[PATCH] D37034: Insert IMPLICIT_DEFS for undef uses in tail merging

[Alberto Magni via Phabricator via llvm-commits]

alberto_magni added a comment.

Hi Matthias,

I have stumbled upon a problem related to this change while working on an out-of-tree target.
It reproduces with Hexagon too.

Is the insertion of the IMPLICIT_DEF legal if one of the two terminators of %bb.0 uses $r0 ?
(Here J2_jumpr uses $r0)

Consider this case:

  ---
  name: func0
  tracksRegLiveness: true
  
  body: |
    bb.0:
      liveins: $r0, $r31
      successors: %bb.2
        J2_jumpt undef $p0, %bb.2, implicit-def $pc
        J2_jumpr killed $r0, implicit-def $pc
  
    bb.1:
      liveins: $r31
      successors: %bb.3
        $r0 = L2_loadruh_io undef $r1, 0
        PS_storerhabs 0, killed $r0
        J2_jump %bb.3, implicit-def $pc
  
    bb.2:
      liveins: $r31
      successors: %bb.3
        PS_storerhabs 0, undef $r0
        J2_jump %bb.3, implicit-def $pc
  
    bb.3:
      liveins: $r31
        PS_jmpret killed $r31, implicit-def $pc
  ...
  ---

Running `llc -march=hexagon -run-pass branch-folder` gives:

  body:             |
    bb.0:
      successors: %bb.1(0x80000000)
      liveins: $r0, $r31
  
      $r0 = IMPLICIT_DEF
      J2_jumpt undef $p0, %bb.1, implicit-def $pc
      J2_jumpr killed $r0, implicit-def $pc
  
    bb.1:
      liveins: $r0
  
      PS_storerhabs 0, $r0
      PS_jmpret killed $r31, implicit-def $pc
  ...

Thus making $r0 undefined in bb.0, when originally it was not.

Thanks,
Alberto


Repository:
  rL LLVM

https://reviews.llvm.org/D37034