[LLVMdev] unwind's permanent residence

[C Bergström]

On Tue, Feb 3, 2015 at 7:07 PM, Renato Golin <renato.golin at linaro.org>
wrote:

> On 30 January 2015 at 20:43, Jonathan Roelofs <jonathan at codesourcery.com>
> wrote:
> > Last time we brought this up, there was only partial consensus, and then
> > someone arbitrarily declared total consensus (without compelling
> arguments
> > in any particular direction) that we were going to move it to
> compiler_rt.
> > Then the discussion fell on the floor because no-one had time to
> actually do
> > the move. Please, let's not let that happen again this time.
>
> So, do we have a consensus?
>
> AFAICS, the most agree solution (with optionals to be defined):
>
> 1. Move Unwinder to its own repository in the LLVM server
> 2. Make the CMake connections from libc++abi and compiler-rt
>   2.1 OPTIONAL 1: err if libunwinder is not there, clang errs if rtlib=RT
>   2.2 OPTIONAL 2: warns if libunwind is not there, clang errs if rtlib=RT
>   2.3 OPTIONAL 3: nothing, make clang smarter to pick existing unwinder
> 3. Change clang to assume -lunwind when --rtlib=compiler-rt
>   3.1 OPTIONAL 4: allow linker error if no -lunwind / -lgcc_s
>   3.2 OPTIONAL 5: Add option to change unwinder library by not adding
> -lunwind/-lgcc_s, but whatever comes as argument
>
> 1, 2, and 3 must be changed.
>
> I vote for adding { 2.2, 3.1 } for now, { 2.2, 3.1, 3.2 } for later.
>
> My idea for 3.2 is something like --unwinder=libgcc_s / libunwind, or
> something like that.
>
> I personally don't think the front-end scanning existing libraries is
> a good thing to do, but I'm not against the idea, if anyone feels
> strongly about it.
>
> If all of us could agree to a common solution, and make sure all
> interested parties are in, we should do the move before 3.7.
>

I know I'm in some minority, but whatever ends up happening please be aware
that some platforms and targets aren't using the bundled unwind.

Specifically this non-gnu one (aka HP's libunwind permissively licensed)
www.nongnu.org/libunwind/download.html

In my experience when it's used in the compiler the name (libgcc, libeh,
libunwind) isn't always the same. (So searching for a particular lib name
as the unwinder is fragile)
----------
Summary: pretty please allow a cmake option like
UNWIND_LIB which can be null or something user defined. (I think this is
covered in one of the cases above, but just want to be clear)

Thanks for the minute
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