On Mon, 15 Nov 2021 11:21:37 GMT, Alan Hayward <d...@openjdk.java.net> wrote:
>> src/hotspot/cpu/aarch64/c1_Runtime1_aarch64.cpp line 452: >> >>> 450: // patch the return address, this stub will directly return to the >>> exception handler >>> 451: __ str(r0, Address(rfp, 1*BytesPerWord)); >>> 452: >> >> Please explain the reason for this change, that leaves `lr` live across >> `restore_live_registers()`. > > In the original code: > *save r0 to the lr location on the stack > *restore_live_registers > *Standard return: remove stack frame, load lr and fp off the stack, jump to > lr. > > With PAC it would now be: > *Sign r0 then save it to the lr location on the stack > *restore_live_registers > *Standard return: remove stack frame, load lr and fp off the stack, auth lr, > jump to lr. > > After reading the code in restore_live_registers, it doesn't touch lr and so > seemed odd to have the save to the stack, only to restore it directly > afterwards. That's an optimization, though. You shouldn't need to read the code in `restore_live_registers()` to see if it's safe to keep the return address in LR: at best it's pathological coupling, in the sense that the correctness of this code depends on the internal details of `restore_live_registers()`. Let's keep LR live ranges as short as possible. ------------- PR: https://git.openjdk.java.net/jdk/pull/6334
