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Re: [PATCH] Fix optimization regression in constant folder

Richard Kenner wrote:

>> Be more specific.  Much more.  What pathological cases, what optimizations 
>> do we suppress currently, which of those are allowed in your system.  What 
>> properties of the type guarantee this.
> We supress reassociation.  For a type with undefined overflow, we can't
> convert (a + b) + c into a + (b + c) for fear of the latter expression
> overflowing where the former couldn't.  However, this can only very rarely
> happen in practice and only in circumstance I'm calling "pathalogical",
> where A and B have opposite signs, both are large, and where B and C have
> the same sign.  But this just can't come up in size and offset computations:
> any negative values are always small.

I've seen people declare C++ objects that map on to huge chunks of the
entire object space, e.g.:

  class Everything {
    char [2 * 1024 * 1024 * 1024];

Then, all you need is a derived class, and the offsets from that to the
base class will be very large negative numbers.  I'm not sure whether we
give those ptrdiff_t type or a signed sizetype type -- but if we're
going to have a special type for sizetype, this ought to be one.

Where does this "negative values are always small" guarantee come from?

Mark Mitchell
(650) 331-3385 x713

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