[llvm-dev] Floor-integer-div and integer sign operations?

[Stephen Canon via llvm-dev]

I should note that your flooring division is incorrect for exact cases, e.g. a = –2, b = 1 should produce –2 but you’ll get –3. Assuming you know b > 0 you want something more like:

mask = a >> 31
(a – (b–1 & mask)) / b

and you may or may not need to handle the possibility of the subtract overflowing, depending on how robust you need to be.

– Steve

> On Mar 30, 2018, at 12:57 PM, John Regehr via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hacker's Delight or this web page are probably the best references for tricks like this:
> 
> https://graphics.stanford.edu/~seander/bithacks.html#CopyIntegerSign
> 
> John
> 
> 
> On 03/30/2018 03:58 AM, edA-qa mort-ora-y via llvm-dev wrote:
>> I'm looking for ways to do some basic operations without using branches.
>> The key operation I want is a floored/round-to-negative-infinity integer
>> division (as opposed to the default round-to-zero).
>>     7 floordiv 5 = 1
>>     -3 floordiv 5 = -1
>>     -6 floordiv 5 = -2
>> As I guess that doesn't exist, the operation can be constructed as:
>>    (a/b) + (a>>31)
>> Assuming a is 32 bits. I can probably check the bit count, but perhaps
>> there's an op that does this already.
>> I'm also looking for a `sign(a) => (-1,0,1)` operation. Is there some
>> easy way to do this without branches? That is, have I overlooked from IR
>> function?
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