count modulo 32,
which gives different results on illegal programs than say a 68000,
which takes the shift count modulo 64.

So the bottom line is that you need to use conditionals when your
shift count is of larger range.  If C shifts were defined for shifts
>= size of the type, then all variable shifts would require the
compiler to generate the conditional, on all architectures, and you
probably wouldn't like the performance of that.
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