[LLVMdev] How to force the creation of arrays with zeroes?

[Stepan Dyatkovskiy]

Hi all. I propose to use wrapper for this kind of constant objects. 

Consider next code:

Constant* C = reinterpret_cast<Constant*>(I->getOperand(idx));
Constant* ArrayItem;
if (!isa<ConstantAggregateZero>(C))
  ArrayItem = reinterpret_cast<Constant*>(I->getOperand(ArrItemIdx));
else
  ArrayItem = ZeroInt;

Code analog within wrapper:

 ConstOrZero C(I->getOperand(idx));
 Constant* ArrayItem = C.getOperand(ArrItemIdx); // Unpleasant work with isa<> inside.

Advantages of this approach is that ConstantAggregateZero saves memory and we use it everywhere.
And in the same time we needn't dance with isa<ConstantAggregateZero>.

If you find it fine, please look at the patch in attachment.

-Stepan

20.01.2012, 14:50, "Duncan Sands" <baldrick at free.fr>:
> Hi Anton,
>
>>>  you can't, you can only get a ConstantAggregateZero.  This is actually kind of
>>>  annoying, and means that places expecting a ConstantArray have to remember to
>>>  also check for ConstantAggregateZero.  Perhaps there's a good reason for the
>>>  current design, but if not it would be great to eliminate this wart.
>>  Well, I think the main reason it so reduce the size of .ll / .bc when
>>  all operands are zero.
>>  If the array / struct is really big then this can save a lot of space.
>
> isn't this an orthogonal issue?  A ConstantVector consisting of only zeros
> doesn't have to be represented by storing a vast array of zero values, it
> could be special cased internally by setting some kind of flag.  If done right,
> an all zero ConstantVector would then act exactly the same as every other kind
> of ConstantVector as far as users of ConstantVector can tell, but wouldn't take
> up a lot of storage.  Instead, what we have now is essentially the same system
> only with the flag exposed to users (ConstantAggregateZero playing the role of
> the flag) so that all users have to worry about it, rather than this flag being
> hidden in the implementation of ConstantVector and ConstantStruct.
>
> Ciao, Duncan.
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