Everything passes except 4 conformance tests.

upb/decode_internal.h

@@ -142,10 +142,11 @@ bool decode_isdone(upb_decstate *d, const char **ptr) {
   }
 }
 
+#include <stdio.h>
 UPB_INLINE
 const char *fastdecode_tagdispatch(upb_decstate *d, const char *ptr,
                                     upb_msg *msg, intptr_t table,
-                                    uint64_t hasbits, uint32_t tag) {
+                                    uint64_t hasbits, uint64_t tag) {
   const upb_msglayout *table_p = decode_totablep(table);
   uint8_t mask = table;
   uint64_t data;
@@ -153,7 +154,9 @@ const char *fastdecode_tagdispatch(upb_decstate *d, const char *ptr,
   UPB_ASSUME((idx & 7) == 0);
   idx >>= 3;
   data = table_p->fasttable[idx].field_data ^ tag;
-  return table_p->fasttable[idx].field_parser(d, ptr, msg, table, hasbits, data);
+  //fprintf(stderr, "Dispatch at: %p\n", ptr);
+  UPB_MUSTTAIL return table_p->fasttable[idx].field_parser(d, ptr, msg, table,
+                                                           hasbits, data);
 }
 
 UPB_INLINE uint32_t fastdecode_loadtag(const char* ptr) {

upb/port_def.inc

@@ -167,8 +167,10 @@
 
 /* Configure whether fasttable is switched on or not. *************************/
 
-#if (defined(__x86_64__) || defined(__aarch64__)) && defined(__GNUC__)
+#if (defined(__x86_64__) || defined(__aarch64__)) && defined(__clang__) && \
+    __has_attribute(musttail)
 #define UPB_FASTTABLE_SUPPORTED 1
+#define UPB_MUSTTAIL __attribute__((musttail))
 #else
 #define UPB_FASTTABLE_SUPPORTED 0
 #endif
@@ -178,7 +180,7 @@
  * for example for testing or benchmarking. */
 #if defined(UPB_ENABLE_FASTTABLE)
 #if !UPB_FASTTABLE_SUPPORTED
-#error fasttable is x86-64 + Clang/GCC only
+#error fasttable is x86-64/ARM64 only and requires a recent Clang that supports __attribute__((musttail))
 #endif
 #define UPB_FASTTABLE 1
 /* Define UPB_TRY_ENABLE_FASTTABLE to use fasttable if possible.

upb/port_undef.inc

@@ -52,6 +52,7 @@
 #undef UPB_SETJMP
 #undef UPB_LONGJMP
 #undef UPB_PTRADD
+#undef UPB_MUSTTAIL
 #undef UPB_FASTTABLE_SUPPORTED
 #undef UPB_FASTTABLE
 #undef UPB_FASTTABLE_INIT