Re: GCC for 8051 based architectures

[Dave Hudson <dave at cyclicode dot net>]

Hi Joe,

Joe Buck wrote:
> On Tue, Jul 15, 2003 at 10:35:37AM -0700, Julia Nilsson wrote:
>
> There has been lots of research on techniques for dealing with
> irregular processors with limited register sets, multiple memory
> banks, and odd addressing modes, and commercial compilers for
> DSPs and 8-bit microcontrollers aggressively use such techniques
> (for example, offset assignment of local variables so that they
> can quickly be accessed through address registers with auto
> increment or decrement).  GCC does not implement such techniques
> because they are not needed for the 32- and 64-bit CISC and RISC
> processors that are its main target.  So, GCC lacks the algorithms
> needed to get really good code on such processors (though it
> is possible to get correct but slow code).

While it's true that gcc assumes the resources, there's no problem as 
long as you can emulate them.  gcc is the sole compiler for the IP2022 
and it has one 8-bit accumulator, one 16-bit non-offsettable address reg 
and 2 offsettable ones (one used as the stack).  The code without some 
of the more interesting RTL hackery that has been added is not actually 
that bad (with the extra stuff the code is abou 20% smaller than without 
it).

FWIW the major wins are in fact are not related to any intrinsic 
characteristic of gcc, but mainly because there is huge pressure on the 
offsettable address register that is not used as a stack and reload 
insists on generating redundant reloads.  The other major win is that 
integer promotion on an 8-bit target is *really* expensive.  In both 
cases a reasonably simple RTL-level optimizer solves the problems.

If I were to start doing a lot of work on this port again I'd probably 
rewrite it to emulate more address registers - this would almost 
certainly give dramatic simplifications and better code, but right now 
the code quality is extremely good.

This solution isn't that far from what's been done with LLVM if memory 
serves.


Regards,
Dave