[llvm-dev] Question about MachineCSE optimization
Matthias Braun via llvm-dev
llvm-dev at lists.llvm.org
Tue Oct 23 10:17:00 PDT 2018
> On Oct 22, 2018, at 11:29 PM, Yuchao (Michael) via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> As far as I know, dead register is not only means none would read it , it may means register is clobbered.
Yes, any definition "clobbers" a register.
>
> http://lists.llvm.org/pipermail/llvm-dev/2018-February/120994.html <http://lists.llvm.org/pipermail/llvm-dev/2018-February/120994.html>
>
> From: Matthias Braun [mailto:matze at braunis.de]
> Sent: Tuesday, October 23, 2018 2:04 AM
> To: Yuchao (Michael) <michael.yuchao at huawei.com>
> Cc: llvm-dev at lists.llvm.org; sunqiang (I) <sunqiang13 at huawei.com>
> Subject: Re: [llvm-dev] Question about MachineCSE optimization
>
> 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 <mailto: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> [mailto:mbraun at apple.com <mailto:mbraun at apple.com>]
> Sent: Monday, October 22, 2018 5:51 AM
> To: Matthias Braun <mbraun at apple.com <mailto:mbraun at apple.com>>
> Cc: Yuchao (Michael) <michael.yuchao at huawei.com <mailto:michael.yuchao at huawei.com>>; llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>; sunqiang (I) <sunqiang13 at huawei.com <mailto: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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