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Date: Mon, 28 Mar 2011 16:29:25 -0400
From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: Pascal Calling Convention
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Date: 2011-03-28T16:29:25-04:00
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On 3/26/2011 4:46 PM, Robert A Duff wrote:
> It doesn't seem reasonable to me.  Uninitialized variables do not
> contain random data.  They contain arbitrary data.  And it's quite
> likely that such arbitrary data happens to be the same every time,
> or has some other non-random pattern.  Bad Guys sometimes exploit
> such patterns to cause security problems.

Time for my favorite data structure! This is a structure which
represents an unordered set of the integers [0,N) and which has
constant time initialization, insertion, deletion, and membership
test. It relies on reading uninitialized data as some arbitrary
value. Compilers which regard doing this as undefined behavior
will break the code. I'm going to write it in C++, but it should
be straightforward to translate to Ada:

template <unsigned N>
class set
{
     unsigned n;    // number of members in set
     unsigned d[N]; // members are in d[0..n-1]
     unsigned s[N]; // d[s[k]] == k if k is a member
     // unspecified elements of d and s are arbitrary

public:
     set() : n(0) { }
     bool has(unsigned k)
         { return k < N && s[k] < n && d[s[k]] == k; }
     void add(unsigned k)
         { if (k < N && !has(k)) { d[n] = k; s[k] = n++; }
     void del(unsigned k)
         { if (has(k)) { unsigned i = s[k]; d[i] = d[--n]; s[d[i]] = i; }
     void clr() { n = 0; }
};