diff --git a/Makefile b/Makefile
index f988657f50..f2bd34e5a6 100644
--- a/Makefile
+++ b/Makefile
@@ -77,6 +77,9 @@ STREAM= \
   src/upb_msg.c \
   src/upb_glue.c \
 
+ASMCORE= \
+  src/upb_decoder_x64.asm
+
 # Parts of core that are yet to be converted.
 OTHERSRC=src/upb_encoder.c src/upb_text.c
 
@@ -114,8 +117,8 @@ LIBUPB=src/libupb.a
 LIBUPB_PIC=src/libupb_pic.a
 lib: $(LIBUPB)
 
-OBJ=$(patsubst %.c,%.o,$(SRC))
-PICOBJ=$(patsubst %.c,%.lo,$(SRC))
+OBJ=$(patsubst %.c,%.o,$(SRC)) src/upb_decoder_x64.o
+PICOBJ=$(patsubst %.c,%.lo,$(SRC)) src/upb_decoder_x64.lo
 $(LIBUPB): $(OBJ)
 	$(E) AR $(LIBUPB)
 	$(Q) ar rcs $(LIBUPB) $(OBJ)
@@ -135,12 +138,19 @@ $(LIBUPB_PIC): $(PICOBJ)
 # critical path but gets very large when -O3 is used.
 src/upb_def.o: src/upb_def.c
 	$(E) CC $<
-	$(Q) $(CC) $(CFLAGS) $(CPPFLAGS) -O0 -c -o $@ $<
+	$(Q) $(CC) $(CFLAGS) $(CPPFLAGS) -Os -c -o $@ $<
 
 src/upb_def.lo: src/upb_def.c
 	$(E) 'CC -fPIC' $<
-	$(Q) $(CC) $(CFLAGS) $(CPPFLAGS) -O0 -c -o $@ $< -fPIC
+	$(Q) $(CC) $(CFLAGS) $(CPPFLAGS) -Os -c -o $@ $< -fPIC
+
+src/upb_decoder_x64.o: src/upb_decoder_x64.asm
+	$(E) NASM $<
+	$(Q) nasm -Ox src/upb_decoder_x64.asm -o src/upb_decoder_x64.o -f macho64
 
+src/upb_decoder_x64.lo: src/upb_decoder_x64.asm
+	$(E) NASM $<
+	$(Q) nasm -Ox src/upb_decoder_x64.asm -o src/upb_decoder_x64.o -f macho64
 
 # Function to expand a wildcard pattern recursively.
 rwildcard=$(strip $(foreach d,$(wildcard $1*),$(call rwildcard,$d/,$2)$(filter $(subst *,%,$2),$d)))
diff --git a/src/upb.h b/src/upb.h
index c5b4310b4e..0698748cdc 100644
--- a/src/upb.h
+++ b/src/upb.h
@@ -29,6 +29,16 @@ extern "C" {
 #define UPB_MIN(x, y) ((x) < (y) ? (x) : (y))
 #define UPB_INDEX(base, i, m) (void*)((char*)(base) + ((i)*(m)))
 
+INLINE void nop_printf(const char *fmt, ...) {
+  (void)fmt;
+}
+
+#ifdef NDEBUG
+#define DEBUGPRINTF nop_printf
+#else
+#define DEBUGPRINTF printf
+#endif
+
 // Rounds val up to the next multiple of align.
 INLINE size_t upb_align_up(size_t val, size_t align) {
   return val % align == 0 ? val : val + align - (val % align);
diff --git a/src/upb_decoder.c b/src/upb_decoder.c
index 44674407de..0e6866baa8 100644
--- a/src/upb_decoder.c
+++ b/src/upb_decoder.c
@@ -14,7 +14,7 @@
 // If the return value is other than UPB_CONTINUE, that is what the last
 // callback returned.
 extern upb_flow_t upb_fastdecode(const char **p, const char *end,
-                                 upb_value_handler_t *value_cb, void *closure,
+                                 upb_value_handler_t value_cb, void *closure,
                                  void *table, int table_size);
 
