Common Lisp is not just "a lisp". I can probably count on the fingers of one hand the number of Common Lisp compilers I've seen in my life. I can write "a lisp" in a couple dozen lines of code, but my copy of CLTL2 on the shelf is about 2 inches thick.
I'm not saying GPU offload is easy. As you point out, no general-purpose language today does that (and LLVM doesn't seem to offer it as a feature), so you'd have to implement that either way. The choice is "new compiler + GPU target" versus "mature portable compiler + GPU target". Why is the latter "not realistic"?
I find it hard to believe that the architecture of every (free) compiler is so horribly suited to this that we need to start from scratch. We've forked CL compilers many times, for new architectures, but I've never seen any architecture that required starting over.
I'm not saying GPU offload is easy. As you point out, no general-purpose language today does that (and LLVM doesn't seem to offer it as a feature), so you'd have to implement that either way. The choice is "new compiler + GPU target" versus "mature portable compiler + GPU target". Why is the latter "not realistic"?
I find it hard to believe that the architecture of every (free) compiler is so horribly suited to this that we need to start from scratch. We've forked CL compilers many times, for new architectures, but I've never seen any architecture that required starting over.