Re: Migrating to LLVM binutils tools (ar, nm, addr2line, etc.)

On Mon, 5 Jul 2021 at 16:09, Ed Maste <emaste@freebsd.org> wrote:
>
> FreeBSD migrated from GNU binutils to versions from ELF Tool Chain,
> starting in 2014. At that time there were no usable LLVM versions of
> those tools, but they have been developing rapidly since then. Now I
> think it may be prudent to migrate to the LLVM tools where they exist,
> for both functionality and maintainability reasons.
>
> I'd like to allow use of link-time optimization (LTO) in the FreeBSD
> base system. LTO runs optimization passes over the entire executable
> (or library) at link time and thus allows for more effective
> optimization than when performed on individual compilation units.
>
> When using LTO object files (.o) including those contained in static
> library archives (.a) contain LLVM IR bitcode rather than target
> object code. This means that utilities that operate on object files
> need to support LLVM IR; we currently use a number of bespoke tools
> and ones obtained from ELF Tool Chain that do not have this support.
>
> Alex Richardson also pointed out that asan (address sanitizer)
> produces a useful backtrace only if addr2line is llvm-symbolizer.
>
> Like ELF Tool Chain the LLVM tools aim for command line and output
> format compatibility with GNU binutils, although there are a few minor
> differences. Where these cause a material issue (breaking a port or
> eliminating required functionality) we can submit LLVM bugs and work
> on patches.
>
> In the past we provided build knobs to control individual utilities
> (e.g. WITH_LLD_IS_LD); I'd like to avoid perpetuating that here. It
> seems individual knobs (WITH_LLVM_AR_IS_AR, WITH_LLVM_NM_IS_NM,
> WITH_LLVM_SYMBOLIZER_IS_ADDR2LINE etc.) will introduce extra
> complexity without adding much value.
>
> Alex is working on a patch now and will follow up shortly, but I
> wanted to email the list as a heads-up, and see if there are any
> comments or concerns.

Thanks for writing this up, I've posted the initial patch as
https://reviews.freebsd.org/D31060.


>
> Potential next steps are:
> - Introduce new build knob
> - Iterate on exp-runs and call for testing
> - Switch to LLVM tools by default
> - Major release (14.0)
> - Retire knob, leaving only the LLVM implementation.