Page 76 - The Art of Designing Embedded Systems

Real Time Means Right Now! 63

250-275 nsec for(i=O; i<count; ++i)
508-580 nsec {if (start-count !=
end-c oun t )
250 nsec end_point=head;
I

where a range of values cover possible differences in execution
paths depending on how the statement operates (for example, if the
FOR statement iterates or terminates).
To get actual times, of course, the compiler needs to know a lot
about our system, including clock rates and wait states. Another op-
tion is to display T states, or even just number of instructions exe-
cuted (since that would give us at least some sort of view of the
code’s performance in the time domain).
Vendors tell me that cache, pipelines, and prefetchers make
modeling code performance too difficult. I disagree. Most small em-
bedded CPUs don’t have these features, and of them, only cache is
truly tough to model.
Please, Mr. Compiler Vendor, give us some sort of indication
about the sort of performance we can expect! Give us a clue about
how long a runtime routine or floating-point operation takes.
A friend told me how his DOD project uses an antique lan-
guage called CMSP. The compiler is so buggy they have to look for
bugs in the assembly listing after each and every compile-and then
make a more or less random change and recompile, hoping to lure
the tool into creating correct code. I laughed until I realized that’s
exactly the situation we’re in when using a high-quality C compiler
in performance-bound applications.

Be especially wary of using complex data structures in ISRs. Watch
what the compiler generates. You may gain an enormous amount of per-
formance by sizing an array at an even power of 2, perhaps wasting some
memory, but avoiding the need for the compiler to generate complicated
and slow indexing code.
An old software adage recommends coding for functionality first,
and speed second. Since 80% of the speed problems are usually in 20% of
the code, it makes sense to get the system working and then determine
where the bottlenecks are. Unfortunately, real-time systems by their nature
usually don’t work at all if things are slow. You’ve often got to code for
speed up front.