diff --git a/intl/icu/source/common/utext.cpp b/intl/icu/source/common/utext.cpp
index a89e7a62b..994b5dac5 100644
--- a/intl/icu/source/common/utext.cpp
+++ b/intl/icu/source/common/utext.cpp
@@ -847,11 +847,18 @@ U_CDECL_END
 //------------------------------------------------------------------------------
 
 // Chunk size.
-//     Must be less than 85, because of byte mapping from UChar indexes to native indexes.
-//     Worst case is three native bytes to one UChar.  (Supplemenaries are 4 native bytes
-//     to two UChars.)
+//     Must be less than 42  (256/6), because of byte mapping from UChar indexes to native indexes.
+//     Worst case there are six UTF-8 bytes per UChar.
+//         obsolete 6 byte form fd + 5 trails maps to fffd
+//         obsolete 5 byte form fc + 4 trails maps to fffd
+//         non-shortest 4 byte forms maps to fffd
+//         normal supplementaries map to a pair of utf-16, two utf8 bytes per utf-16 unit
+//     mapToUChars array size must allow for the worst case, 6.
+//     This could be brought down to 4, by treating fd and fc as pure illegal,
+//     rather than obsolete lead bytes. But that is not compatible with the utf-8 access macros.
 //
 enum { UTF8_TEXT_CHUNK_SIZE=32 };
+enum { SIZEOF_MAPTOUCHARS=UTF8_TEXT_CHUNK_SIZE*6+6 };
 
 //
 // UTF8Buf  Two of these structs will be set up in the UText's extra allocated space.
@@ -889,7 +896,7 @@ struct UTF8Buf {
                                                      //  Requires two extra slots,
                                                      //    one for a supplementary starting in the last normal position,
                                                      //    and one for an entry for the buffer limit position.
-    uint8_t   mapToUChars[UTF8_TEXT_CHUNK_SIZE*3+6]; // Map native offset from bufNativeStart to
+    uint8_t   mapToUChars[SIZEOF_MAPTOUCHARS]; // Map native offset from bufNativeStart to
                                                      //   correspoding offset in filled part of buf.
     int32_t   align;
 };
@@ -1032,6 +1039,7 @@ utf8TextAccess(UText *ut, int64_t index, UBool forward) {
             // Requested index is in this buffer.
             u8b = (UTF8Buf *)ut->p;   // the current buffer
             mapIndex = ix - u8b->toUCharsMapStart;
+            U_ASSERT(mapIndex < (int32_t)SIZEOF_MAPTOUCHARS);
             ut->chunkOffset = u8b->mapToUChars[mapIndex] - u8b->bufStartIdx;
             return TRUE;
 
@@ -1298,6 +1306,10 @@ fillReverse:
         // Can only do this if the incoming index is somewhere in the interior of the string.
         //   If index is at the end, there is no character there to look at.
         if (ix != ut->b) {
+            // Note: this function will only move the index back if it is on a trail byte
+            //       and there is a preceding lead byte and the sequence from the lead 
+            //       through this trail could be part of a valid UTF-8 sequence
+            //       Otherwise the index remains unchanged.
             U8_SET_CP_START(s8, 0, ix);
         }
 
@@ -1311,7 +1323,10 @@ fillReverse:
         UChar   *buf = u8b->buf;
         uint8_t *mapToNative = u8b->mapToNative;
         uint8_t *mapToUChars = u8b->mapToUChars;
-        int32_t  toUCharsMapStart = ix - (UTF8_TEXT_CHUNK_SIZE*3 + 1);
+        int32_t  toUCharsMapStart = ix - SIZEOF_MAPTOUCHARS + 1;
+        // Note that toUCharsMapStart can be negative. Happens when the remaining
+        // text from current position to the beginning is less than the buffer size.
+        // + 1 because mapToUChars must have a slot at the end for the bufNativeLimit entry.
         int32_t  destIx = UTF8_TEXT_CHUNK_SIZE+2;   // Start in the overflow region
                                                     //   at end of buffer to leave room
                                                     //   for a surrogate pair at the
@@ -1338,6 +1353,7 @@ fillReverse:
             if (c<0x80) {
                 // Special case ASCII range for speed.
                 buf[destIx] = (UChar)c;
+                U_ASSERT(toUCharsMapStart <= srcIx);
                 mapToUChars[srcIx - toUCharsMapStart] = (uint8_t)destIx;
                 mapToNative[destIx] = (uint8_t)(srcIx - toUCharsMapStart);
             } else {
@@ -1367,6 +1383,7 @@ fillReverse:
                 do {
                     mapToUChars[sIx-- - toUCharsMapStart] = (uint8_t)destIx;
                 } while (sIx >= srcIx);
+                U_ASSERT(toUCharsMapStart <= (srcIx+1));
 
