[llvm-dev] Question about MachineCSE optimization

[mbraun via llvm-dev]

> On Oct 21, 2018, at 2:48 PM, mbraun via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> As far as I understand it (though I didn’t write the code so I may be missing something) this is about making sure liveness flags (kill, dead) are updated correctly when merging instruction. The situations to consider are probably:
> 
> 1)
> OP_X …, implicit-def %X
> with
> OP_X …, implicit-def dead %X
> 
> The merged instruction probably should not have a dead flag set. The 2nd if appears to be about this situation:
> 
> 
> 2)
> OP_X …, implicit-def dead %X
> Use killed %X
> …
> OP_X …, implicit-def %X    ; this gets merged with the previous OP_X
> Use %X
This example should of course read:

OP_X …, implicit-def %X
Use killed %X
…
OP_X …, implicit-def %X    ; this gets merged with the previous OP_X
Use %X

Once we merge the 2nd OP_X with the first one, we have to drop the `killed` flags on all users of %X between the first OP_X and the position where we had the 2nd OP_X before it was merged.

> 
> Once we merge the 2nd OP_X with the first one, we have to drop the `killed` flags on the %X users.
> 
> 
> 
> Though admittedly I am at a loss right now why this is only performed for implicit operands and not for all operands…
> 
> - Matthias
> 
>> On Oct 20, 2018, at 1:38 AM, Yuchao (Michael) via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> Hi 
>>  
>> I am trying to fix a bug in MachineCSE and have one question about following code:
>>  
>> *********************************************
>> // Go through implicit defs of CSMI and MI, if a def is not dead at MI,
>> // we should make sure it is not dead at CSMI.
>> if (MO.isImplicit() && !MO.isDead() && CSMI->getOperand(i).isDead())
>>   ImplicitDefsToUpdate.push_back(i);
>>  
>> // Keep track of implicit defs of CSMI and MI, to clear possibly
>> // made-redundant kill flags.
>> if (MO.isImplicit() && !MO.isDead() && OldReg == NewReg)
>>   ImplicitDefs.push_back(OldReg);
>>  
>> if (OldReg == NewReg) {
>>   --NumDefs;
>>   continue;
>> }
>> **********************************************
>>  
>> From Above source code, it seems that CSE does not handle instruction which
>> have one operand that is ‘implicit+isDead+isDef’.
>>  
>> Base on my understanding, implicit dead register is mean clobbered, so it is not safe to
>> do CSE. Is this right?
>>  
>> Best wishes
>> Michael
>>  
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181021/de51f41f/attachment.html>