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

[Daniel Sanders]

dsanders added inline comments.

================
Comment at: lib/CodeGen/RegisterCoalescer.cpp:1541
@@ -1534,1 +1540,3 @@
+          return false;
+      }
     }
----------------
dsanders wrote:
> 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.
While making the change, I noticed that I can't remove the loop on line 1497. Isn't it needed for the if-statement on line 1510?

If I understand isFlipped() correctly, the true path is trying to account for:
  %B = COPY %A
  ...
  %A = COPY %B
We still need to check for defines of %A in the '...' region.

Also, suppose there's a JAL that clobbers A between them. In this case %A can't be coalesced with %A because that would move the second %A def above the clobber.

Shouldn't we be moving the check for regmask clobbers up into the loop on line 1497 somehow?


http://reviews.llvm.org/D11649