 /* Pure Decoding **************************************************************/
@@ -131,6 +131,7 @@ INLINE int64_t upb_zzdec_64(uint64_t n) { return (n >> 1) ^ -(int64_t)(n & 1); }
 
 INLINE void upb_decoder_advance(upb_decoder *d, size_t len) {
   d->ptr += len;
+  //d->bytes_parsed_slow += len;
 }
 
 INLINE size_t upb_decoder_bufleft(upb_decoder *d) {
@@ -214,6 +215,7 @@ INLINE bool upb_decode_tag(upb_decoder *d, upb_tag *tag) {
   if ((*(p++) & 0x80) == 0) goto done;  // likely
 slow:
   // Decode a full varint starting over from ptr.
+  DEBUGPRINTF("tag was >2 bytes.\n");
   if (!upb_decode_varint_slow(d, &val)) return false;
   tag_int = upb_value_getint64(val);
   p = d->ptr;  // Trick the next line into not overwriting us.
@@ -317,6 +319,8 @@ void upb_decoder_run(upb_src *src, upb_status *status) {
   d->end = NULL;  // Force a buffer pull.
   d->submsg_end = (void*)0x1;  // But don't let end-of-message get triggered.
   d->msgdef = d->top->msgdef;
+  //d->bytes_parsed_fast = 0;
+  //d->bytes_parsed_slow = 0;
 
 // TODO: handle UPB_SKIPSUBMSG
 #define CHECK_FLOW(expr) if ((expr) == UPB_BREAK) { /*assert(!upb_ok(status));*/ goto err; }
@@ -335,11 +339,25 @@ void upb_decoder_run(upb_src *src, upb_status *status) {
       CHECK_FLOW(upb_pop(d));
     }
 
+    //const char *ptr = d->ptr;
     // Decodes as many fields as possible, updating d->ptr appropriately,
     // before falling through to the slow(er) path.
-    //CHECK_FLOW(upb_fastdecode(&d->ptr, d->end,
-    //                          d->dispatcher->top->handlers.set->value,
-    //                          d->top->handlers.closure));
+    const char *end = UPB_MIN(d->end, d->submsg_end);
+#ifdef USE_ASSEMBLY_FASTPATH
+    CHECK_FLOW(upb_fastdecode(&d->ptr, end,
+                              d->dispatcher.top->handlers.set->value,
+                              d->dispatcher.top->handlers.closure,
+                              d->top->msgdef->itof.array,
+                              d->top->msgdef->itof.array_size));
+    if (end - d->ptr < 12) {
+      DEBUGPRINTF("Off the fast path because <12 bytes of data\n");
+    } else {
+      DEBUGPRINTF("Off the fast path for some other reason.\n");
+    }
+#endif
+    //if (d->ptr > ptr) {
+    //  d->bytes_parsed_fast += (d->ptr - ptr);
+    //}
 
     // Parse/handle tag.
     upb_tag tag;
@@ -348,6 +366,7 @@ void upb_decoder_run(upb_src *src, upb_status *status) {
         // Normal end-of-file.
         upb_clearerr(status);
         CHECK_FLOW(upb_dispatch_endmsg(&d->dispatcher));
+        //DEBUGPRINTF("bytes parsed fast: %d, bytes parsed slow; %d\n", d->bytes_parsed_fast, d->bytes_parsed_slow);
         return;
       } else {
         if (status->code == UPB_EOF) {
@@ -363,8 +382,10 @@ void upb_decoder_run(upb_src *src, upb_status *status) {
     upb_value val;
     switch (tag.wire_type) {
       case UPB_WIRE_TYPE_START_GROUP:
+        DEBUGPRINTF("parsing start group\n");
         break;  // Nothing to do now, below we will push appropriately.
       case UPB_WIRE_TYPE_END_GROUP:
+        DEBUGPRINTF("parsing end group\n");
         if(d->top->end_offset != UPB_GROUP_END_OFFSET) {
           upb_seterr(status, UPB_ERROR, "Unexpected END_GROUP tag.");
           goto err;
@@ -395,6 +416,8 @@ void upb_decoder_run(upb_src *src, upb_status *status) {
     }
 
