oopMap.hpp is included by about 585 out of about 1000 .o files in HotSpot. In turn, it pulls in other complex headers such as vmreg.hpp, register.hpp and their cpu-dependent headers.

There are only 4 header files that include oopMap.hpp. All of these can be replaced for forward declarations.

According to https://github.com/iklam/tools/tree/main/headers

[before]

/jdk/src/hotspot$ whoincludes oopMap.hpp
scanning 585 oopMap.hpp
   2 found 553 method.hpp
   3 found 407 codeBlob.hpp
   4 found 262 debugInfoRec.hpp
   5 found 232 ciEnv.hpp
/jdk/src/hotspot$ whoincludes vmreg.hpp
scanning 597 vmreg.hpp
   2 found 585 oopMap.hpp
   3 found 281 location.hpp
   4 found 118 regmask.hpp
   5 found 109 registerMap.hpp
   ...
/jdk/src/hotspot$ whoincludes register.hpp
scanning 611 register.hpp
   2 found 611 register_x86.hpp
   3 found 597 vmreg.hpp
   4 found 364 assembler.hpp
   5 found 364 assembler_x86.hpp
   ...

[after]

/jdk/src/hotspot$ whoincludes oopMap.hpp
scanning 39 oopMap.hpp
   2 found 6 c1_LinearScan.hpp
/jdk/src/hotspot$ whoincludes vmreg.hpp
scanning 333 vmreg.hpp
   2 found 281 location.hpp
   3 found 181 sharedRuntime.hpp
   4 found 118 regmask.hpp
   5 found 109 registerMap.hpp
   ...
/jdk/src/hotspot$ whoincludes register.hpp
scanning 386 register.hpp
   2 found 386 register_x86.hpp
   3 found 364 assembler.hpp
   4 found 364 assembler_x86.hpp
   5 found 333 vmreg.hpp
   ...