[LLVMdev] How doesn't llvm generate IR for logical negate operation

[Bruce Hoult]

In that case, I'm pretty sure that if you use <N x i1> in LLVM IR then the
generated code will use 8 bits in the vector registers for each i1, which
is probably not what you want.

But hopefully someone who actually knows this stuff will come along
shortly...

On Sat, Apr 11, 2015 at 3:14 PM, zhi chen <zchenhn at gmail.com> wrote:

> I see. My CPU is a general Core i7 Ivy bridge CPU.
>
> On Fri, Apr 10, 2015 at 6:48 PM, Bruce Hoult <bruce at hoult.org> wrote:
>
>> I suppose that depends on your CPU.  Do you even have a CPU that supports
>> operations on <N x i1> as packed bits in vector registers?
>>
>>
>> On Sat, Apr 11, 2015 at 12:43 PM, zhi chen <zchenhn at gmail.com> wrote:
>>
>>> Yes, but my point is that there would be some overhead to do cast the <N
>>> x i1> vectortype to an integerNty. Is there any good way to check not all
>>> of these N bits in the vectortype are 0s?
>>>
>>> On Fri, Apr 10, 2015 at 5:37 PM, Bruce Hoult <bruce at hoult.org> wrote:
>>>
>>>> Sure, if you actually just want an i1 saying whether or not at least
>>>> one bit is set to 1, then comparing against 0 is the right thing.
>>>>
>>>> That should end up generating a unary TST instruction on ISAs that have
>>>> one.
>>>>
>>>> On Sat, Apr 11, 2015 at 12:29 PM, zhi chen <zchenhn at gmail.com> wrote:
>>>>
>>>>> Thanks, Bruce. So, what is the easiest way to check if there is any
>>>>> bit set to 1 in a <N x i1> vector type? I used bitcast instruction to cast
>>>>> it into "iN" first and them compare iN to 0. Do you have a better way to do
>>>>> it? Thanks again.
>>>>>
>>>>> On Fri, Apr 10, 2015 at 5:22 PM, Bruce Hoult <bruce at hoult.org> wrote:
>>>>>
>>>>>> LLVM doesn't have a "logical neg" (or "not") operator. That's a C
>>>>>> thing. Do a compare against 0 to create an i1 result, then zero extend the
>>>>>> i1 to the size of integer result you want.
>>>>>>
>>>>>>
>>>>>> On Sat, Apr 11, 2015 at 12:07 PM, zhi chen <zchenhn at gmail.com> wrote:
>>>>>>
>>>>>>> How can I generate LLVM IR for both logical NEG (!)? For example, if
>>>>>>> I have Int32Ty a,
>>>>>>>
>>>>>>> For the bitwise NEG(~):
>>>>>>>
>>>>>>> c = ~a ;
>>>>>>>
>>>>>>> I can use the following API from LLVM:
>>>>>>>
>>>>>>> BinaryOperator *neg = BinaryOperator::CreateNeg(nbits, "bitwiseNEG", insertBefore);
>>>>>>>
>>>>>>> How, if I want to generate logical NEG:
>>>>>>>
>>>>>>> c = !a;
>>>>>>>
>>>>>>> what should I do for this?
>>>>>>>
>>>>>>> Thanks
>>>>>>>
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>>>>>>>
>>>>>>>
>>>>>>
>>>>>
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