There is no obvious reason why these are macros; especially the latter two. Here are the uses:

$ ack SCAN_AND_FORWARD src/
src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp
2099: SCAN_AND_FORWARD(cp,end,block_is_obj,block_size);

src/share/vm/memory/space.cpp
441: SCAN_AND_FORWARD(cp, end, block_is_obj, block_size);
446: SCAN_AND_FORWARD(cp, top, block_is_always_obj, obj_size);

src/share/vm/memory/space.hpp
471:#define SCAN_AND_FORWARD(cp,scan_limit,block_is_obj,block_size) { \


$ ack SCAN_AND_ADJUST_POINTERS src/
src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp
2115: SCAN_AND_ADJUST_POINTERS(adjust_obj_size);

src/share/vm/memory/space.cpp
490: SCAN_AND_ADJUST_POINTERS(adjust_obj_size);

src/share/vm/memory/space.hpp
592:#define SCAN_AND_ADJUST_POINTERS(adjust_obj_size) { \


$ ack SCAN_AND_COMPACT src/
src/share/vm/gc_implementation/concurrentMarkSweep/compactibleFreeListSpace.cpp
2120: SCAN_AND_COMPACT(obj_size);

src/share/vm/memory/space.cpp
494: SCAN_AND_COMPACT(obj_size);

src/share/vm/memory/space.hpp
658:#define SCAN_AND_COMPACT(obj_size) { \


Making them (inline) methods would make it easier to read, change and debug this code.