Re: Stack based machines

[The comp.compilers moderator wrote]:
>[There have been lots of stack architectures that kept the top of the
>stack in registers, but it's my impression that performance is worse
>than real registers. Registers hold common subexpression values and
>hot loop variables as well as expression temporaries, and stack
>caching doesn't easily get either of those. -John]


Stacks work pretty well if someone takes the trouble to write a
stack-scheduling compiler. The problem has always been that
register-based machines with whizzy compilers were compared with
idiot-simple stack code -- not a fair comparison. Similarly, most of
the old "stacks are worse than registers/memory" arguments were based
on small code snippets that didn't exploit reuse of on-stack
variables, or didn't use realistic instruction sets that permitted
nondestructive accesses to the top couple stack elements. The stack
machines I worked on a decade ago were optimized for cost/performance,
not just raw performance. (But they did pretty well at raw
performance too.)


I published a paper on a first cut at such an optimizing stack
compiler back in 1994; I've since moved on to other pursuits. See:
http://www.ices.cmu.edu/koopman/stack_compiler/index.html It's
suitable for stack architectures that keep the top 2 or 3 values in
registers, which is commonly the case.


-- Phil


Phil Koopman -- koopman@cmu.edu -- http://www.ices.cmu.edu/koopman

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