[llvm-commits] patch: go crazy, compute bits for an entire add instruction

[Nick Lewycky]

Pardon me, fixed patch attached. I had a buggy test.

Nick

Nick Lewycky wrote:
> Jay Foad wrote:
>>> In general though, will I be
>>> allowed to use an O(n) loop in ComputeMaskedBits if needed? I don't
>>> think
>>> we'll be able to solve multiply without it--though we can try!
>>
>> I'd be very surprised if you get any real world benefit from trying to
>> calculate the middle bits of the result of a multiply. Surely it's
>> good enough to calculate just the low bits and the high bits?
>
> No, I do want middle bits, but ...
>
>> Or maybe you do want the middle bits, but only in the special case
>> where one operand is a completely known constant. That might be easier
>> to solve than the completely general case, but I haven't thought about
>> it much.
>
> ... all the cases I care about right now have a constant RHS.
>
> Jay, would you be willing to review this updated patch, now for add+sub?
> I think I'd like to do the rest (overflow intrinsics and bswap) next
> before moving on to mul, actually. I have piles of missed optz'ns of due
> to overflow intrinsics that I haven't solved.
>
> In previous review over IRC, Chris complained that the change to
> InstSimplify was unclear and I promised to add comments. I have
> reconsidered, and consider the code clear enough with the same comments
> it had before (but with a bugfix). In particular the 'and' analysis only
> requests masked bits because we knows that if the whole result is
> constant than the result bits not in the mask must be zero, while an
> 'or' could be setting all the unknown bits to one, but we still need
> ComputeMaskedBits to tell us what the other bits actually are. In
> addition, I've added four tests, one for each "return" statement in
> InstSimplify (another one of Chris' comments was that one of the returns
> was unreachable).
>
> Nick
>
>
>
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