[sdiy] The Chameleon Guitar

[Eric Brombaugh]

Matthew Smith wrote:
> Quoth Csaba Zvekan at 2009-04-09 07:14...
> <snip/>
>> Sure you loose performance  
>> the code is not as optimized as it would be in Assembler 
> <snip/>
> 
> I should probably point out at this juncture that the output of a DECENT
> C compiler can be more efficient than hand-coded assembly.

Good point - for general-purpose processor architectures this can often 
be true. In most cases these days, compiler output is so tight that it's 
almost not worth bothering to look at the disassembly to check. Trouble 
is that DSPs contain a lot of highly specialized structures that are 
fairly difficult to use in general computing applications, and hence the 
compiler writers ignore them. MACs in particular don't get the love they 
deserve, and so need assembly to use to their fullest potential.

An emerging trend over the last few years has been the use of hinting, 
pragmas and intrinsics which are ways to to explicitly tell the compiler 
"Hey - I'm trying to do something odd here - pay attention!". Most often 
these look like comments or function calls that cause special behavior 
to help optimize the output.

Another approach is the use of in-line assembly, with special syntax to 
allow the low-level code to merge almost seamlessly with the surrounding 
high-level constructs. This allows things like using variable names 
instead of register names when specifying operands of assembly 
instructions and makes coding a lot easier.

Finally, the DSP56 used in the Symphony Soundbite has a couple of 
accelerators for speeding up filter and buffer processing which no 
compiler could know about. This has to be treated as a separate 
resource, and while assembly isn't required to use it, some serious 
research is needed to grok its function.

Eric