RFR: 8316694: Implement relocation of nmethod within CodeCache [v18]

[Chad Rakoczy]

On Wed, 28 May 2025 20:41:43 GMT, Chad Rakoczy <duke at openjdk.org> wrote:

>> If a trampoline is null, it is a critical situation. The patched call instruction will be incorrect.
>> `NativeCall::set_destination` does not check whether a destination is reachable:
>> ```c++
>>   void set_destination(address dest) {
>>     int offset = dest - instruction_address();
>>     unsigned int insn = 0b100101 << 26;
>>     assert((offset & 3) == 0, "should be");
>>     offset >>= 2;
>>     offset &= (1 << 26) - 1; // mask off insn part
>>     insn |= offset;
>>     set_int_at(displacement_offset, insn);
>>   }
>> 
>> 
>> So higher bits will be masked out.
>
>> The patched call instruction will be incorrect.
> 
> That's not entirely correct. The null trampoline check is needed because on debug builds branches of distance >2M will fall into the `if (!Assembler::reachable_from_branch_at(addr(), x))` block but Graal would not have generated a trampoline for that call because it is still <128M. It is still safe to use that distance but it is just different than what HotSpot expects

If we want to guarantee that a trampoline exists if `Assembler::reachable_from_branch_at` fails we would need to update Graal to use the check as well

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r2112717962