diff --git a/benchmarks/parsestream.upb.c b/benchmarks/parsestream.upb.c
index 37ccb42dcb..80f94440d2 100644
--- a/benchmarks/parsestream.upb.c
+++ b/benchmarks/parsestream.upb.c
@@ -75,7 +75,8 @@ static size_t run(int i)
   (void)i;
   upb_status status = UPB_STATUS_INIT;
   upb_stringsrc_reset(&stringsrc, input_str, input_len);
-  upb_decoder_reset(&decoder, upb_stringsrc_bytesrc(&stringsrc), 0, UINT64_MAX, NULL);
+  upb_decoder_reset(&decoder, upb_stringsrc_bytesrc(&stringsrc),
+                    0, UPB_NONDELIMITED, NULL);
   upb_decoder_decode(&decoder, &status);
   if(!upb_ok(&status)) goto err;
   return input_len;
diff --git a/benchmarks/parsetoproto2.upb.cc b/benchmarks/parsetoproto2.upb.cc
index af3e1f236a..74d068091e 100644
--- a/benchmarks/parsetoproto2.upb.cc
+++ b/benchmarks/parsetoproto2.upb.cc
@@ -283,7 +283,8 @@ static size_t run(int i)
   (void)i;
   upb_status status = UPB_STATUS_INIT;
   msg[i % NUM_MESSAGES].Clear();
-  upb_decoder_reset(&d, upb_stringsrc_bytesrc(&strsrc), 0, UINT64_MAX, &msg[i % NUM_MESSAGES]);
+  upb_decoder_reset(&d, upb_stringsrc_bytesrc(&strsrc),
+                    0, UPB_NONDELIMITED, &msg[i % NUM_MESSAGES]);
   upb_decoder_decode(&d, &status);
   if(!upb_ok(&status)) goto err;
   return len;
diff --git a/benchmarks/parsetostruct.upb.c b/benchmarks/parsetostruct.upb.c
index 64a4d35c36..f44b6502ae 100644
--- a/benchmarks/parsetostruct.upb.c
+++ b/benchmarks/parsetostruct.upb.c
@@ -69,7 +69,8 @@ static size_t run(int i)
   upb_status status = UPB_STATUS_INIT;
   i %= NUM_MESSAGES;
   upb_msg_clear(msg[i], def);
-  upb_decoder_reset(&d, upb_stringsrc_bytesrc(&strsrc), 0, UINT64_MAX, msg[i]);
+  upb_decoder_reset(&d, upb_stringsrc_bytesrc(&strsrc),
+                    0, UPB_NONDELIMITED, msg[i]);
   upb_decoder_decode(&d, &status);
   if(!upb_ok(&status)) goto err;
   return len;
diff --git a/tests/test_decoder.c b/tests/test_decoder.c
index 88b87416f8..e3fc1ead68 100644
--- a/tests/test_decoder.c
+++ b/tests/test_decoder.c
@@ -53,7 +53,7 @@ int main(int argc, char *argv[]) {
 
   upb_decoder d;
   upb_decoder_initforhandlers(&d, handlers);
-  upb_decoder_reset(&d, upb_stdio_bytesrc(&in), 0, UINT64_MAX, p);
+  upb_decoder_reset(&d, upb_stdio_bytesrc(&in), 0, UPB_NONDELIMITED, p);
 
   upb_status_clear(&status);
   upb_decoder_decode(&d, &status);
diff --git a/upb/handlers.c b/upb/handlers.c
index 05300c0cc1..d02a32a05b 100644
--- a/upb/handlers.c
+++ b/upb/handlers.c
@@ -16,8 +16,10 @@ static upb_mhandlers *upb_mhandlers_new() {
   upb_inttable_init(&m->fieldtab, 8, sizeof(upb_fhandlers));
   m->startmsg = NULL;
   m->endmsg = NULL;
-  m->tablearray = NULL;
   m->is_group = false;
+#ifdef UPB_USE_JIT_X64
+  m->tablearray = NULL;
+#endif
   return m;
 }
 
