Re: programmer optimizations?

hbaker@netcom.com (Henry Baker) wrote:


> Before everyone gets too upset here, it is quite difficult--in
> general--for a compiler to keep track of whether a number is positive,
> negative or zero.
> When it comes to range information in which one or both ends of the range
> are a run-time variable, the complexity of static analysis skyrockets.


I was referring to the negative constant divisor, not variable dividend.
For example, consider the following expression.


foo / -8


All the power-of-two optimizations I listed in the previous message are
easy to implement (e.g. do not require flow information).


In early releases of Nullstone, we only tested a few combinations of
power-of-two optimizations, thinking that if a compiler optimized
(foo / (1 << 3)), that it probably optimized (foo / (1 << 15)).
Our analysis of twenty compilers on a variety of platforms revealed
that very few compilers generate efficient code for all combinations
of positive/negative signed/unsigned power-of-two multiply/divide/mod.


Regards,
Christopher Glaeser cdg@nullstone.com
Nullstone Corporation


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