[LLVMdev] Self-referential instruction from jump threading

[Hal Finkel]

----- Original Message -----
> From: "Nick Lewycky" <nicholas at mxc.ca>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: llvmdev at cs.uiuc.edu
> Sent: Monday, October 22, 2012 4:25:36 AM
> Subject: Re: [LLVMdev] Self-referential instruction from jump threading
> 
> Hal Finkel wrote:
> > Hello,
> >
> > After investigating PR14133, I've discovered that jump threading
> > can output self-referential instructions:
> > %inc.us = add nsw i32 %inc.us, 1
> >
> > At least in the test case for that bug report, the relevant code is
> > later deleted (perhaps it is unreachable), and so this does not
> > cause a problem. Unfortunately, when vectorization is enabled,
> > this instruction causes BBVectorize to hang. Should I make
> > BBVectorize ignore this kind of thing, on the assumption that it
> > occurs only in unreachable (or otherwise dead) code, or should
> > this be fixed at the source (in jump threading in this case)?
> 
> The way to check whether it's valid is with the verifier. If jump
> threading were emitting that in a reachable block it would fail the
> verifier, in an unreachable block it would pass the verifier. Your
> pass
> gets to assume its input passes the verifier, and must produce output
> that does as well. That leads to the conclusion that if you see one
> of
> these in your input, then its parent block must be unreachable.

Makes sense, thanks!

 -Hal

> 
> Nick
> 

-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory