[PATCH 1/2] IEEE-754R 2008 nextUp/nextDown implementation: tcDecrement

[Stephen Canon]

LGTM now that my Top Secret Offline Comments have been addressed.

On May 28, 2013, at 3:36 PM, Michael Gottesman <mgottesman at apple.com> wrote:

> Fixed up some comments/etc.
> 
> <0001-APInt-Implement-tcDecrement-as-a-counterpart-to-tcIn.patch>
> 
> On May 28, 2013, at 11:40 AM, Michael Gottesman <mgottesman at apple.com> wrote:
> 
>> Truth.
>> 
>> See attached revised patch:
>> 
>> <0001-APInt-Implement-tcDecrement-as-a-counterpart-to-tcIn.patch>
>> 
>> On May 28, 2013, at 3:57 AM, Stephen Canon <scanon at apple.com> wrote:
>> 
>>> Hi Michael —
>>> 
>>> +  integerPart carry = 0;
>>> +  const integerPart allones = ~integerPart(0);
>>> +  for (unsigned int i = 0; i < parts; i++) {
>>> +    integerPart oldvalue = dst[i];
>>> +    if (carry) {
>>> +      dst[i] += allones + 1;
>>> +      carry = (dst[i] <= oldvalue);
>>> +    } else {
>>> +      dst[i] += allones;
>>> +      carry = (dst[i] < oldvalue);
>>> +    }
>>> +  }
>>> +
>>> +  // We only borrow when dst == 0. Conclude since:
>>> +  //   1. dst != 0 => dst + ~0 will overflow.
>>> +  //   2. dst == 0 => dst + ~0 = ~0 no overflow.
>>> +  // that if the ``carry'' flag is not set, we need to borrow.
>>> +  return !carry;
>>> +}
>>> 
>>> While this is correct, it can be made much, much simpler by taking advantage of the fact that the addend is -1.  Once you get a carry-out, the result of the operation is a no-op:
>>> 
>>> 	for (unsigned int i = 0; i < parts; i++) {
>>> 		// If the current word is non-zero, then the decrement has no effect on
>>> 		// higher-order words of the integer, and there is no borrow.
>>> 		if (dst[i]--) return 0; 
>>> 	}
>>> 	// If every word was zero, then there is a borrow.
>>> 	return 1;
>>> 
>>> – Steve
>>> 
>>> On May 27, 2013, at 9:50 PM, Michael Gottesman <mgottesman at apple.com> wrote:
>>> 
>>>> The attached patch is the first in a series of 2 patches which implement IEEE-754R 2008 nextUp/nextDown via the function APFloat::next.
>>>> 
>>>> This first patch implements the static function tcDecrement on APInt which decrements a bignum represented by a part count and an integerPart and returns the borrow flag. Unittests are included as well.
>>>> 
>>>> Please Review,
>>>> Michael
>>>> 
>>>> <0001-APInt-Implement-tcDecrement-as-a-counterpart-to-tcIn.patch>_______________________________________________
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>> 
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