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Subject: Re: Hand-written assembly and Python API
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To: Jan Vrany <jan@vrany.io>, GDB mailing list <gdb@sourceware.org>
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From: Simon Marchi <simark@simark.ca>
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On 2022-04-29 08:32, Jan Vrany via Gdb wrote:
> Hi folks,
>
> I'm trying to debug a code that uses a handful of
> hand-written assembly routines. For example, one of
> them looks like (its RISC-V but that does not matter):
>
>     .text
>     .globl returnFromJIT1
>     .type returnFromJIT1,function
>     .align 2
> returnFromJIT1:
>     .cfi_startproc
>     sd a0,248(s10)
>     sd s11,32(s10)
>     li a0, 17
>     li a1, 1
>     j cInterpreterFromJIT
>     .cfi_endproc
>     .size   returnFromJIT1, .-returnFromJIT1
>
>    .text
>    .globl ...
>
> GDB clealy knows "something" about the assembly routines, it shows
> the source properly and 'info symbol' works too:
>
>    (gdb) info symbol 0x3ff75eca24
>    returnFromJIT1 in section .text of /opt/riscv/sysroot/tmp/jdk/lib/default/libj9jit29.so
>
> The problem is how to figure out I'm in (say) `returnFromJIT1` routine
> using Python API:
>
>    (gdb) py print(gdb.block_for_pc(0x3ff75eca24).function)
>    None
>
> The only way I can think of is to parse value of `gdb.format_address()`:
>
>    (gdb) py print(gdb.format_address(0x3ff75eca24))
>    0x3ff75eca24 <returnFromJIT1>
>
> which is bit awkward (but doable!).
>
> Question is: is there a better way? I can modify the assembly source
> too if there's some directive that may help GDB (.cfi_startproc / .cfi_endproc
> is clearly not enough). Or do I have to roll up sleeves and implement python
> API for minimal symbols?

I was going to say "returnFromJIT1" is a minimal symbol, but you clearly
know that already.  There is DWARF debug info for assembly when building
with -g (at least when building with gcc / gas), but it only contains
line statements, which allows GDB to show where you are in the source
file (instead of showing disassembly).  You won't be able to look up a
block from that.

I don't know of a way to do it with the current API, unfortunately.

Simon