[llvm-dev] RegAlloc Q: spill when implicit-def physreg is also the output reg of instruction

[Kevin Choi via llvm-dev]

Hi Quentin,

MyInst is a custom instruction that has implicit-defs of fixed 
registers. The implicit-defs are seen at the end of Instruction Selection.
I'd like to add a report, but I am working on an out-of-tree backend 
based on 7.0. I can try to help reduce the testcase down.
Filed https://bugs.llvm.org/show_bug.cgi?id=41790

Regards,
Kevin

On 2019-05-07 3:45 p.m., Quentin Colombet wrote:
> Hi Kevin,
>
> That sounds like a bug :).
>
> When is the implicit-def added?
>
> What I am thinking is if it gets added after we computed the live 
> intervals, then we may miss it is there.
>
> Could you file a public report?
>
> Thanks,
> -Quentin
>
>> On May 6, 2019, at 3:13 PM, Kevin Choi via llvm-dev 
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> Hi LLVM,
>>
>> I ran into a case where RegAlloc would insert a spill across 
>> instruction that had same register for output operand and 
>> implicit-def. The effect this had was that spill code would 
>> immediately overwrite the output result. Is this the expected result 
>> of setting up MyInst this way? In other words, does RegAlloc know to 
>> not insert spill in case it sees that output reg is same as one of 
>> implicit-def?
>>
>> If this is intended (always spilling live regs across implicit-def 
>> inst?), I am puzzled on how to use MyInst that has variable output 
>> reg with static implicit-defs. Any tips would be greatly appreciated.
>>
>> Reduced Example:
>>
>> _*Before RegAlloc:*_
>> %1:reg = COPY ...;
>> %2:reg = MyInst %1:reg, ..., implicit-def dead $p1, ...;
>> %3:reg = Use %2:reg
>>
>> _*RegAlloc:*_
>> >> %2:reg = MyInst %1:reg, ..., implicit-def dead $p1, ...;
>> Regs: $p1=%1
>> Killing last use: %1:reg
>> Assigning %2 to $p1
>> Spilling %2 in $p1 to stack slot #2  <-- suspicious if this is 
>> inserting spill, unaware of output reg being same
>> << $p1 =MyInst killed $p1, ..., implicit-def dead $p1, ...;
>>
>> *_After RegAlloc:_*
>> $p1 = COPY ...;
>> Store $p1 %stack.2;
>> $p1 =MyInst killed $p1, ..., implicit-def dead $p1, ...;
>> $p1 = Load %stack.2;
>> $p2 = Use $p1, ...;
>>
>>
>> Best Regards,
>>
>> Kevin
>>
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