[OE-core] RFC: Secondary Toolchain

[Khem Raj]

On Thu, Oct 4, 2012 at 11:02 AM, Mark Hatle <mark.hatle at windriver.com> wrote:
> We have an issue where we'd like to have an alternative toolchain
> (assembler, linker, compiler) available for our customers to selectively
> use.  However, before we just go off and implement something, I'd like at
> least some basic consensus on what the best practice is for doing this work.
> Below is my attempt at an early proposal.
>
> Background
> ----------------
>
> Many companies have commercial / highly optimized toolchains for specific
> purpose, such as ICC from Intel, LLVM, ARM specific toolchain, etc..  For
> (potentially) better performance with some applications a mechanism to both
> provide the hooks for the alternative toolchain as well as a way to active
> it per-package is desired.
>
> This work assumes that the third party toolchain is generally compatible
> with the idea of sysroots, linking to the libc provided by OE, and generally
> compatible with GNU conventions.
>
> However, as part of the third party toolchain, it may not be GNU compatible.
> This means many Open Source applications simply may not work with this
> toolchain.  That means that we need to have a way for a toolchain to
> blacklist (or whitelist) specific recipes.  This way only supported
> components can be built by the user, avoiding numerous complaints.
>
> Currently OE has a method to generate an SDK based on the GNU toolchain.  We
> would like to be able to also export the external toolchain along with the
> SDK, effectively providing both the GNU toolchain and the third party
> toolchain using the common sysroot.
>
> We need a way to active the third party toolchain on a per-package basis.

Per package bases is a bit iffy from my POV if you consider the compiler runtime
and C library runtime and so forth. Is it intended to just compile and
be able to
cope with the default runtimes on images or do you also intend to figure out
ways for multiple runtimes to live together.

> This activation will need to use the existing sysroot, but be able to pass
> different C, C++, LD, AS and other flags as specified by the third party
> toolchain.

that sort of answers the above. So we are assuming that toolchain that
can interoperate
will be using GNU runtime

There are cases where you might want something like ELLCC which avoids
complete GPL runtime e.g.
I think it could be divided into runtime  + tools where runtime lives
in a different layer but
essentially offering to let people use builtin C library and runtime
may not be a bad choice


>
> Finally third party toolchains should be implemented as layers that can
> easily plug into OE.

agreed. I think we need to document the interfaces in OE-Core for this
too so that
people can rely on the interface. Otherwise it will be constant pain
for keeping those
layers working together.

>
> Implementation
> ---------------------
>
> Add an OVERRIDE to specify the alternative toolchain.  Can this be done
> within the layer by doing something like:
>
> OVERRIDE_append = ":toolchain-${TOOLCHAIN}"
>
> TOOLCHAIN = "gnu" (or "icc")
>
> To activate the toolchain you would use things like:
>
> TOOLCHAIN_pn-bash = 'icc'
>
> To define the correct behavior for the toolchain, the following would need
> to be defined (with _toolchain-<toolchain>):
>
> TARGET_CPPFLAGS
> TARGET_CFLAGS
> TARGET_CXXFLAGS
> TARGET_LDFLAGS
> CC
> CXX
> F77
> CPP
> LD
> CCLD
> AR
> AS
> RANLIB
> STRIP
> OBJCOPY
> OBJDUMP
> NM
> FULL_OPTIMIZATIONS
> DEBUG_OPTIMIZATION
>
> Is anyone aware of any other items that may require additional items?  Will
> the above actually work?  Using the override of the TOOLCHAIN_… will that
> actually change the override values or do we get stuck?
>
> Comments/suggestions appreciated!
> --Mark
>
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