On Wed, 22 Nov 2023 18:26:12 GMT, Daniel D. Daugherty <dcu...@openjdk.org>
wrote:
>> src/hotspot/cpu/x86/vm_version_x86.cpp line 819:
>>
>>> 817: }
>>> 818:
>>> 819: _supports_cx8 = supports_cmpxchg8();
>>
>> I think we should leave the runtime check here (under `ifndef`, like in
>> ARM?). This covers the remaining case of running on legacy x86 without CX8
>> implemented: the init guarantee would then fire and prevent any other
>> surprises at runtime. Sure, it would be hard to come up with such a platform
>> today, but it would be safer to refuse to run there right away on the
>> off-chance someone actually has it :)
>
> @shipilev - Do you have a particular legacy x86 in mind?
My point is that it is such an easy thing to do: leave the "cx8" flag sensing
code in, and keep setting up `_supports_cx8` based on it. This both provides
more safety by failing cleanly on non-CX8 platform, and gives other platforms
some guidance: if you can check something is supported, check it.
But now that you nerd-sniped me into this... I think non-CX8 platforms would
probably predate Pentium. The oldest real machine my lab has is Z530, which
already has CX8. But it was easy to also go to my QEMU-driven build-test
server, ask for `i486` as platform there, and et voila, no `cx8` in CPU flags:
buildworker-debian12-32:~$ lscpu
Architecture: i486
CPU op-mode(s): 32-bit
Address sizes: 36 bits physical, 32 bits virtual
Byte Order: Little Endian
CPU(s): 4
On-line CPU(s) list: 0-3
Vendor ID: GenuineIntel
Model name: 486 DX/4
CPU family: 4
Model: 8
Thread(s) per core: 4
Core(s) per socket: 1
Socket(s): 1
Stepping: 0
BogoMIPS: 5699.99
Flags: fpu vme pse apic ht cpuid tsc_known_freq x2apic
hypervisor cpuid_fault
And mainline JDK even starts there! (with interpreter, there are some asserts
firing in compiler code, having to do with odd instruction selection on some
paths):
$ jdk/bin/java -Xint -version
openjdk version "22-testing" 2024-03-19
OpenJDK Runtime Environment (fastdebug build
22-testing-builds.shipilev.net-openjdk-jdk-b627-20231121)
OpenJDK Server VM (fastdebug build
22-testing-builds.shipilev.net-openjdk-jdk-b627-20231121, interpreted mode,
sharing)
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/16625#discussion_r1402738580
