Re: Language Efficiency

[eachus@spectre.mitre.org (Robert I. Eachus)]

In article <MATT.95Apr20113010@physics10.berkeley.edu> matt@physics10.berkeley.edu (Matt Austern) writes:

  > If certain optimizations are possible in principle in two different
  > languages, but if they are vastly more difficult in one language than
  > in the other, then it is both correct and misleading to say that
  > efficiency differences between the two languages are purely issues of
  > implementation.

    I have worked on two different compilers which adopted the same
approach to optimization and gotos.  Normal compilation assumed that
there were no gotos in scope.  If a goto was encountered, a switch was
set and compillation restarted from the beginning.  An elegent
technique for languages where gotos are rare in practice but legal.
(The compilers mentioned above were for Pascal and Ada, and developed
by different companies.)

    In addition I know of at least one Ada compiler which lays out a
record during processing of the declaration, and if it later
encounters rep clauses, it just repeats the work from scratch.

    So, yes, in practice many compilers do ignore language features
which make compilation or optimization more difficult, and have a
completely different mode or routine when those features are
encountered.  With the Pascal compiler mentioned above, since it
supported multiple compilation units, it was a "known rule" that you
should put any gotos in the smallest possible compilation.  An awful
lot of optimization got turned off.  The Ada compiler was much better
behaved, about the only thing affected was spilling of register
variables.

--

					Robert I. Eachus

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