@@ -29,7 +31,11 @@ static upb_fhandlers *_upb_mhandlers_newfhandlers(upb_mhandlers *m, uint32_t n,
   if (f) abort();
   upb_fhandlers new_f = {false, type, repeated,
       repeated && upb_isprimitivetype(type), UPB_ATOMIC_INIT(0),
-      n, m, NULL, UPB_NO_VALUE, NULL, NULL, NULL, NULL, NULL, 0, 0, 0, NULL};
+      n, m, NULL, UPB_NO_VALUE, NULL, NULL, NULL, NULL, NULL,
+#ifdef UPB_USE_JIT_X64
+      0, 0, 0,
+#endif
+      NULL};
   upb_inttable_insert(&m->fieldtab, tag, &new_f);
   f = upb_inttable_lookup(&m->fieldtab, tag);
   assert(f);
@@ -77,7 +83,9 @@ void upb_handlers_unref(upb_handlers *h) {
     for (int i = 0; i < h->msgs_len; i++) {
       upb_mhandlers *mh = h->msgs[i];
       upb_inttable_free(&mh->fieldtab);
+#ifdef UPB_USE_JIT_X64
       free(mh->tablearray);
+#endif
       free(mh);
     }
     free(h->msgs);
@@ -154,7 +162,11 @@ static upb_fhandlers toplevel_f = {
 #else
   {{0}, -1},
 #endif
-  NULL, NULL, NULL, NULL, NULL, 0, 0, 0, NULL};
+  NULL, NULL, NULL, NULL, NULL,
+#ifdef UPB_USE_JIT_X64
+  0, 0, 0,
+#endif
+  NULL};
 
 void upb_dispatcher_init(upb_dispatcher *d, upb_handlers *h,
                          upb_skip_handler *skip, upb_exit_handler *exit,
diff --git a/upb/handlers.h b/upb/handlers.h
index b9e120aaa1..db28705874 100644
--- a/upb/handlers.h
+++ b/upb/handlers.h
@@ -149,9 +149,11 @@ typedef struct _upb_fieldent {
   upb_endfield_handler *endsubmsg;
   upb_startfield_handler *startseq;
   upb_endfield_handler *endseq;
+#ifdef UPB_USE_JIT_X64
   uint32_t jit_pclabel;
   uint32_t jit_pclabel_notypecheck;
   uint32_t jit_submsg_done_pclabel;
+#endif
   void (*decode)(struct _upb_decoder *d, struct _upb_fieldent *f);
 } upb_fhandlers;
 
@@ -184,16 +186,18 @@ typedef struct _upb_mhandlers {
   upb_startmsg_handler *startmsg;
   upb_endmsg_handler *endmsg;
   upb_inttable fieldtab;  // Maps field number -> upb_fhandlers.
+  bool is_group;
+#ifdef UPB_USE_JIT_X64
   uint32_t jit_startmsg_pclabel;
   uint32_t jit_endofbuf_pclabel;
   uint32_t jit_endofmsg_pclabel;
   uint32_t jit_unknownfield_pclabel;
-  bool is_group;
   int32_t jit_parent_field_done_pclabel;
   uint32_t max_field_number;
   // Currently keyed on field number.  Could also try keying it
   // on encoded or decoded tag, or on encoded field number.
   void **tablearray;
+#endif
 } upb_mhandlers;
 
 // mhandlers are created as part of a upb_handlers instance, but can be ref'd
diff --git a/upb/pb/decoder.c b/upb/pb/decoder.c
index df74b4815f..20838498c1 100644
--- a/upb/pb/decoder.c
+++ b/upb/pb/decoder.c
@@ -13,10 +13,6 @@
 #include "upb/pb/decoder.h"
 #include "upb/pb/varint.h"
 
-// Used for frames that have no specific end offset: groups, repeated primitive
-// fields inside groups, and the top-level message.
-#define UPB_NONDELIMITED UINT32_MAX
-
 #ifdef UPB_USE_JIT_X64
 #define Dst_DECL upb_decoder *d
 #define Dst_REF (d->dynasm)
@@ -33,7 +29,11 @@
 #define FORCEINLINE static __attribute__((always_inline))
 #define NOINLINE static __attribute__((noinline))
 
-static void upb_decoder_exit(upb_decoder *d) { siglongjmp(d->exitjmp, 1); }
+static void upb_decoder_exit(upb_decoder *d) {
+  // If/when we support resumable decoding, we would want to back our progress
+  // up to completed_ptr and possibly get a previous buffer.
+  siglongjmp(d->exitjmp, 1);
+}
 static void upb_decoder_exit2(void *_d) {
   upb_decoder *d = _d;
   upb_decoder_exit(d);
@@ -43,7 +43,12 @@ static void upb_decoder_abort(upb_decoder *d, const char *msg) {
   upb_decoder_exit(d);
 }
 
