deftio / absolute_this

Different bit twiddlings on abs() for C (or C++) -- more of a historical note than practical code

Absolute..This A few thought bitwiddling absolute values

(c) 2000 M. A. Chatterjee, < deftio [at] deftio [dot] com >

release notes:

• cleaned up comments 2012
• changed to markdown 2016

About this code and what it does

Actually this code does .. virtually nothing and is more of a historical exploration of different compiler/processor issues I was investigating in the mid 1990s. Both embedded (e.g. ARM, mcore, 68xxx) and desktop (PowerPC, 80x86) compilers were being target for work I was doing in speech and handwriting recognition. Many of the embedded platforms were running in the low single digit to low 10s of MHz.

These musings on bit shifting and absolute values come from the mid 1990s when processors could have pipeline stalls and I was experimenting with ways to avoid cache/pipeline misses on branch predictors.

This started a subset line of thinking on measuring branch penalties vs statements. Some processors then didn't have hardware absolute value (ABS) instructions so with a little fun the macros below were created. I make no statements that they were useful and I looked at these and other statement constructs to study effects of branch-miss, cache fill issues for a specific problem I was dealing with at the time which which I've since forgotten.

Those of you coming from a Java background, bear in mind the difference in Java between >> and >>> as here in C >> means arithmetic shift (e.g. if there is a 1 in the high order bit then the register 1 fills not zero-fills).

I'm publishing this as a way to preserve some thought experiments rather than as a true open-source "library" of any kind. Many times I've run accross some little brainbender of code someone wrote and thought "hmmm cute" and soe here is a small contribution to such thinking.

Cheers- MC

Building the Code

See C language code "absolute_this.c" command line compile via gcc or equivalent. e.g. in gcc

#build it
gcc absolute_this.c -lm -o abs_this

#run it
./abs_this

Note the code uses stdio.h so your platform will need to support printf like 
statements.

License

Copyright (c) 2001-2016, M. A. Chatterjee < deftio at deftio dot com > All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

• Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.