A value number issue

[Gary Oblock <gary@amperecomputing.com>]

I seem to be having a problem with the pre pass.

When eliminate_dom_walker::eliminate_stmt is called with
the gsi to "dedangled_864 = bea_43->tail;" which in turn
calls eliminate_dom_walker::eliminate_avail op of dedangled_864.
This gives VN_INFO (lhs)->valnum of _920. The _920 is not
associated with any SSA variable in the function and I don't
see how it got associated with dedangled_864. This is not
a theoretical issue because it causes an error (the gcc_unreachable
in eliminate_stmt is called.)

Here is how _920 (in function main) is used.

  _920 = arcnew_916->head;
  _921 = MEM[(struct node.reorg.reorder *)_920].firstin;
  MEM[(struct node.reorg.reorder *)_920].firstin = arcnew_916;

Here is how dedangled_864 is used:

  <bb 63> [local count: 2609125]:
  dedangled_863 = bea_43->head;
  dedangled_864 = bea_43->tail;
  goto <bb 65>; [100.00%]

  <bb 64> [local count: 1813121]:
  dedangled_865 = bea_43->tail;
  dedangled_866 = bea_43->head;

  <bb 65> [local count: 4422246]:
  # dedangled_867 = PHI <dedangled_863(63), dedangled_865(64)>
  # dedangled_868 = PHI <dedangled_864(63), dedangled_866(64)>
  delta_461 = 1;
  goto <bb 82>; [100.00%]

Note, dedangled_868 is used in an ever widening net of
phis and operations. Also, the other similar statements

  dedangled_863 = bea_43->head;
  dedangled_865 = bea_43->tail;
  dedangled_866 = bea_43->head;

don't seem to be malformed.

I tried using a watchpoint to see what was happening but that turned
out to be not productive in that it was tripping on something
unrelated even if I set it at the start of the pre pass.

I'm assuming that some of my code is malformed in some
subtle way and I was wondering it anybody had any ideas?
I say subtle because this was all working on a slightly different
version of gcc without the code of some other Ampere optimizations
in the mix (I disabled those optimizations and things still failed.)

Note, if you guys don't have any ideas the next approach is adding
tons of brute force instrumentation and special purpose sanity
checking to the value numbering routine... please help me avoid that.

Thanks,

Gary


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