We previously avoided inserting these moves during isel in a few cases which is implemented using a whitelist of opcodes. But it's too difficult to generate a perfect list of opcodes to whitelist. Especially with AVX512F without AVX512VL using 512 bit vectors to implement some 128/256 bit operations. Since isel is done bottoms up, we'd have to check the VT and opcode and subtarget in order to determine whether an EXTRACT_SUBREG would be generated for some operations.
So instead of doing that, this patch adds a post processing step that detects when the moves are unnecesssary after isel. At that point any EXTRACT_SUBREGs would have already been created and appear in the DAG. So then we just need to ensure the input to the move isn't one.
Differential Revision: https://reviews.llvm.org/D44289
llvm-svn: 327724
e6913ec340