[llvm-dev] generate vectorized code

[Mehdi Amini via llvm-dev]

> On Mar 18, 2016, at 12:45 PM, Rail Shafigulin <rail at esenciatech.com> wrote:
> 
> On Thu, Mar 17, 2016 at 2:41 PM, Rail Shafigulin <rail at esenciatech.com <mailto:rail at esenciatech.com>> wrote:
> On Thu, Mar 17, 2016 at 10:10 AM, Rail Shafigulin <rail at esenciatech.com <mailto:rail at esenciatech.com>> wrote:
> On Wed, Mar 16, 2016 at 6:38 PM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> 
>> On Mar 16, 2016, at 5:38 PM, Rail Shafigulin <rail at esenciatech.com <mailto:rail at esenciatech.com>> wrote:
>> 
>> On Wed, Mar 16, 2016 at 11:48 AM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>> Hi Rail,
>> 
>> Two hints to begin with:
>> 
>> 1) Makes sure you example is vectorized on X86 for example
>> 2) Is your target correctly overriding the TTI (declaring the vector register size for example) so that the vectorizer can kicks-in (see X86TTIImpl::getRegisterBitWidth for instance). Alternatively you can test the SLP vectorizer by passing to clang: -mllvm -slp-max-reg-size -mllvm 512  (I don't see an equivalent option for the loop vectorizer though).
>> 
>> Well, it sort of worked. I added a getRegisterBitWidth(...) but then I got this error:
>> 
>> fatal error: error in backend: Cannot select: 0x5e949a8: v4i32 = BUILD_VECTOR 0x5e91ae8, 0x5e91ae8, 0x5e91ae8, 0x5e91ae8 [ORD=16] [ID=16]
>>   0x5e91ae8: i32 = Constant<0> [ID=5]
>>   0x5e91ae8: i32 = Constant<0> [ID=5]
>>   0x5e91ae8: i32 = Constant<0> [ID=5]
>>   0x5e91ae8: i32 = Constant<0> [ID=5]
>> 
>> What am I missing?
> 
> I means that you have a vectorized IR that reached your backend, but your backend is not ready to deal with all the vector constructs in SelectionDAG. 
> You need to express how to legalize/select the BUILD_VECTOR in SelectionDAG to instructions that your target supports. You can look at what other targets are doing.
> 
> -- 
> Mehdi
> 
> 
> So I've added setOperationAction(ISD::BUILD_VECTOR,      MVT::v4i32, Expand); to my code but that generated a following error:
> 
> fatal error: error in backend: Cannot select: 0x6a84dc8: i32 = extract_vector_elt 0x6a85388, 0x6a813b0 [ORD=9] [ID=16]
>   0x6a85388: v4i32 = add 0x6a81098, 0x6a81e00 [ORD=8] [ID=15]
>     0x6a81098: v4i32 = add 0x6a81bf0, 0x6a84168 [ORD=6] [ID=12]
>       0x6a81bf0: v4i32,ch = CopyFromReg 0x6a2b7f0, 0x6a819e0 [ORD=5] [ID=8]
>         0x6a819e0: v4i32 = Register %vreg4 [ID=1]
>       0x6a84168: v4i32 = vector_shuffle 0x6a81bf0, 0x6a857a8<2,3,u,u> [ORD=5] [ID=10]
>         0x6a81bf0: v4i32,ch = CopyFromReg 0x6a2b7f0, 0x6a819e0 [ORD=5] [ID=8]
>           0x6a819e0: v4i32 = Register %vreg4 [ID=1]
>         0x6a857a8: v4i32 = undef [ID=2]
>     0x6a81e00: v4i32 = vector_shuffle 0x6a81098, 0x6a857a8<1,u,u,u> [ORD=7] [ID=14]
>       0x6a81098: v4i32 = add 0x6a81bf0, 0x6a84168 [ORD=6] [ID=12]
>         0x6a81bf0: v4i32,ch = CopyFromReg 0x6a2b7f0, 0x6a819e0 [ORD=5] [ID=8]
>           0x6a819e0: v4i32 = Register %vreg4 [ID=1]
>         0x6a84168: v4i32 = vector_shuffle 0x6a81bf0, 0x6a857a8<2,3,u,u> [ORD=5] [ID=10]
>           0x6a81bf0: v4i32,ch = CopyFromReg 0x6a2b7f0, 0x6a819e0 [ORD=5] [ID=8]
>             0x6a819e0: v4i32 = Register %vreg4 [ID=1]
>           0x6a857a8: v4i32 = undef [ID=2]
>       0x6a857a8: v4i32 = undef [ID=2]
>   0x6a813b0: i32 = Constant<0> [ID=3]
> In function: main
> 
> Then I've added
> setOperationAction(ISD::EXTRACT_VECTOR_ELT,      MVT::i32, Expand);
> 
> but I still got the same error. So removed setOperationAction(ISD::EXTRACT_VECTOR_ELT,      MVT::i32, Expand); and added
> 
> setOperationAction(ISD::EXTRACT_VECTOR_ELT,      MVT::v4i32, Expand);
> 
> which produced a following error:
> 
> fatal error: error in backend: Cannot select: 0x7389250: v4i32 = vector_shuffle 0x73884e8, 0x738cbf8<1,u,u,u> [ORD=7] [ID=15]
>   0x73884e8: v4i32 = add 0x7389040, 0x738b5b8 [ORD=6] [ID=13]
>     0x7389040: v4i32,ch = CopyFromReg 0x73327f0, 0x7388e30 [ORD=5] [ID=9]
>       0x7388e30: v4i32 = Register %vreg4 [ID=1]
>     0x738b5b8: v4i32 = vector_shuffle 0x7389040, 0x738cbf8<2,3,u,u> [ORD=5] [ID=11]
>       0x7389040: v4i32,ch = CopyFromReg 0x73327f0, 0x7388e30 [ORD=5] [ID=9]
>         0x7388e30: v4i32 = Register %vreg4 [ID=1]
>       0x738cbf8: v4i32 = undef [ID=2]
>   0x738cbf8: v4i32 = undef [ID=2]
> In function: main
> 
> Then I'v added setOperationAction(ISD::VECTOR_SHUFFLE,      MVT::v4i32, Expand);
> 
> and then my clang just hang. There is no error, no warning clang just sits there and nothing happens. 


I think you created a cycle, this is easy to do with SelectionDAG :)
Basically SelecitonDAG will iterate until it does not see anything to change. So if you insert a transformation on a pattern A, that generates pattern B, while you have another transformation that matches B and generates somehow A, you run into an infinite loop.



> 
> I'm doing a lot of guess work in trying to understand what is going on. I would really appreciate any help on this.

Here is how I started with SelectionDAG: 

- small IR (bugpoint can help)
- the magic flag: -debug 
- read the output of SelectionDAG debugging (especially with cycles)
- matching the log to source code
- single stepping in a debugger sometimes.


-- 
Mehdi

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