[clang] [llvm] [BPF] Add load-acquire and store-release instructions under -mcpu=v4 (PR #108636)

[Peilin Ye via cfe-commits]

================
@@ -703,6 +715,39 @@ SDValue BPFTargetLowering::LowerSELECT_CC(SDValue Op, 
SelectionDAG &DAG) const {
   return DAG.getNode(BPFISD::SELECT_CC, DL, VTs, Ops);
 }
 
+SDValue BPFTargetLowering::LowerATOMIC_LOAD(SDValue Op,
+                                            SelectionDAG &DAG) const {
+  const char *Msg =
+      "sequentially consistent (seq_cst) atomic load is not supported";
+  SDNode *N = Op.getNode();
+  SDLoc DL(N);
+
+  if (cast<AtomicSDNode>(N)->getMergedOrdering() ==
+      AtomicOrdering::SequentiallyConsistent)
+    fail(DL, DAG, Msg);
+
+  return Op;
+}
+
+SDValue BPFTargetLowering::LowerATOMIC_STORE(SDValue Op,
+                                             SelectionDAG &DAG) const {
+  const char *Msg =
+      "sequentially consistent (seq_cst) atomic store is not supported";
+  EVT VT = Op.getOperand(1).getValueType();
+  SDNode *N = Op.getNode();
+  SDLoc DL(N);
+
+  // Promote operand #1 (value to store) if necessary.
+  if (!isTypeLegal(VT))
+    return SDValue();
+
+  if (cast<AtomicSDNode>(N)->getMergedOrdering() ==
+      AtomicOrdering::SequentiallyConsistent)
+    fail(DL, DAG, Msg);
----------------
peilin-ye wrote:

> Let me change it to (encodings unchanged):
> ``` 
>               | imm{0-3}          | imm{4-7}        |
> ------------- | ----------------- | --------------- |
> load-acquire  | BPF_ACQUIRE (0x0) | BPF_LOAD (0x1)  |
> store-release | BPF_RELEASE (0x0) | BPF_STORE (0x2) |
> ```

Actually, let's change the encoding and define [all 6 of 
them](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/n4950.pdf#page=1817)
 once and for all:
```
namespace std {
  enum class memory_order : unspecified {
    relaxed, consume, acquire, release, acq_rel, seq_cst
  };
}
```
Will explain more after the push.

https://github.com/llvm/llvm-project/pull/108636
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