[clang] Switch builtin strings to use string tables (PR #118734)

[Chandler Carruth via cfe-commits]

================
@@ -68,23 +69,156 @@ enum ID {
   FirstTSBuiltin
 };
 
+// The info used to represent each builtin.
 struct Info {
-  llvm::StringLiteral Name;
-  const char *Type, *Attributes;
-  const char *Features;
+  // Rather than store pointers to the string literals describing these four
+  // aspects of builtins, we store offsets into a common string table.
+  struct StrOffsets {
+    int Name;
+    int Type;
+    int Attributes;
+    int Features;
+  } Offsets;
+
   HeaderDesc Header;
   LanguageID Langs;
 };
 
+// The storage for `N` builtins. This contains a single pointer to the string
+// table used for these builtins and an array of metadata for each builtin.
+template <size_t N> struct Storage {
+  const char *StringTable;
+
+  std::array<Info, N> Infos;
+
+  // A constexpr function to construct the storage for a a given string table in
+  // the first argument and an array in the second argument. This is *only*
+  // expected to be used at compile time, we should mark it `consteval` when
+  // available.
+  //
+  // The `Infos` array is particularly special. This function expects an array
+  // of `Info` structs, where the string offsets of each entry refer to the
+  // *sizes* of those strings rather than their offsets, and for the target
+  // string to be in the provided string table at an offset the sum of all
+  // previous string sizes. This function walks the `Infos` array computing the
+  // running sum and replacing the sizes with the actual offsets in the string
+  // table that should be used. This arrangement is designed to make it easy to
+  // expand `.def` and `.inc` files with X-macros to construct both the string
+  // table and the `Info` structs in the arguments to this function.
+  static constexpr auto Make(const char *Strings,
+                             std::array<Info, N> Infos) -> Storage<N> {
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chandlerc wrote:

Sorry, a new habit of mine and I hadn't gone through thoroughly enough to fix everywhere.

Should be fixed now throughout, I checked all the `auto` keywords in the diff.

https://github.com/llvm/llvm-project/pull/118734