RFR: 8253795: Implementation of JEP 391: macOS/AArch64 Port [v9]

[Gerard Ziemski]

On Tue, 2 Feb 2021 19:23:16 GMT, Bernhard Urban-Forster <burban at openjdk.org> wrote:

>> src/hotspot/os/posix/signals_posix.cpp line 1297:
>> 
>>> 1295:   kern_return_t kr;
>>> 1296:   kr = task_set_exception_ports(mach_task_self(),
>>> 1297:                                 EXC_MASK_BAD_ACCESS | EXC_MASK_BAD_INSTRUCTION | EXC_MASK_ARITHMETIC,
>> 
>> Could someone elaborate on why we need to add `EXC_MASK_BAD_INSTRUCTION` to the mask here?
>
> See comment above about `gdb`, the same applies to `lldb` today. The AArch64 backend uses `SIGILL` (~= `EXC_MASK_BAD_INSTRUCTION`) to initiate a deoptimization. Without this change you cannot continue debugging once you the debuggee receives `SIGILL`. This wasn't needed before as x86 doesn't use `SIGILL`.

Part of the comment said `This work-around is not necessary for 10.5+, as CrashReporter no longer intercedes on caught fatal signals.` so I thought it was no longer needed, but it sounds like the part about `gdb` still applies then.

We should update the comment to just say the `gdb` relevant part perhaps (and evaluate which of the EXC_MASK_BAD_ACCESS | EXC_MASK_BAD_INSTRUCTION | EXC_MASK_ARITHMETIC) we actually need for gdb:

 `// gdb installs both standard BSD signal handlers, and mach exception`
 `// handlers. By replacing the existing task exception handler, we disable gdb's mach`
 `// exception handling, while leaving the standard BSD signal handlers functional.`

Do you know if this also apply to `lldb` or is it `gdb` only specific? How do you run `gdb` on macOS nowadays anyhow?

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PR: https://git.openjdk.java.net/jdk/pull/2200