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

[Michael Gottesman]

Truth.

See attached revised 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>_______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits

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