[llvm] TableGen: Emit perfect hash function for runtime libcalls (PR #150192)

[Eli Friedman via llvm-commits]

================
@@ -315,8 +325,183 @@ void RuntimeLibcallEmitter::emitGetRuntimeLibcallEnum(raw_ostream &OS) const {
         "#endif\n\n";
 }
 
+// StringMap uses xxh3_64bits, truncated to uint32_t.
+static uint64_t hash(StringRef Str) {
+  return static_cast<uint32_t>(xxh3_64bits(Str));
+}
+
+static void emitHashFunction(raw_ostream &OS) {
+  OS << "static inline uint64_t hash(StringRef Str) {\n"
+        "  return static_cast<uint32_t>(xxh3_64bits(Str));\n"
+        "}\n\n";
+}
+
+/// Return the table size, maximum number of collisions for the set of hashes
+static std::pair<int, int>
+computePerfectHashParameters(ArrayRef<uint64_t> Hashes) {
+  const int SizeOverhead = 10;
+  const int NumHashes = Hashes.size();
+
+  // Index derived from hash -> number of collisions.
+  DenseMap<uint64_t, int> Table;
+
+  for (int MaxCollisions = 1;; ++MaxCollisions) {
+    for (int N = NumHashes; N < SizeOverhead * NumHashes; ++N) {
+      Table.clear();
+
+      bool NeedResize = false;
+      for (uint64_t H : Hashes) {
+        uint64_t Idx = H % static_cast<uint64_t>(N);
+        if (++Table[Idx] > MaxCollisions) {
+          // Need to resize the final table if we increased the collision count.
+          NeedResize = true;
+          break;
+        }
+      }
+
+      if (!NeedResize)
+        return {N, MaxCollisions};
+    }
+  }
+
+  llvm_unreachable("loop should terminate");
----------------
efriedma-quic wrote:

No, I mean, the point of llvm_unreachable is to mark codepaths where the compiler can't prove some aspect of the control flow, like a switch statement without a default.  It's useless to write `if (x) return 1; else return 2; llvm_unreachable("else wasn't an else");`.

https://github.com/llvm/llvm-project/pull/150192