[llvm-dev] Tail merging "undef" with a defined register: wrong code

[Krzysztof Parzyszek via llvm-dev]

The comment in test/CodeGen/X86/branchfolding-undef.mir states that such 
merging is legal, however doing so can actually generate wrong code:

Consider this (valid code):

---
name: fred
tracksRegLiveness: true

body: |
   bb.0:
     successors: %bb.1, %bb.2
       J2_jumpt undef %p0, %bb.2, implicit-def %pc
       J2_jump %bb.1, implicit-def %pc

   bb.1:
     successors: %bb.3
       %r0 = L2_loadruh_io undef %r0, 0
       PS_storerhabs 0, killed %r0
       J2_jump %bb.3, implicit-def %pc

   bb.2:
     successors: %bb.3
       PS_storerhabs 0, undef %r0
       J2_jump %bb.3, implicit-def %pc

   bb.3:
       PS_jmpret killed %r31, implicit-def %pc
...


Run:
llc -march=hexagon -run-pass branch-folder test.mir -o -


We get an invalid code:

body: |
   bb.0:
     successors: %bb.1(0x40000000), %bb.2(0x40000000)
     J2_jumpt undef %p0, %bb.2, implicit-def %pc

   bb.1:
     successors: %bb.2(0x80000000)
     %r0 = L2_loadruh_io undef %r0, 0

   bb.2:
     liveins: %r0       ; Invalid: r0 is not live out of bb.0
     PS_storerhabs 0, %r0
     PS_jmpret killed %r31, implicit-def %pc
...


Should we disallow merging undef with non-undef?

-Krzysztof


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