BPF: provide better error message for unsupported atomic operations

Currently, BPF backend does not support all variants of
  atomic_load_{add,and,or,xor}, atomic_swap and atomic_cmp_swap
For example, it only supports 32bit (with alu32 mode) and 64bit
operations for atomic_load_{and,or,xor}, atomic_swap and
atomic_cmp_swap. Due to historical reason, atomic_load_add is
always supported with 32bit and 64bit.

If user used an unsupported atomic operation, currently,
codegen selectiondag cannot find bpf support and will issue
a fatal error. This is not user friendly as user may mistakenly
think this is a compiler bug.

This patch added Custom rule for unsupported atomic operations
and will emit better error message during ReplaceNodeResults()
callback. The following is an example output.

  $ cat t.c
  short sync(short *p) {
    return  __sync_val_compare_and_swap (p, 2, 3);
  }
  $ clang -target bpf -O2 -g -c t.c
  t.c:2:11: error: Unsupported atomic operations, please use 64 bit version
    return  __sync_val_compare_and_swap (p, 2, 3);
            ^
  fatal error: error in backend: Cannot select: t19: i64,ch =
    AtomicCmpSwap<(load store seq_cst seq_cst 2 on %ir.p)> t0, t2,
    Constant:i64<2>, Constant:i64<3>, t.c:2:11
    t2: i64,ch = CopyFromReg t0, Register:i64 %0
      t1: i64 = Register %0
    t11: i64 = Constant<2>
    t10: i64 = Constant<3>
  In function: sync
  PLEASE submit a bug report ...

Fatal error will still happen since we did not really do proper
lowering for these unsupported atomic operations. But we do get
a much better error message.

Differential Revision: https://reviews.llvm.org/D98471

llvm/lib/Target/BPF/BPFISelLowering.cpp

@@ -78,6 +78,24 @@ BPFTargetLowering::BPFTargetLowering(const TargetMachine &TM,
   setOperationAction(ISD::STACKSAVE, MVT::Other, Expand);
   setOperationAction(ISD::STACKRESTORE, MVT::Other, Expand);
 
+  // Set unsupported atomic operations as Custom so
+  // we can emit better error messages than fatal error
+  // from selectiondag.
+  for (auto VT : {MVT::i8, MVT::i16, MVT::i32}) {
+    if (VT == MVT::i32) {
+      if (STI.getHasAlu32())
+        continue;
+    } else {
+      setOperationAction(ISD::ATOMIC_LOAD_ADD, VT, Custom);
+    }
+
+    setOperationAction(ISD::ATOMIC_LOAD_AND, VT, Custom);
+    setOperationAction(ISD::ATOMIC_LOAD_OR, VT, Custom);
+    setOperationAction(ISD::ATOMIC_LOAD_XOR, VT, Custom);
+    setOperationAction(ISD::ATOMIC_SWAP, VT, Custom);
+    setOperationAction(ISD::ATOMIC_CMP_SWAP_WITH_SUCCESS, VT, Custom);
+  }
+
   for (auto VT : { MVT::i32, MVT::i64 }) {
     if (VT == MVT::i32 && !STI.getHasAlu32())
       continue;
@@ -218,6 +236,30 @@ BPFTargetLowering::getRegForInlineAsmConstraint(const TargetRegisterInfo *TRI,
   return TargetLowering::getRegForInlineAsmConstraint(TRI, Constraint, VT);
 }
 
+void BPFTargetLowering::ReplaceNodeResults(
+  SDNode *N, SmallVectorImpl<SDValue> &Results, SelectionDAG &DAG) const {
+  const char *err_msg;
+  uint32_t Opcode = N->getOpcode();
+  switch (Opcode) {
+  default:
+    report_fatal_error("Unhandled custom legalization");
+  case ISD::ATOMIC_LOAD_ADD:
+  case ISD::ATOMIC_LOAD_AND:
+  case ISD::ATOMIC_LOAD_OR:
+  case ISD::ATOMIC_LOAD_XOR:
+  case ISD::ATOMIC_SWAP:
+  case ISD::ATOMIC_CMP_SWAP_WITH_SUCCESS:
+    if (HasAlu32 || Opcode == ISD::ATOMIC_LOAD_ADD)
+      err_msg = "Unsupported atomic operations, please use 32/64 bit version";
+    else
+      err_msg = "Unsupported atomic operations, please use 64 bit version";
+    break;
+  }
+
+  SDLoc DL(N);
+  fail(DL, DAG, err_msg);
+}
+
 SDValue BPFTargetLowering::LowerOperation(SDValue Op, SelectionDAG &DAG) const {
   switch (Op.getOpcode()) {
   case ISD::BR_CC:

llvm/lib/Target/BPF/BPFISelLowering.h

@@ -99,6 +99,9 @@ private:
                       const SmallVectorImpl<SDValue> &OutVals, const SDLoc &DL,
                       SelectionDAG &DAG) const override;
 
+  void ReplaceNodeResults(SDNode *N, SmallVectorImpl<SDValue> &Results,
+                          SelectionDAG &DAG) const override;
+
   EVT getOptimalMemOpType(const MemOp &Op,
                           const AttributeList &FuncAttributes) const override {
     return Op.size() >= 8 ? MVT::i64 : MVT::i32;