Assume there is a directory "foo" in the current directory and there is a foo.c in ./foo , I saw this on 4.5.1

$ gcc-4.5 -fdump-tree-optimized -save-temps=obj foo/foo.c -S -o foo/foo.s
foo/foo.c: In function ‘foo’:
foo/foo.c:1:6: error: could not open dump file ‘foo/foo/foo.140t.optimized’: No such file or directory