--- dyld/dyld-1340/mach_o/FunctionStarts.cpp
+++ dyld/dyld-1162/mach_o/FunctionStarts.cpp
@@ -75,6 +75,47 @@
 }
 
 
+#if BUILDING_MACHO_WRITER
+FunctionStarts::FunctionStarts(uint64_t prefLoadAddr, std::span<const uint64_t> functionAddresses)
+{
+    uint64_t lastAddr = prefLoadAddr;
+    for (uint64_t addr : functionAddresses) {
+        assert(addr >= lastAddr && "function addresses not sorted");
+        // <rdar://problem/10422823> filter out zero-length atoms, so LC_FUNCTION_STARTS address can't spill into next section
+        if ( addr == lastAddr)
+            continue;
+        // FIXME: for 32-bit arm need to check thumbness
+        uint64_t delta = addr - lastAddr;
+        append_uleb128(delta);
+        lastAddr = addr;
+    }
+    // terminate delta encoded list
+    _bytes.push_back(0);
+    // 8-byte align
+    while ( (_bytes.size() % 8) != 0 )
+        _bytes.push_back(0);
+
+    // set up pointers to data can be parsed
+    _funcStartsBegin = &_bytes[0];
+    _funcStartsEnd   = &_bytes[_bytes.size()];
+}
+
+void FunctionStarts::append_uleb128(uint64_t value)
+{
+    uint8_t byte;
+    do {
+        byte = value & 0x7F;
+        value &= ~0x7F;
+        if ( value != 0 )
+            byte |= 0x80;
+        _bytes.push_back(byte);
+        value = value >> 7;
+    } while( byte >= 0x80 );
+}
+
+#endif // BUILDING_MACHO_WRITER
+
+
 
 
 } // namespace mach_o