[lldb-dev] Stack unwinding fails on x86/x86_64

[Jason Molenda]

Hi Mario, sorry I missed this one over the holidays.

The problem here is the '[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V' function.  It has a mid-function epilogue which lldb can't handle today on x86_64.  This is a common idiom on armv7/arm64 - I have experience with how to solve the problem there but I had never seen a compiler generate code like this on x86_64 so it wasn't handled there.

In your example session, when you're stopped at 0x000000010014bb5f, lldb is no longer able to backtrace.  Looking at the disassembly, we see that 0x10014bb5f is just past a mid-function epilogue.  We'll need to update the x86_64 assembly unwinder to recognize the epilogue sequence and re-install the previous unwind state before the epilogue unwound it.

0x10014bb38 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V>: pushq  %rbp
0x10014bb39 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+1>: movq   %rsp, %rbp
0x10014bb3c <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+4>: subq   $0x20, %rsp
0x10014bb40 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+8>: movq   %rdi, -0x8(%rbp)
0x10014bb44 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+12>: movq   -0x10000(%rsp), %rax
0x10014bb4c <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+20>: movl   %esi, -0xc(%rbp)
0x10014bb4f <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+23>: cmpl   $0x64, -0xc(%rbp)
0x10014bb53 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+27>: movq   %rdi, -0x18(%rbp)
0x10014bb57 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+31>: jle    0x10014bb5f               ; [J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V + 39 at InvalidFrame.java:13
0x10014bb59 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+33>: addq   $0x20, %rsp
0x10014bb5d <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+37>: popq   %rbp
0x10014bb5e <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+38>: retq   
0x10014bb5f <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+39>: movl   -0xc(%rbp), %eax
0x10014bb62 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+42>: addl   $0x1, %eax
0x10014bb65 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+45>: movl   %eax, -0x10(%rbp)
0x10014bb68 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+48>: movl   -0x10(%rbp), %esi
0x10014bb6b <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+51>: movq   -0x18(%rbp), %rdi
0x10014bb6f <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+55>: callq  0x10014bb38               ; [J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V at InvalidFrame.java:10
0x10014bb74 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+60>: addq   $0x20, %rsp
0x10014bb78 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+64>: popq   %rbp
0x10014bb79 <[J]com.robovm.debug.server.apps.InvalidFrame.testRecursion(I)V+65>: retq   





> On Dec 23, 2014, at 6:59 AM, Mario Zechner <badlogicgames at gmail.com> wrote:
> 
> Hi,
> 
> i'm running into stack unwinding issues when trying to get a backtrace for a currently selected thread. You can see the output of diagnose-unwind here: https://gist.github.com/badlogic/99736e5c37f54ea08481
> 
> The simple stack walking algorithm in diagnose-unwind succeeds in reconstructing the correct frames.
> 
> Any idea what I could be doing wrong or how i could fix this issue?
> 
> Thanks,
> Mario
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