On Wed, Oct 17, 2018 at 10:25:21AM -0700, Yonghong Song wrote: > > > On 10/17/18 9:13 AM, Edward Cree wrote: > > On 17/10/18 08:23, Yonghong Song wrote: > >> This patch adds BTF_KIND_FUNC and BTF_KIND_FUNC_PROTO > >> support to the type section. BTF_KIND_FUNC_PROTO is used > >> to specify the type of a function pointer. With this, > >> BTF has a complete set of C types (except float). > >> > >> BTF_KIND_FUNC is used to specify the signature of a > >> defined subprogram. BTF_KIND_FUNC_PROTO can be referenced > >> by another type, e.g., a pointer type, and BTF_KIND_FUNC > >> type cannot be referenced by another type. > > Why are distinct kinds created for these? A function body is > > a value of function type, and since there's no way (in C) to > > declare a variable of function type (only pointer-to- > > function), any declaration of function type must necessarily > > be a BTF_KIND_FUNC, whereas any other reference to a function > > type (e.g. a declaration of type pointer to function type) > > must, as you state above, be a BTF_KIND_FUNC_PROTO. > > In fact, you can tell the difference just from name_off, since > > a (C-legal) BTF_KIND_FUNC_PROTO will always be anonymous (as > > the pointee of a pointer type), while a BTF_KIND_FUNC will > > have the name of the subprogram. > > What you stated is true, BTF_KIND_FUNC_PROTO corresponds to > dwarf subroutine tag which has no name while BTF_KIND_FUNC > must have a valid name. The original design is to have both > since they are corresponding to different dwarf constructs. > > Martin, what do you think? I prefer to have separate kinds. We need a way to distinguish them. For example, the BTF verifier is checking it. Having two kinds is cleaner instead of resorting to other hints from 'struct btf_type'. We don't lack of bits for kind.
> > > > > -Ed > >
