[LLVMdev] The Trouble with Triples

[David Chisnall]

On 9 Jul 2015, at 10:45, Renato Golin <renato.golin at linaro.org> wrote:
> 
> On 9 July 2015 at 10:39, David Chisnall <David.Chisnall at cl.cam.ac.uk> wrote:
>> The use-case that I’d really like to go from mostly-working to actually-working is the ability to create symlinked versions of clang with a triple prefix and have it behave sensibly.  We can symlink clang to mips64-unknown-freebsd-clang and get a working cross-compiler, more or less, except that we also want to specify things like the default sysroot.  Having the bit in the name of the compiler just be a name for a config file containing a set of command-line options would be very nice - we’d have a set of predefined names, and then if someone wanted to provide a androidsdk-v47-arm.conf (or whatever) and just drop it into a known location then they’d be able to use androidsdk-v47-arm-clang as a cross compiler.
> 
> This already works well with Clang, but is restricted to the triples
> that actually make sense. If you need to change anything that the
> triple can't, you're on your own.

Right, the problem is that the triple almost never contains enough information for a cross-compile (which, at a minimum, needs to know where the default sysroot is, where to find cross-linkers, and may also need to target a specific CPU variant or turn on soft float).  It works moderately well for trivial cases (e.g. targeting x86 Linux from x86-64 Linux, and even targeting ARM Linux from x86-64 Linux, as long as ARM Linux doesn’t mean Android or WebOS or some flavour of Linux that’s different from the host).

I would really like to completely remove the triple as something that can be decomposed into meaningful components and just have it become a name, which is only used by the front end to identify a configuration.

David