[llvm-dev] Using ConstantDataVector instead of ConstantVector

[Friedman, Eli via llvm-dev]

On 11/9/2016 1:30 AM, Alex Susu via llvm-dev wrote:
>   Hello.
>     I'm curious why can't I convert a Value like
>       <16 x i16> <i16 2, i16 2, i16 2, i16 2, i16 2, i16 2, i16 2... 
> i16 2>
>      to ConstantVector - using llvm::dyn_cast<llvm::ConstantVector>() 
> returns NULL.
>
>     However, converting it with dyn_cast to ConstantDataVector 
> fortunately works.
>
>     Note that at 
> http://llvm.org/docs/doxygen/html/classllvm_1_1ConstantDataVector.html 
> it is written "A vector constant whose element type is a simple 
> 1/2/4/8-byte integer or float/double", etc. This is a bit misleading, 
> since vectors of bigger sizes can be used.

I think you somehow found some old doxygen pages? 
http://llvm.org/doxygen/classllvm_1_1Constant.html is what it looks like 
on trunk.

The class hierarchy is set up the way it is because it's convenient for 
the implementation; essentially, ConstantAggregate subclasses are 
constants which use other constants, and ConstantData subclasses don't 
use other constants.

You can use Constant::getAggregateElement() if you need to iterate over 
the elements of a constant of vector type.

-Eli

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