AArch64: implement copies to/from NZCV as a last ditch effort.

A test in test/Generic creates a DAG where the NZCV output of an ADCS is used
by multiple nodes. This makes LLVM want to save a copy of NZCV for later, which
it couldn't do before.

This should be the last fix required for the aarch64 buildbot.

llvm-svn: 209651

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -1507,7 +1507,25 @@ void AArch64InstrInfo::copyPhysReg(MachineBasicBlock &MBB,
     return;
   }
 
-  assert(0 && "unimplemented reg-to-reg copy");
+  if (DestReg == AArch64::NZCV) {
+    assert(AArch64::GPR64RegClass.contains(SrcReg) && "Invalid NZCV copy");
+    BuildMI(MBB, I, DL, get(AArch64::MSR))
+      .addImm(AArch64SysReg::NZCV)
+      .addReg(SrcReg, getKillRegState(KillSrc))
+      .addReg(AArch64::NZCV, RegState::Implicit | RegState::Define);
+    return;
+  }
+
+  if (SrcReg == AArch64::NZCV) {
+    assert(AArch64::GPR64RegClass.contains(DestReg) && "Invalid NZCV copy");
+    BuildMI(MBB, I, DL, get(AArch64::MRS))
+      .addReg(DestReg)
+      .addImm(AArch64SysReg::NZCV)
+      .addReg(AArch64::NZCV, RegState::Implicit | getKillRegState(KillSrc));
+    return;
+  }
+
+  llvm_unreachable("unimplemented reg-to-reg copy");
 }
 
 void AArch64InstrInfo::storeRegToStackSlot(

llvm/lib/Target/AArch64/AArch64RegisterInfo.cpp

@@ -138,7 +138,7 @@ AArch64RegisterInfo::getPointerRegClass(const MachineFunction &MF,
 const TargetRegisterClass *
 AArch64RegisterInfo::getCrossCopyRegClass(const TargetRegisterClass *RC) const {
   if (RC == &AArch64::CCRRegClass)
-    return nullptr; // Can't copy NZCV.
+    return &AArch64::GPR64RegClass; // Only MSR & MRS copy NZCV.
   return RC;
 }