[llvm-dev] Ok with mismatch between dead-markings in BUNDLE and bundled instructions?
Mikael Holmén via llvm-dev
llvm-dev at lists.llvm.org
Mon Jul 3 04:13:20 PDT 2017
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.
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
>>>>> _______________________________________________
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