common sub-expressions

[Henry Cejtin]

Speaking  of  common  subexpression  elimination,  I  was looking at the code
generated for an FFT and it REALLY hurt (maybe not in terms of  run-time  but
in  terms  of  pain)  to  see the repeated checks for subscript out of range.
First, the way the code  is  written  it  is  `clear'  that  the  checks  are
unneeded.  Ignoring that, what really hurts is code like
    Array.update(a, i, x + Array.sub(a, i))
where it checks if i is in range twice.

On  an unrelated note, what is the story on not-flattening out array entries?
I.e., if I have an array whose entries are tuples,  it  is  always  an  extra
level of indirection to get to the entries of the tuple.  Why are these never
flattened out?