RFR: 8277948: AArch64: Print the correct native stack if -XX:+PreserveFramePointer when crash

[Andrew Haley]

On Mon, 6 Dec 2021 12:11:42 GMT, Denghui Dong <ddong at openjdk.org> wrote:

> > Thank you for this. I'll have a look.
> > Stack unwinding on AArch64 C/C++ uses call frame information, which is in a separate section in the binary file. This allows the stack to be fully traced, even if there is no frame pointer. There is a library, libunwind, which does this. But that won't work with Java, which has its own way to do it
> > It would be nice to get -XX:+PreserveFramePointer working correctly.
> 
> Thanks for the response.
> 
> I also noticed that a java method will reserve 2 words when this method makes a vm leaf call:

It's not reserving anything, it's saving the PC for the stack unwinder.

> ```
> aarch64.ad
> 
> aarch64_enc_java_to_runtime
> 
>       Label retaddr;
>       __ adr(rscratch2, retaddr);
>       __ lea(rscratch1, RuntimeAddress(entry));
>       // Leave a breadcrumb for JavaFrameAnchor::capture_last_Java_pc()
>       __ stp(zr, rscratch2, Address(__ pre(sp, -2 * wordSize)));
>       __ blr(rscratch1);
>       __ bind(retaddr);
>       __ add(sp, sp, 2 * wordSize);
> ```

I wrote it. If you look at `JavaFrameAnchor::capture_last_Java_pc()` you'll see
it being used.

> ```
> MacroAssembler::call_VM_leaf_base
> 
> 
>   stp(rscratch1, rmethod, Address(pre(sp, -2 * wordSize)));
> 
>   mov(rscratch1, entry_point);
>   blr(rscratch1);
>   if (retaddr)
>     bind(*retaddr);
> 
>   ldp(rscratch1, rmethod, Address(post(sp, 2 * wordSize)));
> ```
> 
> I haven't figured out the specific purpose of this operation, but I think it will make the logic of stack walking more complicated.

All this is doing is saving `rmethod` (which is in a call-clobbered register) around a VM call. `retaddr` is saved for OOP maps.

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PR: https://git.openjdk.java.net/jdk/pull/6597