Re: Concerns about SFrame viability for userspace stack walking

[Fangrui Song]

On Thu, Oct 30, 2025 at 12:30 AM Jakub Jelinek <jakub@xxxxxxxxxx> wrote:
>
> On Wed, Oct 29, 2025 at 11:53:32PM -0700, Fangrui Song wrote:
> > I've been following the SFrame discussion and wanted to share some concerns about its viability for userspace adoption, based on concrete measurements and comparison with existing compact unwind implementations in LLVM.
> >
> > **Size overhead concerns**
> >
> > Measurements on a x86-64 clang binary show that .sframe (8.87 MiB) is approximately 10% larger than the combined size of .eh_frame and .eh_frame_hdr (8.06 MiB total).
> > This is problematic because .eh_frame cannot be eliminated - it contains essential information for restoring callee-saved registers, LSDA, and personality information needed for debugging (e.g. reading local variables in a coredump) and C++ exception handling.
>
> I believe .sframe only provides a subset of the .eh_frame information, so
> can't be used for exception throwing, and you don't want to lose
> .eh_frame_hdr either because then dlopen becomes very costly and it will
> even slow down exception throwing.

Right.

> If .eh_frame is considered too large, rather than inventing a new format I'd
> suggest to work in the DWARF committee and provide further size
> optimizations for .dwarf_frame which can then be used in .eh_frame, or agree
> on .eh_frame extensions to make it smaller.
>
>         Jakub

Thanks for the suggestion.
An effective compact unwinding scheme needs to leverage ISA-specific properties.
This architecture-specific nature makes it likely fall outside the
DWARF's scope.
That said, input from the DWARF committee would certainly be valuable.