[llvm-dev] Ok with mismatch between dead-markings in BUNDLE and bundled instructions?
Quentin Colombet via llvm-dev
llvm-dev at lists.llvm.org
Wed Jul 5 10:02:02 PDT 2017
> On Jul 3, 2017, at 4:13 AM, Mikael Holmén <mikael.holmen at ericsson.com> wrote:
>
> Hi,
>
> Ok, so to solve the problem for our out-of-tree target we currently do:
>
> ------------------------ lib/CodeGen/InlineSpiller.cpp ------------------------
> index 67c7e506add..28245a49477 100644
> @@ -987,38 +987,46 @@ void InlineSpiller::spillAroundUses(unsigned Reg) {
> if (foldMemoryOperand(Ops))
> continue;
>
> // Create a new virtual register for spill/fill.
> // FIXME: Infer regclass from instruction alone.
> unsigned NewVReg = Edit->createFrom(Reg);
>
> if (RI.Reads)
> insertReload(NewVReg, Idx, MI);
>
> // Rewrite instruction operands.
> bool hasLiveDef = false;
> for (const auto &OpPair : Ops) {
> MachineOperand &MO = OpPair.first->getOperand(OpPair.second);
> MO.setReg(NewVReg);
> if (MO.isUse()) {
> if (!OpPair.first->isRegTiedToDefOperand(OpPair.second))
> MO.setIsKill();
> } else {
> + // For bundled instructions: Only examine defs in the BUNDLE
> + // instruction itself if it exists.
> + MachineInstr *DefMI = OpPair.first;
> + if (DefMI->isBundled() &&
> + getBundleStart(DefMI->getIterator())->isBundle() &&
> + !DefMI->isBundle())
> + continue;
> +
> if (!MO.isDead())
> hasLiveDef = true;
> }
> }
> DEBUG(dbgs() << "\trewrite: " << Idx << '\t' << *MI << '\n');
>
>
> Is there any point of me putting this patch in Phabricator? I have very little hope of managing to trigger the problem on any in-tree target.
You could use a .mir file, with NOOPs that clobber a lot of regs and fake bundles here and there.
>
> Regards,
> Mikael
>
>
> On 06/29/2017 02:13 AM, Quentin Colombet via llvm-dev wrote:
>>> On Jun 28, 2017, at 5:10 PM, Quentin Colombet via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>
>>> Oh wait, vreg1 is indeed used.
>>> Yeah, having a dead flag here sounds wrong.
>> I mean on the instruction itself.
>> On the bundle, that’s debatable. That would fit the semantic “if no side effect you can kill it” (here there is side effect, we define other vregs).
>>>
>>>> On Jun 28, 2017, at 2:28 AM, Björn Pettersson A <bjorn.a.pettersson at ericsson.com> wrote:
>>>>
>>>> Not sure if I could follow everything in this discussion regarding subregisters. But I think the problem posted by Mikael just happened to involve subregisters, and the discussions about subregisters is confusing when it comes to Mikaels original question/problem.
>>>>
>>>> I think that the bundle could look something like this just as well:
>>>>
>>>> BUNDLE %vreg1<def,dead>
>>>> * %vreg1<def> = add %vreg2, %vreg3
>>>> * call @foo, %vreg1<internal-use>
>>>>
>>>> No subregisters involved.
>>>> %vreg1 is dead after the bundle.
>>>> %vreg1 is not dead when defined at the "add", because it is used later in the same bundle.
>>>>
>>>> Should perhaps the %vreg1 not be included in the BUNDLE head at all here?
>>>> (but shouldn't the BUNDLE head be a summary of what is going on inside the bundle, so leaving out information about %vreg1 being defined seems wrong)
>>>>
>>>> To me it seems wrong to add "dead" to the def of %vreg1 at the add (considering the internal-use).
>>>> Maybe that even answers the question that the "mismatch" between dead-markings should be OK.
>>>> Or would it be OK to mark %vreg1 as dead at the add, even though we have a later internal-use?
>>>>
>>>> Regards,
>>>> Björn
>>>>
>>>>> -----Original Message-----
>>>>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
>>>>> Quentin Colombet via llvm-dev
>>>>> Sent: den 28 juni 2017 00:02
>>>>> To: Matthias Braun <mbraun at apple.com>
>>>>> Cc: llvm-dev at lists.llvm.org
>>>>> Subject: Re: [llvm-dev] Ok with mismatch between dead-markings in
>>>>> BUNDLE and bundled instructions?
>>>>>
>>>>>
>>>>>> On Jun 27, 2017, at 2:51 PM, Matthias Braun via llvm-dev <llvm-
>>>>> dev at lists.llvm.org> wrote:
>>>>>>
>>>>>>
>>>>>>> On Jun 27, 2017, at 2:44 PM, Krzysztof Parzyszek via llvm-dev <llvm-
>>>>> dev at lists.llvm.org> wrote:
>>>>>>>
>>>>>>> On 6/27/2017 4:35 PM, Quentin Colombet via llvm-dev wrote:
>>>>>>>> Yeah I was reading this as “only the non-touched part are dead”, and
>>>>> that’s what I’d like to see in the representation longer. Obviously, the
>>>>> register is not dead as a whole here :)
>>>>>>>
>>>>>>> I think that having two defs for the same register, one dead and one not
>>>>> dead simply doesn't make sense. We already assume that a register is live if
>>>>> at least a part of it is live, so if it's "dead", it should mean that the whole thing
>>>>> is dead.
>>>>>> Without subregister I would agree. However with subregisters and aliases
>>>>> in play you can express more situations. Like for example:
>>>>>>
>>>>>> %rax<dead>, %eax = ...
>>>>>>
>>>>>> could mean the instruction writes the full rax register but we are only
>>>>> gonna read eax later.
>>>>>
>>>>> That sounds like an alias to:
>>>>> %rax<def-undef, subeax> = …
>>>>>
>>>>> Which sounds fine. Though I am not suggesting we want to move to this
>>>>> dead model for such situation.
>>>>>
>>>>>> That said I am not sure whether we actually need it, and if llvm works that
>>>>> way today. Given how subtle all of this is there is also a high danger that we
>>>>> won't get the bahviour consistent.
>>>>>
>>>>> I agree that consistent behavior is important and I also think we probably
>>>>> cannot model what we want with the current representation. What I would
>>>>> like to see if that we don’t sit on potentially useful information, like this part
>>>>> of the register is dead, because it is convenient implementation-wise. I am
>>>>> not saying that’s what you're suggesting!
>>>>> I agree that at the end of the day we want something that works and that is
>>>>> understandable. To me having the semantic of dead being this can be killed if
>>>>> the instruction does not have side effects sounded easy enough to
>>>>> understand.
>>>>>
>>>>> What is your proposal for the semantic?
>>>>>
>>>>> (IIRC the dead flag is required for values that are never used and the
>>>>> proposed fix somehow goes against that.)
>>>>>
>>>>>>
>>>>>> - Matthias
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> llvm-dev at lists.llvm.org
>>>>>> 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
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> 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
>
More information about the llvm-dev
mailing list