[LLVMdev] Vectorization of loops with conditional dereferencing

[Nadav Rotem]

> 
> I'm not. :(
> 

I think that this is probably the most important feature for the vectorizer right now.  Other features require adding complexity to the vectorizer while this feature is relatively simple. 

> What kind of pragmas would work for this loop? Something telling that it's safe to speculatively read from m[] at any position? In this reduction case it might be enough. But if this would be an induction store like:
> 
> for () {
>   if (a[i] > 0)
>     x[i] = ... + m[i];
> 

Sure.  Vectorization of stores is done by loading the current value from memory, blending the new value and saving it back to memory.  


> then, the store would be a more complicated way to write to memory and you'd need the read-pragma to not affect such cases.


There is no need for read pragma or even a special attribute.   The ‘vectorize’ pragma tells the vectorizer that it is safe to access the predicated memory (read or write).