Re: Ada subtypes and base types

[kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)]

     Given an expression, we have to do computations in some other type than
     the type of the expression? Now that's just silly.  

Sure, but that's not what I said.

     If the expression has some type X, then we should be doing our
     computations in type X.  

Right.

Let me try again and take a simpler example.  If we have

	subtype T is Integer range 20..50;

	Y: T;

	   ... Y + 1 ...

What the tree looks like is a PLUS_EXPR of type "Integer" (the base type of
T), not T, whose first operand is a NOP_EXPR converting Y to Integer and
whose second operand is the constant 1 also of type Integer, not T.

So the expression is of type Integer and that's what we do the
computation in.

If the context of that expression is
	
	Y := Y + 1;

then there'll be a conversion to type T (and possibly a bounds check)
of the RHS of the MODIFY_EXPR.  VRP will know that the results of the
PLUS_EXPR are [21,51] (not [21,50]!).  The bounds check will be against
[20,50], so VRP could convert it to a test of "!= 51" if it wanted.