[PATCH] D80077: [LiveVariables] Don't set undef reg PHI used as live for FromMBB

Wed Jun 3 00:30:56 PDT 2020

ZhangKang added a comment.

In D80077#2045630 <https://reviews.llvm.org/D80077#2045630>, @lkail wrote:

> It looks like a bug in phi-node-elimination to me. When critical edge `bb.0 -> bb.2` is splitted and `bb.3` is inserted, `undef %undef` is transformed into `%3 = IMPLICIT_DEF` in `bb.3`. LV in `bb.2` is not updated in memory(If we run `phi-node-elimination` and `twoaddressinstruction` separately, we won't run into crash).

It's not a bug, I have checked the LiveVariables has been updated before exit the `machineverifier` pass.
If you use below command you can also get the error.

  llc -verify-machineinstrs -mtriple=aarch64-linux-gnu -verify-machineinstrs PHIElimination-crash.mir    -run-pass=livevars,phi-node-elimination,phi-node-elimination -no-phi-elim-live-out-early-exit=1 -phi-elim-split-all-critical-edges=1 -o runpass-phi-node-elimination.mir

If the last pass is `machineverifier` pass, the LiveVariables will not be preserved, so the last `machineverifier` pass won't get the LiveVariables and it doesn't check LiveVariables. 
Above command run `phi-node-elimination ` pass twice.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D80077/new/

https://reviews.llvm.org/D80077