Yes I believe they're using a subset of LLVM for PNaCl. The whole idea is to define the semantics of how LLVM should be targeted there for it, since it normally does include some system specific things (how structs and things end up aligned when compiling C and the like). I think they're working on some "sane" defaults for all of those kinds of things so that they can target that subset with their compilers and then in theory run anywhere supported by LLVM. It's a very very interesting idea and if it does actually get anywhere it'll mean that there's a subset of LLVM that could be targeted by "native" applications for a similar effect, e.g. no need for a fat binary, just use the portable subset of llvm bytecode.