stevenhoving / integer_division Goto Github PK

Just a two-cent suggestion.

When writing issue #2, I expected the provided programs not only to do the computations and check for rounding errors, but also to generate the header files.

This repo currently only holds the generates headers and the sources to generate them. It might be useful for other peeps to throw in some examples on how to use the headers.

https://github.com/stevenhoving/integer_division/blob/master/integer_division_uint8.h show results that are technically correct for powers-of-two, although they can be obviously much simpler.

As we all know here, dividing by 2^n is just right shift by n bits. Current implementation shifts left by 7, then right by 8, then right by n-1.

Some compilers for regular platforms will most certainly simplify and generate code for the one simple shift.

Simpler compilers, especially for embedded machines which needs this kind of hack more than others, may not generate optimal assembly.

One might say "these are compile-time macros, if the divisor is a power of two, just do the usual shift".

Still, this is especially interesting for usage where the source code calling those macros is not written by a human but generated by some preprocessing step.

Also this work can be a basis for a division by a variable not known at compile time, via a table lookup of parameters. This case needs power-of-two to be handled also.

Both programs are very similar. Only variable width and magic constants change.

The suggestion is to make a common source to be preprocessed to generate the two variants.

Pros:

• would allow to have only one file to maintain. This is interesting when implementing #2 and #3.

Cons:

• source code may be somewhat less readable when uint16_t becomes e.g. reg_int_t and most_significant_18_bits becomes most_significant_k_bits ?
• emacs diff mode makes it easy to propagate changes between versions, which in practice erodes the benefit of the change (if the maintainer is comfortable with that).