Generate slightly more efficient code

src/core/ext/transport/chttp2/transport/http2_settings.c

@@ -41,25 +41,14 @@
 const uint16_t grpc_setting_id_to_wire_id[] = {1, 2, 3, 4, 5, 6, 65027};
 
 bool grpc_wire_id_to_setting_id(uint32_t wire_id, grpc_chttp2_setting_id *out) {
-  static const uint32_t r[] = {
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0};
   uint32_t i = wire_id - 1;
   uint32_t x = i % 256;
   uint32_t y = i / 256;
   uint32_t h = x;
-  if (y < GPR_ARRAY_SIZE(r)) {
-    uint32_t delta = (uint32_t)r[y];
-    h += delta;
+  switch (y) {
+    case 254:
+      h += 4;
+      break;
   }
   *out = (grpc_chttp2_setting_id)h;
   return h < GPR_ARRAY_SIZE(grpc_setting_id_to_wire_id) &&

tools/codegen/core/gen_settings_ids.py

@@ -118,25 +118,29 @@ print >>C, 'const uint16_t grpc_setting_id_to_wire_id[] = {%s};' % ','.join(
     '%d' % s for s in p.slots)
 print >>H
 print >>H, "bool grpc_wire_id_to_setting_id(uint32_t wire_id, grpc_chttp2_setting_id *out);"
+cgargs = {
+      'r': ','.join('%d' % (r if r is not None else 0) for r in p.r),
+      't': p.t,
+      'offset': abs(p.offset),
+      'offset_sign': '+' if p.offset > 0 else '-'
+  }
 print >>C, """
 bool grpc_wire_id_to_setting_id(uint32_t wire_id, grpc_chttp2_setting_id *out) {
-  static const uint32_t r[] = {%(r)s};
   uint32_t i = wire_id %(offset_sign)s %(offset)d;
   uint32_t x = i %% %(t)d;
   uint32_t y = i / %(t)d;
   uint32_t h = x;
-  if (y < GPR_ARRAY_SIZE(r)) {
-    uint32_t delta = (uint32_t)r[y];
-    h += delta;
+  switch (y) {
+""" % cgargs
+for i, r in enumerate(p.r):
+  if not r: continue
+  if r < 0: print >>C, 'case %d: h -= %d; break;' % (i, -r)
+  else: print >>C, 'case %d: h += %d; break;' % (i, r)
+print >>C, """
   }
   *out = (grpc_chttp2_setting_id)h;
   return h < GPR_ARRAY_SIZE(grpc_setting_id_to_wire_id) && grpc_setting_id_to_wire_id[h] == wire_id;
-}""" % {
-      'r': ','.join('%d' % (r if r is not None else 0) for r in p.r),
-      't': p.t,
-      'offset': abs(p.offset),
-      'offset_sign': '+' if p.offset > 0 else '-'
-  }
+}""" % cgargs
 
 print >>H, """
 typedef enum {