llvm dev - Aug 2017 - assertion in MachineCopyPropagation::isNopCopy

Hi,

I'm hitting an assertion in MachineCopyPropagation::isNopCopy:

if (Src == PreviousSrc) {
   assert(Def == PreviousDef);
   return true;
}

This code compares two COPY instruction to see whether they are 
effectively "the same". The assert assumes that, if the sources are
the
same register then the destinations must be as well.
My instructions look like this, however:

   %V2<def> = COPY %V6
   %V6<def> = COPY %V2_LO

(isNopCopy is called with src and dest of the first instruction switched.)
So dst2 == src1 but src2 is a subregister of dst1.
Is there some assumption that size(src) == size(dst)?
This second COPY, with size(src) != size(dst) was generated in 
InstrEmitter::EmitSubregNode at

// Create the extract_subreg machine instruction.

So it's not out-of-tree code.
Am I missing something or should the assert condition be part of the 
"if" condition instead?
Thanks,

-- 
Verena Beckham

Senior Principal Software Engineer, Compilers

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