[llvm-dev] Question about MachineCSE optimization

[Matthias Braun via llvm-dev]

Yes
OP_X ..., implicit-def dead %X
... potentially more defs/uses of %X
OP_X ..., implicit-def dead %X

is good to merge. Also note that:

%X = DEF
OP_X ..., implicit-def dead %X
use %X

is not valid! (The def overrides the register value but the "dead" flag promises noone will read it, even though in this case another use comes below).

- Matthias

> On Oct 22, 2018, at 4:59 AM, Yuchao (Michael) <michael.yuchao at huawei.com> wrote:
> 
> Hi mbraun
>  
> Thanks for your information.
>  
> What about following situation:
> 3)
> OP_X …, implicit-def dead %X 
> with
> OP_X …, implicit-def dead %X
>  
> Between two instructions, there may be another instruction(not OP_X) which also def %X,
> Is it  correct to do CSE in this situation?
>  
> Best wishes
> Michael
>  
> From: mbraun at apple.com [mailto:mbraun at apple.com] 
> Sent: Monday, October 22, 2018 5:51 AM
> To: Matthias Braun <mbraun at apple.com>
> Cc: Yuchao (Michael) <michael.yuchao at huawei.com>; llvm-dev at lists.llvm.org; sunqiang (I) <sunqiang13 at huawei.com>
> Subject: Re: [llvm-dev] Question about MachineCSE optimization
>  
>  
> 
> 
> On Oct 21, 2018, at 2:48 PM, mbraun via llvm-dev <llvm-dev at lists.llvm.org <mailto: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
>  
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