[Bug middle-end/71762] [5/6/7 Regression] ~X & Y to X < Y doesn't work for uninitialized values

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71762

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |law at redhat dot com

--- Comment #7 from Jeffrey A. Law <law at redhat dot com> ---
ISTM the transformation is valid on gimple, but that at RTL expansion time we
don't guarantee the range of the uninitialized object is [01].  And yes, if we
have to insert truncations this transformation becomes a lose.

One approach would be something similar to the unswitching on an uninitialized
value fix that I need to get back to -- ensure the values are initialized and
if they aren't, don't allow the transformation.

I wouldn't be surprised if there are other places where using type/vrp
information on a sub-word sized object that is uninitialized is going to cause
problems.

I don't think losing the transformations would be that big of a deal either. 
My recollection is they definitely triggered in GCC itself, but I don't have a
sense of how often.