[llvm] [TLI] Make VecDesc statically initializable (PR #211307)

[Valery Chernov via llvm-commits]

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@@ -48,23 +48,28 @@ class VecDesc {
   ElementCount VectorizationFactor;
   bool Masked;
   StringRef VABIPrefix;
-  std::optional<CallingConv::ID> CC;
+  CallingConv::ID CC;
 
 public:
   VecDesc() = delete;
-  VecDesc(StringRef ScalarFnName, StringRef VectorFnName,
-          ElementCount VectorizationFactor, bool Masked, StringRef VABIPrefix,
-          std::optional<CallingConv::ID> Conv)
+  constexpr VecDesc(StringRef ScalarFnName, StringRef VectorFnName,
+                    ElementCount VectorizationFactor, bool Masked,
+                    StringRef VABIPrefix,
+                    std::optional<CallingConv::ID> Conv = std::nullopt)
       : ScalarFnName(ScalarFnName), VectorFnName(VectorFnName),
         VectorizationFactor(VectorizationFactor), Masked(Masked),
-        VABIPrefix(VABIPrefix), CC(Conv) {}
+        VABIPrefix(VABIPrefix), CC(Conv.value_or(CallingConv::C)) {}
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vvchernov wrote:

Thank you guys for tips! I've updated to 0 / CC+1 encoding so an explicit C calling convention remains representable, and added a unit test for it

> Can you mark the tables constinit to prevent regressions?
LLVM is still on C++17, so we can't use the constinit keyword yet

https://github.com/llvm/llvm-project/pull/211307