[LLVMdev] Making GEP into vector illegal?

Mon Ping Wang monping at apple.com
Wed Oct 15 17:07:15 PDT 2008

Hi Duncan,

I can see your point that arrays are similar except for the alignment  
restrictions when representing them in memory.  However, the two  
constructs serves different purpose for me.   I don't think of a  
vector isA array.  A vector is a construct that supports a form of  
parallelism.  It is a container that allow people to expression some  
element wise parallelism in a controlled fashion.  It supports  
parallel operations plus shuffles.   Since it is intended to be mostly  
a parallel construct, it would be nice to avoid writing scalarize code  
for it when possible  (doing an operation on a per element basis  
instead of the whole vector which GEP seems to imply). An array seems  
more of a scalar construct where we can discover parallelism when we  
analyze the pattern of how the array is used.  This distinction makes  
them different for me so I'm not convinced to change my head yet :->.

If a vector is a specialized array, do we want to see users use a  
vector type whenever one has a fix size arrays?  It would be  
convenient for people to do so as you get all the array operations  
plus you get the vector operations to add two vectors and such.  I  
don't think so as it seems like an abuse of the vector type.  It can  
also be challenging to generate good code for.  If the code generator  
doesn't noticed that it is used mostly scalar, it might break up the  
vector into some scalar code and then rebuild the vectors back into  
registers and so forth.

-- Mon Ping

On Oct 15, 2008, at 10:51 AM, Duncan Sands wrote:

> Hi Chris,
>> I'm happy about factoring the code better, but a vectortype isn't an
>> arraytype (isa<ArrayType>(V) should be false).  Maybe a common base
>> class (like sequential type) would be better?
> currently anything you can do with an array you can do with a vector.
> So from this functional viewpoint it would make sense to have a vector
> be an array with more stuff (i.e. a subclass of ArrayType).  However I
> appreciate that there's a difference: a multi-element vector can be  
> held
> in a machine register, while that's not the case for an array.  But  
> then
> again, why not place first-class arrays of appropriate size in such
> machine registers?  Also, at the IR level is there any practical  
> advantage
> to having vectors not be a kind of array - what does disallowing GEP  
> win
> you?  Is this entirely a codegen issue?
> By the way, deriving from SequentialType doesn't make much sense to me
> because SequentialType only exists to unify the types on which you can
> do GEP (at least that's my understanding) - but you want to disallow  
> on vectors!
>>> From this point of view you have to allow GEP into a
>>> vector; the conclusion I suppose is that codegen needs to replace
>>> GEP+load or GEP+store with an extract or insert operation.
>> With that logic, there is no difference at all between an array and
>> vector... I disagree very strongly about this.
> There is a difference of course: vectors can do more than arrays.   
> That's
> why VectorType would derive from ArrayType :)  I understand that  
> probably
> in your head they feel different.  I say: change your head! :)
> Ciao,
> Duncan.
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