upb_stream: all callbacks registered ahead-of-time.

This is a significant change to the upb_stream
protocol, and should hopefully be the last
significant change.

All callbacks are now registered ahead-of-time
instead of having delegated callbacks registered
at runtime, which makes it much easier to
aggressively optimize ahead-of-time (like with a
JIT).

Other impacts of this change:

- You no longer need to have loaded descriptor.proto
  as a upb_def to load other descriptors!  This means
  the special-case code we used for bootstrapping is
  no longer necessary, and we no longer need to link
  the descriptor for descriptor.proto into upb.

- A client can now register any upb_value as what
  will be delivered to their value callback, not
  just a upb_fielddef*.  This should allow for other
  clients to get more bang out of the streaming
  decoder.

This change unfortunately causes a bit of a performance
regression -- I think largely due to highly
suboptimal code that GCC generates when structs
are returned by value.  See:
  http://blog.reverberate.org/2011/03/19/when-a-compilers-slow-code-actually-bites-you/

On the other hand, once we have a JIT this should
no longer matter.

Performance numbers:

plain.parsestream_googlemessage1.upb_table: 374 -> 396 (5.88)
plain.parsestream_googlemessage2.upb_table: 616 -> 449 (-27.11)
plain.parsetostruct_googlemessage1.upb_table_byref: 268 -> 269 (0.37)
plain.parsetostruct_googlemessage1.upb_table_byval: 215 -> 204 (-5.12)
plain.parsetostruct_googlemessage2.upb_table_byref: 307 -> 281 (-8.47)
plain.parsetostruct_googlemessage2.upb_table_byval: 297 -> 272 (-8.42)
omitfp.parsestream_googlemessage1.upb_table: 423 -> 410 (-3.07)
omitfp.parsestream_googlemessage2.upb_table: 679 -> 483 (-28.87)
omitfp.parsetostruct_googlemessage1.upb_table_byref: 287 -> 282 (-1.74)
omitfp.parsetostruct_googlemessage1.upb_table_byval: 226 -> 219 (-3.10)
omitfp.parsetostruct_googlemessage2.upb_table_byref: 315 -> 298 (-5.40)
omitfp.parsetostruct_googlemessage2.upb_table_byval: 297 -> 287 (-3.37)

2 files changed, 13 insertions, 22 deletions

diff --git a/benchmarks/parsestream.upb_table.c b/benchmarks/parsestream.upb_table.c
index 85c9ff9..89649e6 100644
--- a/benchmarks/parsestream.upb_table.c
+++ b/benchmarks/parsestream.upb_table.c
@@ -47,11 +47,12 @@ static bool initialize()
     fprintf(stderr, "Error reading " MESSAGE_FILE "\n");
     return false;
   }
-  upb_decoder_init(&decoder, def);
+
+  upb_handlers_init(&handlers, def);
+  // Cause all messages to be read, but do nothing when they are.
+  upb_register_all(&handlers, NULL, NULL, NULL, NULL, NULL, NULL);
+  upb_decoder_init(&decoder, &handlers);
   upb_stringsrc_init(&stringsrc);
-  upb_handlers_init(&handlers);
-  static upb_handlerset handlerset = {};  // Empty handlers.
-  upb_register_handlerset(&handlers, &handlerset);
   return true;
 }
 
@@ -61,6 +62,7 @@ static void cleanup()
   upb_def_unref(UPB_UPCAST(def));
   upb_decoder_uninit(&decoder);
   upb_stringsrc_uninit(&stringsrc);
+  upb_handlers_uninit(&handlers);
 }
 
 static size_t run(int i)
@@ -68,10 +70,8 @@ static size_t run(int i)
   (void)i;
   upb_status status = UPB_STATUS_INIT;
   upb_stringsrc_reset(&stringsrc, input_str);
-  upb_decoder_reset(&decoder, upb_stringsrc_bytesrc(&stringsrc));
-  upb_src *src = upb_decoder_src(&decoder);
-  upb_src_sethandlers(src, &handlers);
-  upb_src_run(src, &status);
+  upb_decoder_reset(&decoder, upb_stringsrc_bytesrc(&stringsrc), NULL);
+  upb_decoder_decode(&decoder, &status);
   if(!upb_ok(&status)) goto err;
   return upb_string_len(input_str);
 
diff --git a/benchmarks/parsetostruct.upb_table.c b/benchmarks/parsetostruct.upb_table.c
index dfdd5b2..e778bc5 100644
--- a/benchmarks/parsetostruct.upb_table.c
+++ b/benchmarks/parsetostruct.upb_table.c
@@ -13,7 +13,6 @@ static upb_msg *msg;
 static upb_stringsrc strsrc;
 static upb_decoder d;
 static upb_handlers h;
-static upb_msgpopulator p;
 
 static bool initialize()
 {
@@ -54,9 +53,9 @@ static bool initialize()
   msg = upb_msg_new(def);
 
   upb_stringsrc_init(&strsrc);
-  upb_decoder_init(&d, def);
-  upb_msgpopulator_init(&p);
-  upb_handlers_init(&h);
+  upb_handlers_init(&h, def);
+  upb_msg_regdhandlers(&h);
+  upb_decoder_init(&d, &h);
 
   if (!BYREF) {
     // Pretend the input string is stack-allocated, which will force its data
@@ -79,7 +78,6 @@ static void cleanup()
   upb_def_unref(UPB_UPCAST(def));
   upb_stringsrc_uninit(&strsrc);
   upb_decoder_uninit(&d);
-  upb_msgpopulator_uninit(&p);
   upb_handlers_uninit(&h);
 }
 
@@ -89,16 +87,9 @@ static size_t run(int i)
   upb_status status = UPB_STATUS_INIT;
   upb_msg_clear(msg, def);
   upb_stringsrc_reset(&strsrc, input_str);
-  upb_decoder_reset(&d, upb_stringsrc_bytesrc(&strsrc));
-  upb_msgpopulator_reset(&p, msg, def);
-  upb_handlers_init(&h);
-  upb_msgpopulator_register_handlers(&p, &h);
-  upb_src *src = upb_decoder_src(&d);
-  upb_src_sethandlers(src, &h);
-
-  upb_src_run(src, &status);
+  upb_decoder_reset(&d, upb_stringsrc_bytesrc(&strsrc), msg);
+  upb_decoder_decode(&d, &status);
   if(!upb_ok(&status)) goto err;
-  upb_status_uninit(&status);
   return upb_string_len(input_str);
 
 err: