https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113312
--- Comment #24 from Xin Li <xin at zytor dot com> --- (In reply to H.J. Lu from comment #23) > (In reply to Xin Li from comment #22) > > Per Peter's suggestion, I added __attribute__((no_callee_saved_registers)) > > to a linux source tree containing FRED patches: > > https://github.com/xinli-intel/linux-fred-public/commit/ > > 12c38143a5c33e89f2b3d8906629dd4f23f8d79c. And I compiled the linux code > > with a gcc built from > > https://gitlab.com/x86-gcc/gcc/-/tree/users/hjl/pr113312/gcc-13. > > > > Following are my observations: > > 1) the generated kernel boots fine on both FRED Simics model and bare metal. > > 2) the asm code generated for fred_entry_from_{user,kernel}() are the same, > > i.e., __attribute__((no_callee_saved_registers)) makes no difference (Peter > > said the FRED dispatch points simply do not have significant register > > pressure – intentionally). > > 3) other functions with __attribute__((no_callee_saved_registers)) do get > > rid of pushing/popping clobbered registers, and cause no issues because they > > are top-level functions, only invoked by tail call, meaning the following > > case won't happen: > > > > <func>: > > ... > > mov (%rbx), %r13 > > call foo > > mov %rax, (%r13) > > ... > > > > otherwise foo() is NOT a top-level function. > > Do these mean that __attribute__((no_callee_saved_registers)) works for you? Yes, it works as expected. > BTW, my patches also avoid saving callee-saved registers in > __attribute__((noreturn)) > functions. I also checked do_exit, which is the same as what you posted.
