[llvm] Implement areInlineCompatible for SystemZ using feature bitset (PR #132976)

[Ulrich Weigand via llvm-commits]

================
@@ -422,6 +422,20 @@ bool SystemZTTIImpl::isLSRCostLess(const TargetTransformInfo::LSRCost &C1,
              C2.ScaleCost, C2.SetupCost);
 }
 
+bool SystemZTTIImpl::areInlineCompatible(const Function *Caller,
+                                         const Function *Callee) const {
+  const TargetMachine &TM = getTLI()->getTargetMachine();
+
+  const FeatureBitset &CallerBits =
+      TM.getSubtargetImpl(*Caller)->getFeatureBits();
+  const FeatureBitset &CalleeBits =
+      TM.getSubtargetImpl(*Callee)->getFeatureBits();
+
+  // Check that target features from the callee are subset or
+  // equal to the caller's features.
+  return (CalleeBits == CallerBits) || (CalleeBits < CallerBits);
----------------
uweigand wrote:

As far as I can tell, '<' on the `FeatureBitset` type does *not* implement a subset test.  To test for that, you'd need something like
```
CalleeBits & CallerBits == CalleeBits
```

However, in fact allowing any subsets is semantically incorrect.  In particular, the `vector` feature changes the ABI, so we need to be careful here - we may need something similar to the AVX-512 tests in X86.  (Or, maybe, we can at least support inlining as long as both caller and callee agree on the vector feature.)

There might be other features that could be problematic as well, I need to review in more detail.

https://github.com/llvm/llvm-project/pull/132976