[llvm-commits] patch: generalize vector shuffle

[Mon Ping Wang]

Hi,

This is a patch that implements changing the shuffle vector definition  
to
   <result> = shufflevector <n x <ty>> <v1>, <n x <ty>> <v2>, <m x  
i32> <mask>    ; yields <m x <ty>>

As discussed in llvmdev, the reason for this change is to help  
maintain the incoming vector structure of the user code instead of  
converting to insert and extract.

During the SelectionDAG, we normalize the shuffle vector such that the  
vectors and mask are the same size and then we legalize the shuffle  
vectors.  The normalization process tries to retain the structure of  
the vector shuffle by trying to convert it to a series of concat  
vectors, extract subvector, and vector shuffles. If it can't, it falls  
back to using insert/extracts. This implementation normalizes during  
DAG construction while in a earlier implementation  normalized during  
the Legalize phase.  The pros of doing this way is that after DAG  
construction, none of the DAG passes would have to worry about non- 
normalized shuffles, we might be able to optimize the normalized  
shuffles, and in LegalizeTypes, we wouldn't have to do a pass to  
normalize the vector shuffles. (We could probably do normalization  
when we iterate the DAG to build the worklist in LegalizeTypes).  The  
downside is no DAG optimization can generate a non-normalized  
shufflevector and there may be cases where after normalization, we may  
obscure the what the original structure is.  Here is the patch.  I've  
a few more test cases that I didn't include in the patch.

Thanks,
-- Mon Ping

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