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)

1 files changed, 10 insertions, 28 deletions

diff --git a/src/upb_decoder.h b/src/upb_decoder.h
index 98b1b94..af2124c 100644
--- a/src/upb_decoder.h
+++ b/src/upb_decoder.h
@@ -18,7 +18,6 @@
 
 #include <stdbool.h>
 #include <stdint.h>
-#include "upb_def.h"
 #include "upb_stream.h"
 
 #ifdef __cplusplus
@@ -27,31 +26,17 @@ extern "C" {
 
 /* upb_decoder *****************************************************************/
 
-// The decoder keeps a stack with one entry per level of recursion.
-// upb_decoder_frame is one frame of that stack.
-typedef struct {
-  upb_fielddef *f;
-  size_t end_offset;  // For groups, 0.
-} upb_decoder_frame;
-
 struct _upb_decoder {
-  // Immutable state of the decoder.
-  upb_src src;
-  upb_dispatcher dispatcher;
+  // Bytesrc from which we pull serialized data.
   upb_bytesrc *bytesrc;
 
-  // Mutable state of the decoder.
-
-  // Msgdef for the current level.
-  upb_msgdef *msgdef;
-
-  // Stack entries store the offset where the submsg ends (for groups, 0).
-  upb_decoder_frame *top, *limit;
+  // Dispatcher to which we push parsed data.
+  upb_dispatcher dispatcher;
 
   // Current input buffer.
   upb_string *buf;
 
-  // Temporary string for passing to callbacks.
+  // Temporary string for passing string data to callbacks.
   upb_string *tmp;
 
   // The offset within the overall stream represented by the *beginning* of buf.
@@ -66,12 +51,11 @@ struct _upb_decoder {
   // End of this submessage, relative to *ptr.
   const char *submsg_end;
 
+  // The closure that was passed by the caller for the top-level message.
+  void *closure;
+
   // Where we will store any errors that occur.
   upb_status *status;
-
-  // A fake fielddef for storing the msgdef for the top-level message.
-  upb_fielddef f;
-  upb_decoder_frame stack[UPB_MAX_NESTING];
 };
 
 // A upb_decoder decodes the binary protocol buffer format, writing the data it
@@ -80,18 +64,16 @@ struct _upb_decoder;
 typedef struct _upb_decoder upb_decoder;
 
 // Allocates and frees a upb_decoder, respectively.
-void upb_decoder_init(upb_decoder *d, upb_msgdef *md);
+void upb_decoder_init(upb_decoder *d, upb_handlers *handlers);
 void upb_decoder_uninit(upb_decoder *d);
 
 // Resets the internal state of an already-allocated decoder.  This puts it in a
 // state where it has not seen any data, and expects the next data to be from
 // the beginning of a new protobuf.  Parsers must be reset before they can be
 // used.  A decoder can be reset multiple times.
-void upb_decoder_reset(upb_decoder *d, upb_bytesrc *bytesrc);
+void upb_decoder_reset(upb_decoder *d, upb_bytesrc *bytesrc, void *closure);
 
-// Returns a upb_src pointer by which the decoder can be used.  The returned
-// upb_src is invalidated by upb_decoder_reset() or upb_decoder_free().
-upb_src *upb_decoder_src(upb_decoder *d);
+void upb_decoder_decode(upb_decoder *d, upb_status *status);
 
 #ifdef __cplusplus
 }  /* extern "C" */