[Bug gdb/29128] "internal-error: find_pc_sect_compunit_symtab: Assertion `result != nullptr' failed" while debugging asm program

["simark at simark dot ca" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29128

--- Comment #3 from Simon Marchi <simark at simark dot ca> ---
(In reply to Tom Tromey from comment #2)
> gdb shouldn't crash here, but also this file is weird.
> 
> The .debug_info says:
> 
>  <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
>     <c>   DW_AT_low_pc      : 0x4000b0
>     <14>   DW_AT_high_pc     : 0x4000b0
> 
> and
> 
>  <1><3c>: Abbrev Number: 2 (DW_TAG_subprogram)
>     <3d>   DW_AT_low_pc      : 0x4000b0
> 
> (no high PC for the function)

Agreed, looks like bad debug info.

On Ubuntu 20.04, with NASM version 2.14.02, at least the CU's high pc is right:

0x0000000b: DW_TAG_compile_unit
              DW_AT_low_pc [DW_FORM_addr]       (0x0000000000401000)
              DW_AT_high_pc [DW_FORM_addr]      (0x000000000040101e)

The original report on IRC was about GDB not showing the lines when stepping. 
The root cause must be this NASM bug.  I think it's ok to not produce good
results when working with the bad debug info, but ideally we wouldn't crash.

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