[RFC][PR target/39726 P4 regression] match.pd pattern to do type narrowing

[Jeff Law]

On 02/02/15 09:59, Joseph Myers wrote:
> On Sat, 31 Jan 2015, Jeff Law wrote:
>
>> The nice thing about wrapping the result inside a convert is the types for the
>> inner operations will propagate from the type of the inner operands, which is
>> exactly what we want.  We then remove the hack assigning type and instead the
>> original type will be used for the outermost convert.
>
> Those inner operands still need converting to unsigned for arithmetic.
Yes.


>
>> And FWIW, there's no reason to restrict the pattern to just masking off the
>> sign bit.  That's what the PR complains about, but we can do considerably
>> better here.  That's part of the reason why I put in the iterators -- to
>> generalize this to more cases.
>
> Well, we want to move shorten_binary_op and shorten_compare to the new
> mechanism.
Absolutely.  If we could have match.pd cover those cases, that'd be a 
significant validation of match.pd for this class of problems.  Seems 
like gcc-6 stuff to me though.

I haven't looked at those routines in a long time, but reviewing them 
seems wise both in the immediate term WRT this bug and ensuring we're 
doing the right thing for the various corner cases.

Jeff