     upb_fielddef *f = e->f;
+    assert(e->field_type == f->type);
+    assert(e->native_wire_type == upb_types[f->type].native_wire_type);
 
     if (tag.wire_type != e->native_wire_type) {
       // TODO: Support packed fields.
@@ -416,11 +439,13 @@ void upb_decoder_run(upb_src *src, upb_status *status) {
     // doesn't check this, and it would slow us down, so pass for now.
     switch (e->field_type) {
       case UPB_TYPE(MESSAGE):
+        DEBUGPRINTF("parsing submessage\n");
       case UPB_TYPE(GROUP):
         CHECK_FLOW(upb_push(d, f, val, e->field_type));
         continue;  // We have no value to dispatch.
       case UPB_TYPE(STRING):
       case UPB_TYPE(BYTES):
+        DEBUGPRINTF("parsing string\n");
         CHECK(upb_decode_string(d, &val, &d->tmp));
         break;
       case UPB_TYPE(SINT32):
diff --git a/src/upb_decoder.h b/src/upb_decoder.h
index 5662c9c3fb..d0848f8a2c 100644
--- a/src/upb_decoder.h
+++ b/src/upb_decoder.h
@@ -70,6 +70,8 @@ struct _upb_decoder {
 
   // Msgdef for the current level.
   upb_msgdef *msgdef;
+
+  size_t bytes_parsed_fast, bytes_parsed_slow;
 };
 
