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Re: [PATCH] Fix optimization regression in constant folder


On Mon, 1 Oct 2007, Richard Kenner wrote:

> I'm not trying to figure out a way to make things work now, but to establish
> a framework for the future.  The semantics of sizetypes mean "don't
> suppress *any* optimizations".

Let me show you why the predicate "don't suppress any optimizations" is 
non-sensical by a constructed example: Suppose we were to introduce a type 
flag which can be applied to integral types ensuring that all values of 
that type are even integers, TYPE_EVEN_VALUED.

Presumably we did so because we saw the need for some worthwhile 
transformation applied to such values.  For instance we could use the low 
bit for something else, or increase the range of that type to be twice the 
nominal range.  Let one of these transformations be an optimization.

You are probably not going to argue that such optimization should also be 
applied to your sizetype.  Hence "all" optimizations is a useless 
predicate.  You need to spell out what operations are allowed on that 
type, and what the invariants for them are.  There really is no way around 
this.  Because otherwise someone somewhere will break what you think the 
semantics of it should be, but neglected to specify.

So, there's your path to the future, ensuring nobody will break your 
wishes; spell out what operations are required for your type.  To make 
that easy start with the most similar existing type (probably signed 
integer), and say what the new invariants of operations are.  Then destill 
those invariants (or better the diff to the invariants from the starting 
type) to a small set of orthogonal properties which can then be flags on 
types (and perhaps that will simply lead to only the IGNORE_OVERFLOW from 
Ian).  TYPE_IS_SIZETYPE is not such element of an orthogonal set of 


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