Re: Swap two integers without using temporary variable

James Kanze <>
Fri, 30 Nov 2007 01:23:53 -0800 (PST)
On Nov 29, 7:21 pm, Joe Greer <> wrote:

Kai-Uwe Bux <> wrote

  After all, many compilers detect quite some idioms. For example
compilers detect that something like "a = (a << 8) | (a >> 24);"
is actually a rotation and compile that as one single opcode.

That's because such an expression is *always* a rotation.

old C books say that it is shift,is the behavoir changed since then?

I thaught a << 8 is a shift and a >> 24 is a shift. So,

  ( a << 8 ) | ( a >> 24 )

would be a rotation assuming a bitlength of 32.

Yes. I just sort of assumed that (and that everyone else would,
since it seems so obvious).


I don't know about now, but in the olden days, << always 0
filled and >> was machine dependent.

It depends on the type. As a general rule, I only use shifts
and bitwise operators on unsigned types, so there are no

Juha's expression is wide-spread, and is the idiomatic way to
express a rotation in C/C++. It would, I believe, be
immediately recognized as a rotation by anyone who has worked on
code where rotations were needed. (It's used in the reference
implementations of MD5 and SHA1, for example.)

James Kanze (GABI Software)
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