[PATCH] D15477: [X86][AVX] Only shuffle the lower half of vectors if the upper half is undefined

[Elena Demikhovsky via llvm-commits]

delena added a comment.

Hi Simon,

You have 2 input vectors A and B.
Let's see them as (a0, a1) and (b0, b1). You are looking for the one of the following patterns, based on mask:

1. (a0, u), (b0, u)
2. (u, a1), (b0, u)
3. (a0, u), (u, b1)
4. (u, a1), (b0, u)

One pass through the mask vector will map the original shuffle to one of the 4 patterns, if possible.
Then you extract lower or upper part from A and from B according to the pattern.
And calculate the mask as following:
if (M[i] < NumElts) // vector A

  Newmask[i] =  M[i] % HalfNumElts

else // vector B

  Newmask[i] =  (M[i] % HalfNumElts) + HalfNumElts 


Repository:
  rL LLVM

http://reviews.llvm.org/D15477