[cfe-dev] Small patches to allow fully independent clang/llvm/compiler-rt/libc++

[Renato Golin via cfe-dev]

On 15 October 2015 at 12:05, Daniel Sanders <Daniel.Sanders at imgtec.com> wrote:
> I can see the overlap between this and that. If I understand correctly, it's trying to take a JIT's ability to know exactly what the target hardware is and use that to drive normal compilation. For my case, I want the target to be what the distribution wants it to be (which is often a subset of the actual hardware) but both cases require the same ability to modify the target based on a configuration.

It's actually both. And safe by being an external prototype first.

The first stage is to understand the host/target machines and create a
description (CFLAGS). The second stage is to turn that into a text
database (JSON) so that people can add their own by hand *in addition*
to automatically detect it.

So, in your case, if Clang would understand such database, you could
create fedora-mips-linux-gnu and debian-mips-linux-gnu configuration
*names* (not triples) and use that to set all the flags that make
sense for each.

I believe we have to try very hard to keep compatibility with CFLAGS
even after Clang understands the database because of other compilers.
Once we have meaning inside the database that cannot be represented by
flags, people will be locked in, and I want to avoid that at all
costs. Even if the flags are ugly, they have to be standard. The
ugliness will be hidden by the database anyway.

So, a proposed format would be something like:

[
  { "name": "fedora-mips-linux-gnu",
    "cflags": "-march=mips -mcpu=whatever --blah",
    "arch": "arch",
    "cpu": "cpu",
    "endian": "blah"
  },
]

So, we always keep the cflags / cxxflags, even when all the other
parameters made it redundant. The tool also has to be able to produce
a CFLAGS from the other parameters and update the database. This way,
it would be ludicrously simple to write a script for GCC to use this
without asking them to implement anything on their front-end. We can
also easily add more cflags just for gcc, icc, pcc, armcc, etc.

cheers,
--renato