translations (was Help on disassembler/decompilers)

In article <1990Sep18.103653.24032@dce.ie> ch@dce.ie (Charles Bryant) writes:
>That's funny, I can't see any bug in the original. I can see a bug in the
>Pascal one though: when the list is empty new elements can't be inserted
>because 'head' is never changed.


The ``bug'' was the lack of initialization (_not_ really a bug
in C but it _is_ so in other languages, and therefore in general).


It was a rotten trick anyway. If you initialize ``head'' to 0
and allocate ``mem'' cells starting at 0 the Pascal program's ok.


As for saying ``well ok -- _try_ to perform the translation using
Pascal pointers''. This is the same as asking ``translate this
program, that uses language feature F, into one that _doesn't have_
feature F''. A non sequitur?


As I said in the original post, matters of _efficiency_ are quite distinct
from the original statement that "X->Y is easy and Y->X is a hard
translation".


>From the point of view of formal language thy _all_ translation is _very hard_
(i.e. ``impossible'' and therefore equally ``easy'').


Saying that a Pascal program that implements lists using integers and
arrays is either _time_ inefficient and/or _space_ inefficient is beside
the (original) point.


In any case, efficiency is (usually) a matter of a particular underlying
architecture (WELL off the track of the original question). On some systems
using an array is actually preferred to manipulating pointers; the former is
under more-or-less control, whereas the latter is subject to the vagueries of
the C memory management. It may be possible to maintain locality-of-reference
with integers and arrays and ``cache misses'' can kill dominate program
performance considerations.


Also, on VM systems allocating ``huge arrays'' does not present such an
overhead as you seem to think. In fact, the C malloc routine on _some_ systems
essentially declares a _huge_ static array to which it passes out pointers;
the VM mechanism does the rest.


-Kym Horsell
--

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