 // A upb_decoder decodes the binary protocol buffer format, writing the data it
diff --git a/src/upb_decoder_x64.asm b/src/upb_decoder_x64.asm
new file mode 100644
index 0000000000..17d1ef7b00
--- /dev/null
+++ b/src/upb_decoder_x64.asm
@@ -0,0 +1,219 @@
+DEFAULT REL  ; Default to RIP-relative addressing instead of absolute.
+
+extern _upb_decode_varint_fast64
+
+SECTION .data
+
+; Our dispatch table; used to jump to the right handler, keyed on the field's
+; type.
+dispatch_table:
+  dq _upb_fastdecode.cant_fast_path  ; field not in table (type == 0).  (check_4).
+  dq _upb_fastdecode.fixed64  ; double
+  dq _upb_fastdecode.fixed32  ; float
+  dq _upb_fastdecode.varint   ; int64
+  dq _upb_fastdecode.varint   ; uint64
+  dq _upb_fastdecode.varint   ; int32
+  dq _upb_fastdecode.fixed64  ; fixed64
+  dq _upb_fastdecode.fixed32  ; fixed32
+  dq _upb_fastdecode.varint   ; bool
+  dq _upb_fastdecode.cant_fast_path  ; string (TODO)
+  dq _upb_fastdecode.cant_fast_path  ; group (check_6)
+  dq _upb_fastdecode.cant_fast_path  ; message
+  dq _upb_fastdecode.cant_fast_path  ; bytes (TODO)
+  dq _upb_fastdecode.varint   ; uint32
+  dq _upb_fastdecode.varint   ; enum
+  dq _upb_fastdecode.fixed32  ; sfixed32
+  dq _upb_fastdecode.fixed64  ; sfixed64
+  dq _upb_fastdecode.varint_sint32 ; sint32
+  dq _upb_fastdecode.varint_sint64 ; sint64
+
+  GLOBAL _upb_decode_fast
+
+SECTION .text
+; Register allocation.
+%define BUF rbx       ; const char *p, current buf position.
+%define END rbp       ; const char *end, where the buf ends (either submsg end or buf end)
+%define BUF_ADDR r12  ; upb_decoder *d.
+%define FIELDDEF r13  ; upb_fielddef *f, needs to be preserved across varint decoding call.
+%define CALLBACK r14
+%define CLOSURE r15
+
+; Stack layout: *tableptr, uint32_t maxfield_times_8
+%define STACK_SPACE 24      ; this value + 8 must be a multiple of 16.
+%define TABLE_SPILL [rsp]   ; our lookup table, indexed by field number.
+%define MAXFIELD_TIMES_8_SPILL [rsp+8]
+
+
+; Executing the fast path requires the following conditions:
+; - check_1: there are >=12 bytes left (<=2 byte tag and <=10 byte varint).
+; - check_2: the tag is <= 2 bytes.
+; - check_3: the field number is <= the table size
+;   (ie. it must be an array lookup, not a hash lookup).
+; - check_4: the field is known (found in the table).
+; - check_5: the wire type we read is correct for the field number,
+;   ("packed" fields are not accepted, yet.  this could be handled
+;    efficiently by doing an extra check on the "type check failed"
+;    path that goes into a tight loop if the encoding was packed).
+; - check_6: the field is not a group or a message (or string, TODO)
+;   (this could be relaxed, but due to delegation it's a bit tricky).
+; - if the value is a string, the entire string is available in
+;   the buffer, and our cached string object can be recycled.
+
+
+%macro decode_and_dispatch_ 0
+align 16
+.decode_and_dispatch:
+  ; Load a few values we'll need in a sec.
+  mov r8, TABLE_SPILL
+  mov r9d, MAXFIELD_TIMES_8_SPILL
+
+  mov rax, END
+  sub rax, BUF
+  cmp rax, 12
+  jb _upb_fastdecode.cant_fast_path ; check_1 (<12 bytes left).
+
+  ; Decode a 1 or 2-byte varint -> eax.
+  mov cl, byte [BUF]
+  lea rdi, [BUF+1]
+  movzx rax, cl    ; Need all of rax since we're doing a 64-bit lea later.
+  and eax, 0x7f
+  test cl, cl
+  jns .one_byte_tag ; Should be predictable if fields are in order.
+  movzx ecx, byte [BUF+1]
+  lea rdi, [BUF+2]
+  mov edx, ecx
+  and edx, 0x7f
+  shl edx, 7
+  or eax, edx
+  test al, al
+  js _upb_fastdecode.cant_fast_path ; check_2 (tag was >2 bytes).
+.one_byte_tag:
+  mov BUF, rdi
+
+  ; Decode tag and dispatch.
+  mov ecx, eax
+  and eax, 0x3ff8 ; eax now contains field number * 8
+  lea r11, [r8+rax*2]   ; *2 is really *16, since rax is already *8.
+  and ecx, 0x7    ; ecx now contains wire type.
+  cmp eax, r9d
+  jae _upb_fastdecode.cant_fast_path  ; check_3 (field number > table size)
+  mov FIELDDEF, [r11+8]     ; Lookup fielddef (upb_itof_ent.f)
+  movzx rdx, BYTE [r11+1]   ; Lookup field type.
+  mov rax, qword dispatch_table
+  jmp [rax+rdx*8]
+%endmacro
+
+%macro decode_and_dispatch 0
+  jmp .decode_and_dispatch
+%endmacro
+
+%macro call_callback 0
+  ; Value arg must already be in rdx when macro is called.
+  mov rdi, CLOSURE
+  mov rsi, FIELDDEF
+  mov rcx, 33      ; RAW; we could pass the correct type, or only do this in non-debug modes.
+  call CALLBACK
+  mov [BUF_ADDR], BUF
+  cmp eax, 0
+  jne .done    ; Caller requested BREAK or SKIPSUBMSG.
+%endmacro
+
+%macro check_type 1
+  cmp ecx, %1
+  jne _upb_fastdecode.cant_fast_path  ; check_5 (wire type check failed).
+%endmacro
+
+; extern upb_flow_t upb_fastdecode(const char **p, const char *end,
+;                                  upb_value_handler_t value_cb, void *closure,
+;                                  void *table, int table_size);
+align 16
+global _upb_fastdecode
+_upb_fastdecode:
+  ; We use all callee-save regs.
+  push rbx
+  push rbp
+  push r12
+  push r13
+  push r14
+  push r15
+  sub rsp, STACK_SPACE
+
+  ; Parse arguments into reg vals and stack.
+  mov BUF_ADDR, rdi
+  mov BUF, [rdi]
+  mov END, rsi
+  mov CALLBACK, rdx
+  mov CLOSURE, rcx
+  mov TABLE_SPILL, r8
+  shl r9, 3
+  mov MAXFIELD_TIMES_8_SPILL, r9
+
+  decode_and_dispatch
+
+align 16
+.varint:
+  call _upb_decode_varint_fast64  ; BUF is already in rdi.
+  test rax, rax
+  jz _upb_fastdecode.cant_fast_path  ; Varint was unterminated, slow path will handle error.
+  mov BUF, rax
+  call_callback      ; rdx already holds value.
+  decode_and_dispatch_
+
+align 16
+.fixed32:
+  mov edx, DWORD [BUF]  ; Might be unaligned, but that's ok.
+  add BUF, 4
+  call_callback
+  decode_and_dispatch
+
+align 16
+.fixed64:
+  mov rdx, QWORD [BUF]   ; Might be unaligned, but that's ok.
+  add BUF, 8
+  call_callback
+  decode_and_dispatch
+
+align 16
+.varint_sint32:
+  call _upb_decode_varint_fast64  ; BUF is already in rdi.
+  test rax, rax
+  jz _upb_fastdecode.cant_fast_path  ; Varint was unterminated, slow path will handle error.
+  mov BUF, rax
+
+  ; Perform 32-bit zig-zag decoding.
+  mov ecx, edx
+  shr edx, 1
+  and ecx, 0x1
+  neg ecx
+  xor edx, ecx
+  call_callback
+  decode_and_dispatch
+
+align 16
+.varint_sint64:
+  call _upb_decode_varint_fast64  ; BUF is already in rdi.
+  test rax, rax
+  jz _upb_fastdecode.cant_fast_path  ; Varint was unterminated, slow path will handle error.
+  mov BUF, rax
+
+  ; Perform 64-bit zig-zag decoding.
+  mov rcx, rdx
+  shr rdx, 1
+  and ecx, 0x1
+  neg rcx
+  xor rdx, rcx
+  call_callback
+  decode_and_dispatch
+
+.cant_fast_path:
+  mov rax, 0   ; UPB_CONTINUE -- continue as before.
+.done:
+  ; If coming via done, preserve the user callback's return in rax.
+  add rsp, STACK_SPACE
+  pop r15
+  pop r14
+  pop r13
+  pop r12
+  pop rbp
+  pop rbx
+  ret
diff --git a/src/upb_def.c b/src/upb_def.c
index 01143da00b..6a1e543d50 100644
--- a/src/upb_def.c
+++ b/src/upb_def.c
@@ -615,7 +615,7 @@ static upb_flow_t upb_fielddef_endmsg(void *_b) {
 
