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From: zweig@cs.uiuc.edu (Johnny Zweig)
Newsgroups: comp.lang.c++,comp.std.c++
Subject: Add-with-carry operator
Message-ID: <1991Feb3.202530.14874@julius.cs.uiuc.edu>
Date: 3 Feb 91 20:25:30 GMT
Sender: news@julius.cs.uiuc.edu (USENet News)
Reply-To: zweig@cs.uiuc.edu
Organization: U of Illinois, Dept. of Computer Science, Systems Research Group
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In my work with the TCP/IP protocol suite, I have had to implement 16-bit
1's-complement arithmetic in a protable, efficient way for protocol processing
modules.  In case you're rusty, ones-complement addition is exactly ordinary
twos-complement addition with carry (i.e. 101 + 111 = 101 in 3-bit 1's
complement since the 1 carried out of the left goes "end around" to the low
order bits -- 111 thus is "-0").

Every microprocessor I've ever seen the assembly language instruction set for
has, in addition to ordinary twos-complement addition instructions, some kind
of "add with carry" instruction which, while not actually implementing ones
complement arithmetic (to add A and B in ones complement, you execute A + B
add-with-carry 0), is just what the doctor ordered for doing IP checksum
computations, and many other iterative ones-complement arithmetic
computations.

The problem: there is not an add-with-carry operator in C++, so there isn't
any way of writing portable code to use this. I know, I know, I know, I can be
totally gross and icky and hard and use asm's and come up with a different one
for each processor/compiler combination I want to run on. But if there were a
standard way of letting the compiler know that I want to do add-with-carry
rather than vanilla add I could write portable, efficient code that people
like Van Jacobson would think was swell.  I suppose that a properly inlined
function could do the trick, but it would be nice if there were an operator.

Comments?

-Johnny Ones