Hugo Herbelin writes:
 > Assume more generally that you can modify any local variable as in the
 > (standard) following example:
 > 
 > let fact (mutable n) =
 >   let mutable r = 1 in
 >   while n > 0 do
 >      r <- r * n;
 >      n <- n - 1
 >   done;
 >   r

This doesn't actually make life much easier for the compiler.  On
32-bit machines [see other thread!], `r' must be a reference (in the
C++ sense) to a heap object---64-bit float, plus header.  In general
it is not safe to overwrite the float value in the heap, if it's
possible that other variables have been assigned to it (unless floats
are assigned by value not by reference, but in that case you get heap
allocation at the time of assignment ...).  The analysis necessary for
determining that no such aliasing references can exist, and indeed
that the value can simply be kept in a float register/on the FPU
stack, is already there in the compiler and applies perfectly well to
mutable references.

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