[llvm-dev] [RFC] Strategies for Bootstrapping Compiler-RT builtins

[Martell Malone via llvm-dev]

>
> Sure. I’m not sure if everyone will agree with me, but my general
> assertion is that if you’re doing a full bootstrap of a cross compiler,
> we’re not going to fully support that from a single CMake build tree. We
> didn’t support that in autoconf, so I don’t think CMake should be different
> in that regard. I envision bootstrapping a full cross-platform to be
> something like:
> (1) build in-tree clang
> (2) build out-of-tree builtin library
> (3) build out-of-tree runtime libraries


This seems reasonable to me.
The default int-tree stuff should be for the host.
I don't think many will have issue with doing out of tree builds or the
builtins or the runtime if they are cross compiling.

Today there isn’t really a simple answer. For Linux I think the bootstrap
> only builds for the host architecture by default. For Darwin, it builds all
> supported Darwin architectures, which can be complicated to determine.

Not quite sure why IOS is built by default on darwin it should probably be
OSX only and out of tree for IOS like you said above.
Doubt many will agree with changing this now though.

> This might be mission creep, but it's a drag that the unspoken first
> > step in developing an LLVM based toolchain is "port binutils".  I
> > haven't kept watch on LLD progress, but perhaps it's far enough along
> > that bootstrap process can depend on it.
> On Darwin ar and lld are the biggest pieces that aren’t fully featured
> yet.


Yes Darwin seems to be the biggest blocker here with the least amount of
work gone into it.
The COFF and ELF linkers have been replaced with section based ones as that
makes more sense.
I don't know enough about MachO to say if would be better with switching or
staying with an atom based design.
It would be great if maybe an apple engineer more in the know would
nominate them selves and bring it up to par with the rest and then maybe become
an OWNER.

On other platforms I think there are still places that lld isn’t fully
> fleshed.

Yes there are some parts still missing and that are wip but the new COFF
and ELF section based linkers should bootstrap just fine.

Just as a point for building the builtins shouldn't we just need llvm-ar ?
While not being feasible for the Darwin target as you said above it should
be perfectly fine for windows and linux.
I don't think we should even need the linker to bootstrap the builtins or
am I incorrect in saying this ?
If so I think it think it would be reasonable to have that as the mission
creep.

On Tue, Nov 3, 2015 at 12:03 AM, Chris Bieneman via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> > On Nov 2, 2015, at 2:44 PM, Steve King <steve at metrokings.com> wrote:
> >
> > On Mon, Nov 2, 2015 at 1:02 PM, Chris Bieneman <beanz at apple.com> wrote:
> >> Sadly, I believe there are licensing reasons why the builtins can’t be
> in the LLVM repo.
> >
> > Repos and licenses are orthogonal, but I get the concern.
> >
> > Switching gears to other questions:
> > Should the bootstrap build automatically produce a built-ins library
> > for each target in "llvm-config --targets-built" or is the developer
> > expected to provide an explicit list?  Hopefully the former.
>
> Today there isn’t really a simple answer. For Linux I think the bootstrap
> only builds for the host architecture by default. For Darwin, it builds all
> supported Darwin architectures, which can be complicated to determine.
>
> >
> > Is it reasonable that bootstrap build not depend on GNU binutils?
>
> Not necessarily. To my understanding LLVM doesn’t fully replace bin utils
> on any platform yet, and I think we’re a long way from saying the LLVM
> toolchain is the default builtin toolchain.
>
> > This might be mission creep, but it's a drag that the unspoken first
> > step in developing an LLVM based toolchain is "port binutils".  I
> > haven't kept watch on LLD progress, but perhaps it's far enough along
> > that bootstrap process can depend on it.
>
> On Darwin ar and lld are the biggest pieces that aren’t fully featured
> yet. On other platforms I think there are still places that lld isn’t fully
> fleshed.
>
> -Chris
>
> >
> > Regards,
> > -steve
>
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