Re: Effectiveness of compilers today

jbuck@forney.berkeley.edu (Joe Buck) writes:
>Superoptimizer uses exhaustive search and is thus exponential.


preston@dawn.cs.rice.edu (Preston Briggs) writes:
>In defense, I'll point out that it is used at compiler-generation time
>[thus it can crunch for a week w/o affecting compile time.]


The original Superoptmizer was available for the compiler writer, the
compiler, and the programmer. As I recall, Henry told me the biggest
thing he'd ever tried with it was about 20 instructions and that took a
couple weeks (an Othello move generator; substantial speedup). Henry also
used the Superoptimizer technique on longer sequences by Superoptimizing
windows of a few instructions, then moving the window. As I recall, the
first pass would be run with a small window and would go fast; then the
window size would be increased once the ``obvious'' sequences were removed
by the first pass; and so on. That way, Superoptimizer (which never
considered longer sequences) would only run with the large window when the
code in question was already reasonably tight.


;-D on ( Sequence-ial logic ) Pardo
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