Stepping over a `brk` instruction on ARM64

Subprojects LLDB
1

I posted this question on Stack Overflow, but I’m afraid SO might not be the right forum. Please forgive me for cross posting.

I’m working on a JIT compiler on the ARM64 platform. In order to debug the machine code it emits, sometimes I will have the compiler emit a brk instruction so that I can inspect the state of things using lldb.

Unlike the x86_64 platform, the PC register is at the instruction (not after it):

Process 61859 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=1, subcode=0x1008e8020)
    frame #0: 0x00000001008e8020
->  0x1008e8020: brk    #0x1
    0x1008e8024: ldur   x9, [x9]
    0x1008e8028: mov    x10, #0x4000
    0x1008e802c: movk   x10, #0x8e, lsl #16
Target 0: (ruby) stopped.
(lldb) p/x $pc
(unsigned long) $3 = 0x00000001008e8020
(lldb)

This means I cannot use si in lldb to step over the brk instruction because the brk instruction will just be evaluated over and over:

Process 61859 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=1, subcode=0x1008e8020)
    frame #0: 0x00000001008e8020
->  0x1008e8020: brk    #0x1
    0x1008e8024: ldur   x9, [x9]
    0x1008e8028: mov    x10, #0x4000
    0x1008e802c: movk   x10, #0x8e, lsl #16
Target 0: (ruby) stopped.
(lldb) p/x $pc
(unsigned long) $3 = 0x00000001008e8020
(lldb) si
Process 61859 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=1, subcode=0x1008e8020)
    frame #0: 0x00000001008e8020
->  0x1008e8020: brk    #0x1
    0x1008e8024: ldur   x9, [x9]
    0x1008e8028: mov    x10, #0x4000
    0x1008e802c: movk   x10, #0x8e, lsl #16
Target 0: (ruby) stopped.
(lldb) si
Process 61859 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=1, subcode=0x1008e8020)
    frame #0: 0x00000001008e8020
->  0x1008e8020: brk    #0x1
    0x1008e8024: ldur   x9, [x9]
    0x1008e8028: mov    x10, #0x4000
    0x1008e802c: movk   x10, #0x8e, lsl #16
Target 0: (ruby) stopped.
(lldb)

I know that I can directly write to the $pc register to step over the instruction, but I don’t want to copy / paste the next address every time. I used to use this command:

register write $pc `$pc + 4`

But something changed on macOS such that the above command doesn’t work anymore. I get the following error:

error: Failed to write register 'pc' with value '$pc + 4': '$pc + 4' is not a valid unsigned integer string value

Is there a better or more supported way to step over brk instructions on ARM64? I’m currently using this version of lldb:

$ lldb -v
lldb-1403.0.17.64
Apple Swift version 5.8 (swiftlang-5.8.0.124.2 clang-1403.0.22.11.100)

Thanks, and again I’m really sorry for cross posting. I am not sure where to ask this question. :pensive:

2

Yeah it looks like a regression or unintended side effect of something between Xcode 14.2 (lldb-1400) and Xcode 14.3 (lldb-1403), but does not happen on GitHub top of tree sources.

(lldb) p/x $pc
(unsigned long) $0 = 0x0000000100003e78
(lldb) reg write pc `$pc+4`
(lldb) p/x $pc
(unsigned long) $2 = 0x0000000100003e7c
(lldb) reg write pc `0x0000000100003e7c`
(lldb) reg write pc `0x0000000100003e7cU`

versus lldb-1403 in Xcode 14.3:

(lldb) reg write pc `$pc+4`
error: Failed to write register 'pc' with value '$pc+4': '$pc+4' is not a valid unsigned integer string value
(lldb) reg write pc `0x100003e80UL`
error: Failed to write register 'pc' with value '0x100003e80UL': '0x100003e80UL' is not a valid unsigned integer string value

Interesting. It seems to be an issue with the Xcode lldb, I would file a bug report with feedbackassistant.apple.com .

1 Like
3

Great, thank you so much for the help. I’ll file an issue and in the mean time build from source.

Thank you so much!

4

This was fixed in:

75ca15fcbb2e1b3671e41f971a000c6d59f5e5ae

Jim

On Apr 4, 2023, at 5:52 PM, Aaron Patterson via LLVM Discussion Forums notifications@llvm.discoursemail.com wrote:

tenderlove
April 5

Great, thank you so much for the help. I’ll file an issue and in the mean time build from source.

Thank you so much!

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5

Thank you for fixing it! I really appreciate it.