                 // Set native indexing limit to be the current position.
                 //   We are processing a non-ascii, non-native-indexing char now;
@@ -1541,6 +1558,7 @@ utf8TextMapIndexToUTF16(const UText *ut, int64_t index64) {
     U_ASSERT(index>=ut->chunkNativeStart+ut->nativeIndexingLimit);
     U_ASSERT(index<=ut->chunkNativeLimit);
     int32_t mapIndex = index - u8b->toUCharsMapStart;
+    U_ASSERT(mapIndex < (int32_t)SIZEOF_MAPTOUCHARS);
     int32_t offset = u8b->mapToUChars[mapIndex] - u8b->bufStartIdx;
     U_ASSERT(offset>=0 && offset<=ut->chunkLength);
     return offset;
diff --git a/intl/icu/source/i18n/gregoimp.cpp b/intl/icu/source/i18n/gregoimp.cpp
index 1b313602d..204c3ff81 100644
--- a/intl/icu/source/i18n/gregoimp.cpp
+++ b/intl/icu/source/i18n/gregoimp.cpp
@@ -27,6 +27,11 @@ int32_t ClockMath::floorDivide(int32_t numerator, int32_t denominator) {
         numerator / denominator : ((numerator + 1) / denominator) - 1;
 }
 
+int64_t ClockMath::floorDivide(int64_t numerator, int64_t denominator) {
+    return (numerator >= 0) ?
+        numerator / denominator : ((numerator + 1) / denominator) - 1;
+}
+
 int32_t ClockMath::floorDivide(double numerator, int32_t denominator,
                           int32_t& remainder) {
     double quotient;
diff --git a/intl/icu/source/i18n/gregoimp.h b/intl/icu/source/i18n/gregoimp.h
index 27303a6ac..f6c730aef 100644
--- a/intl/icu/source/i18n/gregoimp.h
+++ b/intl/icu/source/i18n/gregoimp.h
@@ -40,6 +40,17 @@ class ClockMath {
      */
     static int32_t floorDivide(int32_t numerator, int32_t denominator);
 
+    /**
+     * Divide two integers, returning the floor of the quotient.
+     * Unlike the built-in division, this is mathematically
+     * well-behaved.  E.g., <code>-1/4</code> => 0 but
+     * <code>floorDivide(-1,4)</code> => -1.
+     * @param numerator the numerator
+     * @param denominator a divisor which must be != 0
+     * @return the floor of the quotient
+     */
+    static int64_t floorDivide(int64_t numerator, int64_t denominator);
+
     /**
      * Divide two numbers, returning the floor of the quotient.
      * Unlike the built-in division, this is mathematically
diff --git a/intl/icu/source/i18n/persncal.cpp b/intl/icu/source/i18n/persncal.cpp
index 278273fb9..6f46dd9c7 100644
--- a/intl/icu/source/i18n/persncal.cpp
+++ b/intl/icu/source/i18n/persncal.cpp
@@ -213,7 +213,7 @@ void PersianCalendar::handleComputeFields(int32_t julianDay, UErrorCode &/*statu
     int32_t year, month, dayOfMonth, dayOfYear;
 
     int32_t daysSinceEpoch = julianDay - PERSIAN_EPOCH;
-    year = 1 + ClockMath::floorDivide(33 * daysSinceEpoch + 3, 12053);
+    year = 1 + (int32_t)ClockMath::floorDivide(33 * (int64_t)daysSinceEpoch + 3, (int64_t)12053);
 
     int32_t farvardin1 = 365 * (year - 1) + ClockMath::floorDivide(8 * year + 21, 33);
     dayOfYear = (daysSinceEpoch - farvardin1); // 0-based
diff --git a/intl/icu/source/i18n/zonemeta.cpp b/intl/icu/source/i18n/zonemeta.cpp
index fdf333c37..dd3bf202f 100644
--- a/intl/icu/source/i18n/zonemeta.cpp
+++ b/intl/icu/source/i18n/zonemeta.cpp
@@ -683,7 +683,6 @@ ZoneMeta::createMetazoneMappings(const UnicodeString &tzid) {
                     mzMappings = new UVector(deleteOlsonToMetaMappingEntry, NULL, status);
                     if (U_FAILURE(status)) {
                         delete mzMappings;
-                        deleteOlsonToMetaMappingEntry(entry);
                         uprv_free(entry);
                         break;
                     }
