RFR: 8209413: AArch64: NPE in clhsdb jstack command

[Nick Gasson (Arm Technology China)]

Hi Jini,

On 02/02/2019 02:16, Jini George wrote:
> Do we reach here after AARCH64CurrentFrameGuess.run() fails to get the 
> PC ? 

Yes, that's right. It's the else branch (not interpreter and not 
compiler) that sets this.pc to null.

> If so, was wondering if it would make more sense to scan from the 
> top of stack sp obtained from 
> context.getRegisterAsAddress(AARCH64ThreadContext.SP) to the sp of the 
> last known java frame with thread.getLastJavaSP() in 
> AARCH64CurrentFrameGuess.run() -- otherwise was wondering if we are 
> risking an exception by running off the top of the stack while scanning 
> in the upward direction (towards lower addresses) for 
> CALLEE_FRAME_SEARCH_LIMIT * addressSize from the previous Java SP 
> (though the scan range is small).
> 

I think this is much better, thanks. But we still have the problem that 
the two-argument AARCH64Frame constructor is wrong: I don't think it's 
ever correct to assume the PC is at SP[-1]. And so we need to fix the 
other uses of it. I've made another patch that moves the frame scanning 
into LinuxAARCH64JavaThreadPDAccess.java, searching between the 
last-known Java SP and and current thread SP as described above. And we 
now use this to find a PC any time we would have called the two argument 
constructor before.

http://cr.openjdk.java.net/~ngasson/8209413/webrev.02/

Please let me know what you think.

+      Assert.that(jcw.getLastJavaPC() != null, "last Java pc should be 
set");

I believe this is OK because the last Java SP/FP/PC are set by 
MacroAssembler::set_last_Java_frame, and I can't see any case where it 
would set SP and FP but not PC.

Thanks,
Nick