[llvm] Allow SMLoc to be used in constexpr context

[wrengr]

Summary

Since StringRef::empty can be used in constexpr context, it seems reasonable that SMLoc::isValid should be too. The default-ctor and operator== are made constexpr for consistency.

In particular, the constexpr keyword is needed for silencing warnings on Windows (whereas Linux allows constexpr usage without the keyword).

[aartbik]

LGTM for the sparse change, but leaving the core LOC stuff to the experts ;-)

[jpienaar]

This seems reasonable to me SMLoc side.

llvm/include/llvm/Support/SMLoc.h

 
 namespace llvm {
 
 /// Represents a location in source code.
 class SMLoc {
   const char *Ptr = nullptr;
 
 public:
-  SMLoc() = default;
+  constexpr SMLoc() = default;
 
-  bool isValid() const { return Ptr != nullptr; }
+  constexpr bool isValid() const { return Ptr != nullptr; }
 
-  bool operator==(const SMLoc &RHS) const { return RHS.Ptr == Ptr; }
-  bool operator!=(const SMLoc &RHS) const { return RHS.Ptr != Ptr; }
+  constexpr bool operator==(const SMLoc &RHS) const { return RHS.Ptr == Ptr; }
+  constexpr bool operator!=(const SMLoc &RHS) const { return RHS.Ptr != Ptr; }
 
-  const char *getPointer() const { return Ptr; }
+  constexpr const char *getPointer() const { return Ptr; }
 
   static SMLoc getFromPointer(const char *Ptr) {
     SMLoc L;
     L.Ptr = Ptr;
     return L;
   }
 };
 

mlir/lib/Dialect/SparseTensor/IR/Detail/Var.h

private:
   std::optional<Var::Num> num; // The unique `Var`-identifier (if resolved).
   VarKind kind;                // The kind of variable.
 
 public:
   constexpr VarInfo(ID id, StringRef name, llvm::SMLoc loc, VarKind vk,
                     std::optional<Var::Num> n = {})
       : name(name), loc(loc), id(id), num(n), kind(vk) {
     assert(!name.empty() && "null StringRef");
+    assert(loc.isValid() && "null SMLoc");
     assert(isWF(vk) && "unknown VarKind");
     assert((!n || Var::isWF_Num(*n)) && "Var::Num is too large");
-    // NOTE TO Wren: windows did not like loc.isValid constexpr
   }
 
   constexpr StringRef getName() const { return name; }
   constexpr llvm::SMLoc getLoc() const { return loc; }
   Location getLocation(AsmParser &parser) const {
     return parser.getEncodedSourceLoc(loc);
   }
   constexpr ID getID() const { return id; }