[clang] [llvm] [RFC][BPF] Do atomic_fetch_*() pattern matching with memory ordering (PR #107343)

[via cfe-commits]

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@@ -152,22 +152,91 @@ static bool hasLiveDefs(const MachineInstr &MI, const TargetRegisterInfo *TRI) {
   return false;
 }
 
-void BPFMIPreEmitChecking::processAtomicInsts() {
+bool BPFMIPreEmitChecking::processAtomicInsts() {
+  if (!MF->getSubtarget<BPFSubtarget>().getHasJmp32()) {
+    // Only check for cpu version 1 and 2.
+    for (MachineBasicBlock &MBB : *MF) {
+      for (MachineInstr &MI : MBB) {
+        if (MI.getOpcode() != BPF::XADDW && MI.getOpcode() != BPF::XADDD)
+          continue;
+
+        LLVM_DEBUG(MI.dump());
+        if (hasLiveDefs(MI, TRI)) {
+          DebugLoc Empty;
+          const DebugLoc &DL = MI.getDebugLoc();
+          const Function &F = MF->getFunction();
+          F.getContext().diagnose(DiagnosticInfoUnsupported{
+              F, "Invalid usage of the XADD return value", DL});
+        }
+      }
+    }
+  }
+
+  // Check return values of atomic_fetch_and_{add,and,or,xor}.
+  // If the return is not used, the atomic_fetch_and_<op> instruction
+  // is replaced with atomic_<op> instruction.
+  MachineInstr *ToErase = nullptr;
+  bool Changed = false;
+  const BPFInstrInfo *TII = MF->getSubtarget<BPFSubtarget>().getInstrInfo();
   for (MachineBasicBlock &MBB : *MF) {
     for (MachineInstr &MI : MBB) {
-      if (MI.getOpcode() != BPF::XADDW && MI.getOpcode() != BPF::XADDD)
+      if (ToErase) {
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eddyz87 wrote:

This is needed to allow iterator move to the next instruction, right?
A bit tricky, maybe add a comment here?
Also, won't delete the last instruction in the basic block, would this be a problem if instruction is used in a later basic block?

https://github.com/llvm/llvm-project/pull/107343