I found 2 problems:
1) when using "Debug" functions, a dialog box with title "gdborig.exe has stopped working" prompted sometimes. Then the debug session stopped.
2) In the "Debug mode" with default GCC and options, I open watch window and find the pointer's address is NOT 64-bit cononical address format, but a normal 32-bit address such as "0x3131b80". But the built out executable is a 64-bti applicaiton according to the file's attribute info. I think in 64-bit mode, the pointer's address should be "0xFFFFxxxxxxxxxxxx" format.