Re: [PATCH] gdb/disasm: mark functions passed to the disassembler noexcept

[Simon Marchi <simark@simark.ca>]

On 10/24/22 13:45, Andrew Burgess via Gdb-patches wrote:
> 
> I realise I made a bit of a mess, including the updated patch in the
> previous email.  I should have just posted both of these as a new V2
> series.
> 
> Oh well!  The patch below applies on top of the first patch (in the
> parent email), and adds the noexcept keyword in a few places.
> 
> Thanks,
> Andrew
> 
> ---
> 
> commit b1e3243296084566baf443d9c5280918a9d784f3
> Author: Andrew Burgess <aburgess@redhat.com>
> Date:   Mon Oct 24 18:35:41 2022 +0100
> 
>     gdb/disasm: mark functions passed to the disassembler noexcept
>     
>     While working on another patch, Simon pointed out that GDB could be
>     improved by marking the functions passed to the disassembler as
>     noexcept.
>     
>       https://sourceware.org/pipermail/gdb-patches/2022-October/193084.html
>     
>     The reason this is important is the on some hosts, libopcodes, being C
>     code, will not be compiled with support for handling exceptions.  As
>     such, an attempt to throw an exception over libopcodes code will cause
>     GDB to terminate.
>     
>     See bug gdb/29712 for an example of when this happened.
>     
>     In this commit all the functions that are passed to the disassembler,
>     and which might be used as callbacks by libopcodes are marked
>     noexcept.
>     
>     Ideally, I would have liked to change these typedefs:
>     
>       using read_memory_ftype = decltype (disassemble_info::read_memory_func);
>       using memory_error_ftype = decltype (disassemble_info::memory_error_func);
>       using print_address_ftype = decltype (disassemble_info::print_address_func);
>       using fprintf_ftype = decltype (disassemble_info::fprintf_func);
>       using fprintf_styled_ftype = decltype (disassemble_info::fprintf_styled_func);
>     
>     which are declared in disasm.h, as including the noexcept keyword.
>     However, when I tried this, I ran into this warning/error:
>     
>       In file included from ../../src/gdb/disasm.c:25:
>       ../../src/gdb/disasm.h: In constructor ‘gdb_printing_disassembler::gdb_printing_disassembler(gdbarch*, ui_file*, gdb_disassemble_info::read_memory_ftype, gdb_disassemble_info::memory_error_ftype, gdb_disassemble_info::print_address_ftype)’:
>       ../../src/gdb/disasm.h:116:3: error: mangled name for ‘gdb_printing_disassembler::gdb_printing_disassembler(gdbarch*, ui_file*, gdb_disassemble_info::read_memory_ftype, gdb_disassemble_info::memory_error_ftype, gdb_disassemble_info::print_address_ftype)’ will change in C++17 because the exception specification is part of a function type [-Werror=noexcept-type]
>         116 |   gdb_printing_disassembler (struct gdbarch *gdbarch,
>             |   ^~~~~~~~~~~~~~~~~~~~~~~~~
>     
>     So I've left that change out.  This does mean that if somebody adds a
>     new use of the disassembler classes in the future, and forgets to mark
>     the callbacks as noexcept, this will compile fine.  We'll just have to
>     manually check for that during review.

Like I said in my other email, I think this warning could safely be
ignored (silenced).  We are not writing a library with a stable ABI, we
don't care if the mangling is different in two different builds.

Simon