[clang] [llvm] [RFC][CodeGen] Add generic target feature checks for intrinsics (PR #201470)

[Matt Arsenault via cfe-commits]

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@@ -62,6 +63,18 @@ LLVM_ABI StringRef getName(ID id);
 /// overloading, such as "llvm.ssa.copy".
 LLVM_ABI StringRef getBaseName(ID id);
 
+/// \returns the target feature expression required by an intrinsic.
+LLVM_ABI StringRef getRequiredTargetFeatures(ID id);
+
+/// Sentinel value for an intrinsic's required-target-features string. When an
+/// intrinsic's \c TargetFeatures is set to this keyword, whether the intrinsic
+/// is supported is decided by a target-specific hook
+/// (TargetSubtargetInfo::isIntrinsicSupportedByTarget) rather than by
+/// evaluating a subtarget feature expression. Use it when support depends on
+/// more than the subtarget features, such as a particular overload/mangling or
+/// argument combination accepted by some targets but not others.
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arsenm wrote:

I think for the subtarget specific operand rules, those should be restricted to immarg. And probably shouldn't be enforced by the verifier, either treat as UB or codegen error 

https://github.com/llvm/llvm-project/pull/201470