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

[Quentin Colombet]

> 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
> 
> 
>