[PATCH] [ConstantRange] Add a smultiply method for signed multiply

[Sanjoy Das]

> Sanjoy, I’m not entirely sure - it’s a bit late for me to have my effective thinking cap on. At the moment it does seem to me that if we swapped ZExt for Sext in multiply() things might just come out correctly, but I’d need to prove it to myself.

My suggestion would be (echoing what Hal said) to simply make
ConstantRange::multiply smarter.  There is no difference between
signed and unsigned multiplication in 2's complement, so there
shouldn't be a difference in ConstantRange's multiplication either.
For instance, [-1, 2) * [-1, 2) should be [-1, 2) in both signed and
unsigned interpretations (I don't know what ConstantRange::multiply
does today, it may be possible that it already handles this).

-- Sanjoy

>
> James
>
> On 20 Feb 2015, at 23:48, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
>
>> Actually, I don't think there is any need for a separate smultiply
>> when we're working on two's complement.  IMHO, the right solution is
>> to just make ConstantRange::multiply smarter about handing cases like
>> [-1,2) * [-1,2) etc.
>>
>> -- Sanjoy
>>
>> On Fri, Feb 20, 2015 at 3:40 PM, Sanjoy Das
>> <sanjoy at playingwithpointers.com> wrote:
>>> I don't think so:
>>>
>>> [-3,0) * [-3, 0) should be [1, 10) but using your formula I get [3,
>>> <something>).
>>>
>>> On Fri, Feb 20, 2015 at 3:31 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>>>> ----- Original Message -----
>>>>> From: "Sanjoy Das" <sanjoy at playingwithpointers.com>
>>>>> To: reviews+D7789+public+ffa7549bb8a7b441 at reviews.llvm.org
>>>>> Cc: "james molloy" <james.molloy at arm.com>, "Hal Finkel" <hfinkel at anl.gov>, "Commit Messages and Patches for LLVM"
>>>>> <llvm-commits at cs.uiuc.edu>
>>>>> Sent: Friday, February 20, 2015 5:11:46 PM
>>>>> Subject: Re: [PATCH] [ConstantRange] Add a smultiply method for signed multiply
>>>>>
>>>>>> ConstantRange
>>>>>> +ConstantRange::smultiply(const ConstantRange &Other) const {
>>>>>> +  // TODO: If either operand is a single element and the multiply
>>>>>> is known to
>>>>>> +  // be non-wrapping, round the result min and max value to the
>>>>>> appropriate
>>>>>> +  // multiple of that element. If wrapping is possible, at least
>>>>>> adjust the
>>>>>> +  // range according to the greatest power-of-two factor of the
>>>>>> single element.
>>>>>> +
>>>>>> +  if (isEmptySet() || Other.isEmptySet())
>>>>>> +    return ConstantRange(getBitWidth(), /*isFullSet=*/false);
>>>>>> +
>>>>>> +  APInt this_min = getSignedMin().sext(getBitWidth() * 2);
>>>>>> +  APInt this_max = getSignedMax().sext(getBitWidth() * 2);
>>>>>> +  APInt Other_min = Other.getSignedMin().sext(getBitWidth() * 2);
>>>>>> +  APInt Other_max = Other.getSignedMax().sext(getBitWidth() * 2);
>>>>>> +
>>>>>> +  ConstantRange Result_zext = ConstantRange(this_min * Other_min,
>>>>>> +                                            this_max * Other_max +
>>>>>> 1);
>>>>>> +  return Result_zext.truncate(getBitWidth());
>>>>>
>>>>> I'm not sure if this is correct:  if we multiply [-1, 2) with itself,
>>>>> won't this return [1, 2)?  Shouldn't [-1,2) * [-1,2) be [-1,2)?
>>>>
>>>> Indeed. The minimum should not be this_min * Other_min, but rather min(this_min * Other_max, this_max * Other_min), right?
>>>>
>>>> -Hal
>>>>
>>>>>
>>>>> -- Sanjoy
>>>>>
>>>>
>>>> --
>>>> Hal Finkel
>>>> Assistant Computational Scientist
>>>> Leadership Computing Facility
>>>> Argonne National Laboratory
>>
>
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