diff --git a/mozilla/js/src/jsemit.c b/mozilla/js/src/jsemit.c
index 01e9d59923c..162d7831858 100644
--- a/mozilla/js/src/jsemit.c
+++ b/mozilla/js/src/jsemit.c
@@ -4162,8 +4162,8 @@ js_AddToSrcNoteDelta(JSContext *cx, JSCodeGenerator *cg, jssrcnote *sn,
     intN index;
 
     /*
-     * Called only from OptimizeSpanDeps to add to main script note deltas,
-     * and only by a small positive amount.
+     * Called only from OptimizeSpanDeps and js_FinishTakingSrcNotes to add to
+     * main script note deltas, and only by a small positive amount.
      */
     JS_ASSERT(cg->current == &cg->main);
     JS_ASSERT((unsigned) delta < (unsigned) SN_XDELTA_LIMIT);
@@ -4304,6 +4304,7 @@ JSBool
 js_FinishTakingSrcNotes(JSContext *cx, JSCodeGenerator *cg, jssrcnote *notes)
 {
     uintN prologCount, mainCount, totalCount;
+    ptrdiff_t offset, delta;
 
     JS_ASSERT(cg->current == &cg->main);
 
@@ -4314,6 +4315,21 @@ js_FinishTakingSrcNotes(JSContext *cx, JSCodeGenerator *cg, jssrcnote *notes)
             return JS_FALSE;
         prologCount = cg->prolog.noteCount;
         CG_SWITCH_TO_MAIN(cg);
+    } else {
+        /*
+         * Either no prolog srcnotes, or no line number change over prolog.
+         * We don't need a SRC_SETLINE, but we may need to adjust the offset
+         * of the first main note, by adding to its delta and possibly even
+         * prepending SRC_XDELTA notes to it to account for prolog bytes.
+         */
+        offset = CG_PROLOG_OFFSET(cg) - cg->prolog.lastNoteOffset;
+        JS_ASSERT(offset >= 0);
+        while (offset > 0) {
+            delta = JS_MIN(offset, SN_XDELTA_MASK);
+            if (!js_AddToSrcNoteDelta(cx, cg, cg->main.notes, delta))
+                return JS_FALSE;
+            offset -= delta;
+        }
     }
 
     mainCount = cg->main.noteCount;
diff --git a/mozilla/js/src/jsemit.h b/mozilla/js/src/jsemit.h
index b00874d1787..523cce96802 100644
--- a/mozilla/js/src/jsemit.h
+++ b/mozilla/js/src/jsemit.h
@@ -480,16 +480,23 @@ js_SetSrcNoteOffset(JSContext *cx, JSCodeGenerator *cg, uintN index,
  */
 #define CG_COUNT_FINAL_SRCNOTES(cg, cnt)                                      \
     JS_BEGIN_MACRO                                                            \
-        cnt = (cg)->main.noteCount + 1;                                       \
-        if ((cg)->prolog.noteCount) {                                         \
-            cnt += (cg)->prolog.noteCount;                                    \
-            if ((cg)->prolog.currentLine != (cg)->firstLine) {                \
-                ptrdiff_t diff_ = CG_PROLOG_OFFSET(cg) -                      \
-                                  (cg)->prolog.lastNoteOffset;                \
-                if (diff_ > SN_DELTA_MASK)                                    \
-                    cnt += JS_HOWMANY(diff_ - SN_DELTA_MASK, SN_XDELTA_MASK); \
-                cnt += 2 + (((cg)->firstLine > SN_3BYTE_OFFSET_MASK) << 1);   \
+        ptrdiff_t diff_ = CG_PROLOG_OFFSET(cg) - (cg)->prolog.lastNoteOffset; \
+        cnt = (cg)->prolog.noteCount + (cg)->main.noteCount + 1;              \
+        if ((cg)->prolog.noteCount &&                                         \
+            (cg)->prolog.currentLine != (cg)->firstLine) {                    \
+            if (diff_ > SN_DELTA_MASK)                                        \
+                cnt += JS_HOWMANY(diff_ - SN_DELTA_MASK, SN_XDELTA_MASK);     \
+            cnt += 2 + (((cg)->firstLine > SN_3BYTE_OFFSET_MASK) << 1);       \
+        } else {                                                              \
+            if (cg->main.noteCount) {                                         \
+                jssrcnote *sn_ = (cg)->main.notes;                            \
+                ptrdiff_t delta_ = SN_IS_XDELTA(sn_)                          \
+                                   ? SN_XDELTA_MASK - (*sn_ & SN_XDELTA_MASK) \
+                                   : SN_DELTA_MASK - (*sn_ & SN_DELTA_MASK);  \
+                diff_ -= JS_MIN(delta_, diff_);                               \
             }                                                                 \
+            if (diff_)                                                        \
+                cnt += JS_HOWMANY(diff_, SN_XDELTA_MASK);                     \
         }                                                                     \
     JS_END_MACRO