   // Field was successfully read, add it as a field of the msgdef.
   upb_msgdef *m = upb_defbuilder_top(b);
-  upb_itof_ent itof_ent = {0, upb_types[f->type].native_wire_type, f->type, f};
+  upb_itof_ent itof_ent = {0, f->type, upb_types[f->type].native_wire_type, f};
   upb_ntof_ent ntof_ent = {{f->name, 0}, f};
   upb_inttable_insert(&m->itof, f->number, &itof_ent);
   upb_strtable_insert(&m->ntof, &ntof_ent.e);
diff --git a/src/upb_def.h b/src/upb_def.h
index cca908bcab..db33fbb337 100644
--- a/src/upb_def.h
+++ b/src/upb_def.h
@@ -163,8 +163,8 @@ typedef struct _upb_msgdef {
 // Hash table entries for looking up fields by name or number.
 typedef struct {
   bool junk;
-  upb_wire_type_t native_wire_type;
   upb_fieldtype_t field_type;
+  upb_wire_type_t native_wire_type;
   upb_fielddef *f;
 } upb_itof_ent;
 typedef struct {
diff --git a/src/upb_msg.h b/src/upb_msg.h
index 8a3c63f296..3246971d80 100644
--- a/src/upb_msg.h
+++ b/src/upb_msg.h
@@ -97,11 +97,13 @@ INLINE upb_value upb_value_read(upb_valueptr ptr, upb_fieldtype_t ft) {
 
