RegScavenger: Remove used regs from scavenge candidates

[arsenm]

Summary

In a future change, AMDGPU will have 2 emergency scavenging indexes in
some situations. The secondary scavenging index ends up being used
recursively when the scavenger calls eliminateFrameIndex for the
emergency spill slot. Without this, it would end up seeing the same
register which was just scavenged in the parent call as free, inserts
a second emergency spill to the same location and returns the same
register when 2 unique free registers are required.

We need to only do this if the register is used. SystemZ uses 2
scavenging slots, but calls the scavenger twice in sequence and not
recursively. In this case the previously scavenged register can be
re-clobbered, but is still tracked in the scavenger until it sees the
deferred restore instruction.

[arsenm]

ping

[arsenm]

5f39a02ea950dbbc146f2e22a7a51e5dea430ce6

llvm/lib/CodeGen/RegisterScavenging.cpp

Register RegScavenger::scavengeRegister(const TargetRegisterClass *RC,
   // Exclude all the registers being used by the instruction.
   for (const MachineOperand &MO : MI.operands()) {
     if (MO.isReg() && MO.getReg() != 0 && !(MO.isUse() && MO.isUndef()) &&
         !Register::isVirtualRegister(MO.getReg()))
       for (MCRegAliasIterator AI(MO.getReg(), TRI, true); AI.isValid(); ++AI)
         Candidates.reset(*AI);
   }
 
+  // If we have already scavenged some registers, remove them from the
+  // candidates. If we end up recursively calling eliminateFrameIndex, we don't
+  // want to be clobbering previously scavenged registers or their associated
+  // stack slots.
+  for (ScavengedInfo &SI : Scavenged) {
+    if (SI.Reg) {
+      if (isRegUsed(SI.Reg)) {
+        LLVM_DEBUG(
+          dbgs() << "Removing " << printReg(SI.Reg, TRI) <<
+          " from scavenging candidates since it was already scavenged\n");
+        for (MCRegAliasIterator AI(SI.Reg, TRI, true); AI.isValid(); ++AI)
+          Candidates.reset(*AI);
+      }
+    }
+  }
+
   // Try to find a register that's unused if there is one, as then we won't
   // have to spill.
   BitVector Available = getRegsAvailable(RC);
   Available &= Candidates;
   if (Available.any())
     Candidates = Available;
 
   // Find the register whose use is furthest away.
   MachineBasicBlock::iterator UseMI;
   Register SReg = findSurvivorReg(I, Candidates, 25, UseMI);
 
   // If we found an unused register there is no reason to spill it.
   if (!isRegUsed(SReg)) {
     LLVM_DEBUG(dbgs() << "Scavenged register: " << printReg(SReg, TRI) << "\n");
     return SReg;
   }
 
   if (!AllowSpill)
     return 0;
 
+#ifndef NDEBUG
+  for (ScavengedInfo &SI : Scavenged) {
+    assert(SI.Reg != SReg && "scavenged a previously scavenged register");
+  }
+#endif
+
   ScavengedInfo &Scavenged = spill(SReg, *RC, SPAdj, I, UseMI);
   Scavenged.Restore = &*std::prev(UseMI);
 
   LLVM_DEBUG(dbgs() << "Scavenged register (with spill): "
                     << printReg(SReg, TRI) << "\n");
 
   return SReg;
 }