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

[Krzysztof Parzyszek via llvm-dev]

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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