value range propagation for _bitwise_ OR
SeeWebsiteForEmail at erdani.org
Fri Apr 16 20:23:24 PDT 2010
On 04/12/2010 12:57 PM, "Jérôme M. Berger" wrote:
> Andrei Alexandrescu wrote:
>> On 04/11/2010 09:18 PM, Adam D. Ruppe wrote:
>>> On Sun, Apr 11, 2010 at 10:00:41PM -0400, Steven Schveighoffer wrote:
>>>> We are talking about range propagation, a function of the compiler,
>>>> not a function of the compiled program. Therefore, slower but more
>>>> exact functions should be preferred, since they only affect the
>>>> compile time.
>>> I agree with this. While my solution has a certain beauty to it and is
>>> fast, your solution accurately covers all the bases, whereas mine
>>> fails with
>>> min> 1. I say we go with your's.
>> Agreed. Steve's solution is the best we have for unsigned numbers.
> Nope, not anymore...
Looks great, thanks Jerome!
Now, how about that case in which some or all of the ranges involved
include negative values? A simple approach is to define them in terms of
ranges for unsigned numbers. Here are the cases I identified:
a) If both ranges cross zero, then:
minOR == setmsb(min(
minOR(nomsb(min_a), nomsb(-1), nomsb(min_b), nomsb(max_b)),
minOR(nomsb(min_a), nomsb(max_a), nomsb(min_b), nomsb(-1))))
maxOR == maxOR(0, max_a, 0, max_b)
b) If both ranges are negative, then:
minOR == setmsb(minOR(nomsb(max_a), nomsb(min_a), nomsb(max_b),
maxOR == setmsb(maxOR(nomsb(max_a), nomsb(min_a), nomsb(max_b),
c) If a crosses zero and b doesn't:
minOR == setmsb(minOR(nomsb(min_a), nomsb(-1), min_b, max_b))
maxOR == maxOR(0, max_a, min_b, max_b)
The primitive nomsb clears off the sign bit in a number, and setmsb sets it.
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