[llvm-dev] Resend: assertion in MachineCopyPropagation::isNopCopy

Verena Beckham via llvm-dev llvm-dev at lists.llvm.org
Mon Sep 18 04:10:56 PDT 2017

Hi, anyone know anything about copy propagation? Matthias, I see this 
was your code originally? Was there some assumptions you made?

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?

Verena Beckham

Senior Principal Software Engineer, Compilers

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