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

Gerard Ziemski gziemski at openjdk.java.net
Wed Feb 3 20:32:47 UTC 2021


On Wed, 3 Feb 2021 20:04:18 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

>> 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?

To answer my own question, it seems that code is still needed on `x86_64` for `lldb` with `EXC_MASK_BAD_ACCESS` or we keep tripping over `EXC_BAD_ACCESS`

Remaining questions:

a) why we need `EXC_MASK_ARITHMETIC` ?
b) we hit `signal SIGSEGV` in debugger even with the code in place, any way to avoid that?
c) does `BSD aarch64` need only `EXC_MASK_BAD_INSTRUCTION` or does it need `EXC_MASK_BAD_ACCESS` as well?
d) can we `#ifdef` the `EXC_MASK_BAD_INSTRUCTION` part of the mask only to apply to `aarch64`?

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


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