[llvm] [RISCV] Initial ISel support for the experimental zacas extension (PR #67918)

[Craig Topper via llvm-commits]

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@@ -728,6 +807,72 @@ bool RISCVExpandAtomicPseudo::expandAtomicCmpXchg(
   return true;
 }
 
+static Register getGPRPairEvenReg(Register PairedReg) {
+  switch (PairedReg) {
+  case RISCV::X0_PD:
+    return RISCV::X0;
+  case RISCV::X2_PD:
+    return RISCV::X2;
+  case RISCV::X4_PD:
+    return RISCV::X4;
+  case RISCV::X6_PD:
+    return RISCV::X6;
+  case RISCV::X8_PD:
+    return RISCV::X8;
+  case RISCV::X10_PD:
+    return RISCV::X10;
+  case RISCV::X12_PD:
+    return RISCV::X12;
+  case RISCV::X14_PD:
+    return RISCV::X14;
+  case RISCV::X16_PD:
+    return RISCV::X16;
+  case RISCV::X18_PD:
+    return RISCV::X18;
+  case RISCV::X20_PD:
+    return RISCV::X20;
+  case RISCV::X22_PD:
+    return RISCV::X22;
+  case RISCV::X24_PD:
+    return RISCV::X24;
+  case RISCV::X26_PD:
+    return RISCV::X26;
+  case RISCV::X28_PD:
+    return RISCV::X28;
+  case RISCV::X30_PD:
+    return RISCV::X30;
+  default:
+    llvm_unreachable("Unexpected GPR pair");
+  }
+}
+
+bool RISCVExpandAtomicPseudo::expandAMOCAS(
+    MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI, bool IsPaired,
+    int Width, MachineBasicBlock::iterator &NextMBBI) {
+  MachineInstr &MI = *MBBI;
+  DebugLoc DL = MI.getDebugLoc();
+
+  Register DestReg = MI.getOperand(0).getReg();
+  if (IsPaired)
+    DestReg = getGPRPairEvenReg(DestReg);
+  Register AddrReg = MI.getOperand(1).getReg();
+  Register NewValReg = MI.getOperand(3).getReg();
+  if (IsPaired)
+    NewValReg = getGPRPairEvenReg(NewValReg);
+  AtomicOrdering Ordering =
+      static_cast<AtomicOrdering>(MI.getOperand(4).getImm());
+
+  MachineInstr *NewMI =
+      BuildMI(MBB, MBBI, DL, TII->get(getAMOCASForRMW(Ordering, Width, STI)))
+          .addReg(DestReg)
+          .addReg(AddrReg)
+          .addReg(NewValReg);
+  NewMI->getOperand(0).setIsDef(true);
----------------
topperc wrote:

Can we do `.addReg(DestReg, RegState::Define)` on the BuildMI so we don't need to do this?

https://github.com/llvm/llvm-project/pull/67918