> With, I presume, a eye toward further producing:
x.a[dat[i]] = i;
y.a[dat[i]] = i;

Bingo.

> I assume you mean zero-extending; otherwise `x=255` would result in `result=-1`, which is clearly wrong.

Naturally.

> None of this involves that the compiler infering objective (and frequently false) properties of the input program (such as "this loop will terminate" or "p != NULL"), though.

Thus the need to use an abstraction model which allows optimizations to alter observable aspects of a program whose behavior is, generally, defined. I wouldn't describe such things as "behavior characteristic of the environment", though the environment would affect the ways in which the effects of optimizations might be likely to manifest themselves.

Note that programs intended for different tasks on different platforms will benefit from slightly--but critically--different abstraction models, and there needs to be a way for programs to specify when deviations from the "load/store machine model" which would normally be acceptable, aren't. For example, there should be a way of indicating that a program requires that automatic objects always behave as though initialized with Unspecified rather than Indeterminate Value.

A good general-purpose abstraction model, however, should allow a compiler to make certain assumptions about the behaviors of constructs, or substitute alternative constructs whose behaviors would be allowed to differ, but would not allow a compiler to make assumptions about the behaviors of constructs it has changed to violate them.

Consider, for example:

typedef void proc(int); // Ever seen this shorthand for prototypes?
proc do_something1, do_something2, do_something3;

void test2(int z)
{
if (z < 60000) do_something3(z);
}

int q;
void test1(int x)
{
q = x*60000/60000;
if (q < 60000) do_something1(q);
int y = x*60000/60000;
if (y < 60000) do_something2(y);
test2(y);
}

Under a good general-purpose model, a compiler could generate code that could never set q to a value greater than INT_MAX/60000, and a 32-bit compiler that did so could assume that q's value would always be in range and thus omit the comparison. A compiler could also generate code that would simply set q to x, but would forfeit the right to assume that it couldn't be greater than INT_MAX/60000.

There could be optimization value in allowing a compiler to treat automatic objects "symbolically", allowing the second assignment/test combination to become:

if (x*60000/60000 < 60000)
do_something2(x*60000/60000);

even though the effect of the substituted expression might not be consistent. I wouldn't favor allowing inconsistent substitutions by default, but would favor having a means of waiving normal behavioral guarantees against them for
local automatic objects whose address is not taken. On the other hand, there would need to be an operator which, when given an operand with a non-determinisitic value, would choose in Unspecified fashion from among the possibilities; to minimize security risks that could be posed by such values, I would say that function arguments should by default behave as though passed through that operator.

The guiding principle I would use in deciding that the value substitution would be reasonable when applied to y but not q or z would be that a programmer would be able to see how y's value is assigned, and see that it could produce something whose behavior would be "unusual", but a programmer looking at test2() would have no reason to believe such a thing about z.