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

[Quentin Colombet via llvm-dev]

Yeah, that’s want I was expecting from the verifier.
I’d say we should add an implicit_def (assuming it gets kill at code emission) in the part where it is undef.

> On Jul 27, 2017, at 11:52 AM, Krzysztof Parzyszek <kparzysz at codeaurora.org> wrote:
> 
> Yes, immediately after branch folding the code would still behave the same as the original.  At the same time, any subsequent optimization may "exploit" the incorrect liveness information to do something bad.  If you add -run-pass if-converter, you'll get:
> 
> # After If Converter
> # Machine code for function fred: IsSSA, NoPHIs, TracksLiveness, NoVRegs
> 
> BB#0:
>        %R0<def> = L2_ploadruhf_io %P0<undef>, %R0<undef>, 0, %R0<imp-use>
>        PS_storerhabs 0, %R0
>        PS_jmpret %R31<kill>, %PC<imp-def>
> 
> # End machine code for function fred.
> 
> *** Bad machine code: Using an undefined physical register ***
> - function:    fred
> - basic block: BB#0  (0x411e8e8)
> - instruction: %R0<def> = L2_ploadruhf_io
> - operand 4:   %R0<imp-use>
> LLVM ERROR: Found 1 machine code errors.
> 
> -Krzysztof
> 
> 
> On 7/27/2017 1:47 PM, Quentin Colombet wrote:
>> Why do you say the code is wrong?
>> I can see why the MachineVerifier would complain, but the code seems to do the same thing as it was doing before the transformation, isn’t it?
>>> On Jul 27, 2017, at 11:40 AM, Krzysztof Parzyszek via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>> 
>>> 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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> 
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