jaykang10 added a comment.

> The motivation doesn't seem solid to me, who else is going to benefit from 
> this flag? You also didn't explain why doing this transformation yourself 
> (looking through the shuffle) on your downstream pass isn't enough for you. 
> We generally try to avoid adding flags if not for a good reason.

If you turn on optimization with -O option, the shufflevector is gone on 
'InstructionCombining' pass. I showed you the IR example and source code on 
previous comment. You could mention bitcast instead of shufflevector. The 
bitcast could be constant expression with global variables and it makes code 
dirty. I think Anastasia has already explained the motivation well on her 
comments. If you need something more, please let me know.


https://reviews.llvm.org/D30810



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