[llvm] [IR][TLI] Cache getLibFunc() result on Function (NFC) (PR #72867)

[Dhruv Chawla via llvm-commits]

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@@ -120,6 +122,13 @@ class LLVM_EXTERNAL_VISIBILITY Function : public GlobalObject,
   void setIsNewDbgInfoFormat(bool NewVal);
 
 private:
+  friend class TargetLibraryInfoImpl;
+
+  /// Cache for TLI::getLibFunc() result without prototype validation.
+  /// -1 if uninitialized. NotLibFunc if definitely not lib func.
+  /// Otherwise may be libfunc if prototype validation passes.
+  mutable LibFunc LibFuncCache = LibFunc(-1);
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dc03 wrote:

`LibFunc(-1)` seems like a slight code smell to me, because of the implicit unsigned wrap-around and it being reused multiple times as well. I think it would be better to have a constant `UnsetLibFuncValue` or something similar.

https://github.com/llvm/llvm-project/pull/72867