[PATCH] D11649: [regalloc] Make RegMask clobbers prevent merging vreg's into PhysRegs when hoisting def's upwards.

[Daniel Sanders]

Thanks.

Because we're in rc2 now and the rules are stricter: Hans: Is this ok with you too?
________________________________________
From: Quentin Colombet [qcolombet at apple.com]
Sent: 31 July 2015 17:59
To: reviews+D11649+public+c6823873e6bbbad6 at reviews.llvm.org
Cc: Daniel Sanders; matze at braunis.de; hans at chromium.org; llvm-commits at cs.uiuc.edu
Subject: Re: [PATCH] D11649: [regalloc] Make RegMask clobbers prevent merging vreg's into PhysRegs when hoisting def's upwards.

> On Jul 31, 2015, at 6:08 AM, Daniel Sanders <Daniel.Sanders at imgtec.com> wrote:
>
> dsanders added a comment.
>
> I've used the earlier LGTM to commit so that I have something to merge to the release branch. I will of course follow up on the comment I received after the LGTM.
>
> I fixed a silly mistake before the commit. The inner if had two statements and no '{ }' so the return false was always executed. The mistake showed up as failures in four PowerPC tests.
>
> Ok to merge to the branch?

Yep, OK to merge to the branch!

Q.

>
>
> ================
> Comment at: lib/CodeGen/RegisterCoalescer.cpp:1541
> @@ -1534,1 +1540,3 @@
> +          return false;
> +      }
>     }
> ----------------
> qcolombet wrote:
>> MatzeB wrote:
>>> dsanders wrote:
>>>> qcolombet wrote:
>>>>> This check should have been handled by the loop starting line 1497, IIRC.
>>>>> I.e., this shouldn’t be necessary.
>>>> I agree that that loop should have caught this but it doesn't. I haven't looked into why that loop doesn't catch it yet but if the information is based on MachineInstr::definesRegister() then it could be that that doesn't seem to account for regmasks.
>>>>
>>>> I also tried:
>>>>    if (MI->definesRegister(DstReg, TRI)) {
>>>>      DEBUG(dbgs() << "\t\tInterference (read): " << *MI);
>>>>      return false;
>>>>    }
>>>> in this loop but that doesn't catch it either.
>>> From what I can see we do not create mini liverange segments for clobbers. This is both a bit scary but also understandable from a performance point of view as I imagine creating countless small segments would increase the LiveIntervals quite a bit.
>>>
>>> The register allocators (the LiveRegMatrix class) has a special code paths to deal with regmasks.
>>>
>>> That definesRegister() doesn't account for regmasks is understandable as the register isn't defined to something sensible, but there is certainly a case to make for MachineInstr::killsRegister() to check for register masks.
>> I guess Matthias is right about the small segments.
>>
>> Then, I would suggest a slightly different API to use.
>> Use the analyzePhysReg on the MachineOperand iterator.
>> This will tell you if the physreg is read, write, or clobbered, and you can get rid of the previous loop to do the interference check.
> I've looked at that function and I think it will do the right thing. I'll post another patch to make this change shortly.
>
>
> http://reviews.llvm.org/D11649
>
>
>