 INLINE void upb_value_write(upb_valueptr ptr, upb_value val,
                             upb_fieldtype_t ft) {
+#ifndef NDEBUG
   if (ft == UPB_VALUETYPE_ARRAY) {
     assert(val.type == UPB_VALUETYPE_ARRAY);
-  } else {
+  } else if (val.type != UPB_VALUETYPE_RAW) {
     assert(val.type == upb_types[ft].inmemory_type);
   }
+#endif
 #define CASE(t, member_name) \
   case UPB_TYPE(t): *ptr.member_name = val.val.member_name; break;
 
diff --git a/src/upb_table.c b/src/upb_table.c
index 9a6cf7a622..d2cb5dacb2 100644
--- a/src/upb_table.c
+++ b/src/upb_table.c
@@ -188,7 +188,7 @@ void upb_inttable_compact(upb_inttable *t) {
   while ((1UL << lg2_array) < largest_key) ++lg2_array;
   ++lg2_array;  // Undo the first iteration.
   size_t array_size;
-  int array_count;
+  int array_count = 0;
   while (lg2_array > 0) {
     array_size = (1 << --lg2_array);
     //printf("Considering size %d (btw, our table has %d things total)\n", array_size, upb_inttable_count(t));
diff --git a/src/upb_table.h b/src/upb_table.h
index 3903c757f6..c658a6e38d 100644
--- a/src/upb_table.h
+++ b/src/upb_table.h
@@ -123,18 +123,21 @@ INLINE void *_upb_inttable_fastlookup(upb_inttable *t, uint32_t key,
   upb_inttable_value *arrval =
       (upb_inttable_value*)UPB_INDEX(t->array, key, value_size);
   if (_upb_inttable_isarrkey(t, key)) {
-    //printf("array lookup for key %d, &val=%p, has_entry=%d\n", key, val, val->has_entry);
+    //DEBUGPRINTF("array lookup for key %d, &val=%p, has_entry=%d\n", key, val, val->has_entry);
     return (arrval->has_entry) ? arrval : NULL;
   }
   uint32_t bucket = _upb_inttable_bucket(t, key);
   upb_inttable_entry *e =
       (upb_inttable_entry*)UPB_INDEX(t->t.entries, bucket, entry_size);
-  //printf("looking in first bucket %d, entry size=%zd, addr=%p\n", bucket, entry_size, e);
+  //DEBUGPRINTF("looking in first bucket %d, entry size=%zd, addr=%p\n", bucket, entry_size, e);
   while (1) {
-    //printf("%d, %d, %d\n", e->val.has_entry, e->hdr.key, key);
-    if (e->hdr.key == key) return &e->val;
+    //DEBUGPRINTF("%d, %d, %d\n", e->val.has_entry, e->hdr.key, key);
+    if (e->hdr.key == key) {
+      DEBUGPRINTF("returning val from hash part\n");
+      return &e->val;
+    }
     if ((bucket = e->hdr.next) == UPB_END_OF_CHAIN) return NULL;
-    //printf("looking in bucket %d\n", bucket);
+    //DEBUGPRINTF("looking in bucket %d\n", bucket);
     e = (upb_inttable_entry*)UPB_INDEX(t->t.entries, bucket, entry_size);
   }
 }