[clang] [llvm] [NVPTX] Support __usAtomicCAS builtin (PR #99646)

[via cfe-commits]

================
@@ -871,7 +871,8 @@ NVPTXTargetLowering::NVPTXTargetLowering(const 
NVPTXTargetMachine &TM,
   // actions
   computeRegisterProperties(STI.getRegisterInfo());
 
-  setMinCmpXchgSizeInBits(32);
+  bool Allow16BitCAS = STI.getSmVersion() >= 70 && STI.getPTXVersion() >= 63;
+  setMinCmpXchgSizeInBits(Allow16BitCAS ? 16 : 32);
----------------
gonzalobg wrote:

Can we add a: 

```
bool hasAtomCas16() const { return SmVersion >= 70 && PTXVersion >= 63; }
```

to NVPTXSubtarget instead, and call it here: 

```suggestion
  setMinCmpXchgSizeInBits(STI.hasAtomCas16() ? 16 : 32);
```

https://github.com/llvm/llvm-project/pull/99646
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