-/* Decoding/Buffering of wire types *******************************************/
+/* Buffering ******************************************************************/
+
+// We operate on one buffer at a time, which may be a subset of the bytesrc
+// region we have ref'd.  When data for the buffer is gone we pull the next
+// one.  When we've committed our progress we release our ref on any previous
+// buffers' regions.
 
 static size_t upb_decoder_bufleft(upb_decoder *d) { return d->end - d->ptr; }
 static void upb_decoder_advance(upb_decoder *d, size_t len) {
@@ -61,12 +66,11 @@ static void upb_decoder_setmsgend(upb_decoder *d) {
   upb_dispatcher_frame *f = d->dispatcher.top;
   size_t delimlen = f->end_ofs - d->bufstart_ofs;
   size_t buflen = d->end - d->buf;
-  if (f->end_ofs != UINT64_MAX && delimlen <= buflen) {
-    d->delim_end = (uintptr_t)(d->buf + delimlen);
+  if (f->end_ofs != UPB_NONDELIMITED && delimlen <= buflen) {
+    // Delimited message ends in this buffer.
+    d->delim_end = d->buf + delimlen;
   } else {
-    // Buffers must not run up against the end of memory.
-    assert((uintptr_t)d->end < UINTPTR_MAX);
-    d->delim_end = UINTPTR_MAX;
+    d->delim_end = NULL;
   }
 }
 
@@ -111,6 +115,9 @@ void upb_decoder_commit(upb_decoder *d) {
   }
 }
 
+
+/* Decoding of wire types *****************************************************/
+
 NOINLINE uint64_t upb_decode_varint_slow(upb_decoder *d) {
   uint8_t byte = 0x80;
   uint64_t u64 = 0;
@@ -150,7 +157,8 @@ done:
 
 FORCEINLINE bool upb_trydecode_varint32(upb_decoder *d, uint32_t *val) {
   if (upb_decoder_bufleft(d) == 0) {
-    // Check for our two normal end-of-message conditions.
+    // Check for our two successful end-of-message conditions
+    // (user-specified EOM and bytesrc EOF).
     if (d->bufend_ofs == d->end_ofs) return false;
     if (!upb_trypullbuf(d)) return false;
   }
@@ -286,8 +294,8 @@ static void upb_decode_MESSAGE(upb_decoder *d, upb_fhandlers *f) {
 /* The main decoding loop *****************************************************/
 
 static void upb_decoder_checkdelim(upb_decoder *d) {
-  while ((uintptr_t)d->ptr >= d->delim_end) {
-    if ((uintptr_t)d->ptr > d->delim_end)
+  while (d->delim_end != NULL && d->ptr >= d->delim_end) {
+    if (d->ptr > d->delim_end)
       upb_decoder_abort(d, "Bad submessage end");
 
     if (d->dispatcher.top->is_sequence) {
@@ -460,7 +468,7 @@ void upb_decoder_reset(upb_decoder *d, upb_bytesrc *bytesrc, uint64_t start_ofs,
 #ifdef UPB_USE_JIT_X64
   d->jit_end = NULL;
 #endif
-  d->delim_end = UINTPTR_MAX;  // But don't let end-of-message get triggered.
+  d->delim_end = NULL;  // But don't let end-of-message get triggered.
   d->strref.bytesrc = bytesrc;
 }
 
diff --git a/upb/pb/decoder.h b/upb/pb/decoder.h
index 398135903b..9a20d7664b 100644
--- a/upb/pb/decoder.h
+++ b/upb/pb/decoder.h
@@ -50,9 +50,8 @@ typedef struct _upb_decoder {
   // UPB_TRYAGAIN (or in the future, UPB_SUSPEND).
   const char *completed_ptr;
 
-  // End of the delimited region, relative to ptr, or UINTPTR_MAX if not in
-  // this buf.
-  uintptr_t delim_end;
+  // End of the delimited region, relative to ptr, or NULL if not in this buf.
+  const char *delim_end;
 
 #ifdef UPB_USE_JIT_X64
   // For JIT, which doesn't do bounds checks in the middle of parsing a field.
@@ -69,6 +68,10 @@ typedef struct _upb_decoder {
   sigjmp_buf exitjmp;
 } upb_decoder;
 
+// Used for frames that have no specific end offset: groups, repeated primitive
+// fields inside groups, and the top-level message.
+#define UPB_NONDELIMITED UINT32_MAX
+
 // Initializes/uninitializes a decoder for calling into the given handlers
 // or to write into the given msgdef, given its accessors).  Takes a ref
 // on the handlers or msgdef.