Re: [ANNOUNCE] LLVM backend for Sparse

[Pekka Enberg <penberg@kernel.org>]

On Sun, Aug 28, 2011 at 7:01 AM, Pekka Enberg <penberg@kernel.org> wrote:
>> It's lack of PSEUDO_SYM support. I haven't quite figured out what kind of
>> code to generate for that because, frankly, I'm not completely sure what
>> it's all about. ;-)

On Sun, Aug 28, 2011 at 7:54 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> So "PSEUDO_SYM" is just a "link-time constant".
>
> IOW, it's nothing but a pointer to an in-memory variable, and from a
> code generation standpoint generating the symbol should be just about
> the same as generating a constant, except rather than an actual value,
> it's now a linker reference.
>
> Of course, what complicates them is that you also need to generate the
> symbol definition itself (somewhere else). In particular, the
> sym->initializer points to the initializer of a symbol, and they can
> be *complicated*. Generating the output for some symbols can be a
> major pain in the *ss, just think about a complex structure array
> initializer.
>
> The PSEUDO_SYM you hit for the "hello world" program is trivial,
> though. It's an unnamed symbol of type "char []", and it has a trivial
> initializer (the string itself). So symbols can be hard to generate,
> but there are simple cases.
>
> So for a PSUEDO_SYM, you need to look up "struct symbol" for it
> (pseudo->sym), which then has:
>
>  - name of the symbol (sym->ident). Of course, some symbols don't have
> names, like the constant string example.
>
>  - type/size/alignment information (sym->ctype)
>
>  - initializer information (sym->initializer - which can be a really
> complex expression). Of course, for external symbols - or symbols with
> no initializer - this is just empty.
>
> There's way more to symbol types in sparse than that, but apart from
> the initializer expression, most of the symbol complexity has been
> handled by earlier stages (ie a *lot* of the front-end of sparse is
> about symbols and their types).

Right. Will the "link time constant" 'struct symbol' be part of the
symbol_list? If it is, then we already do LLVMAddGlobal in
output_data() on it and could probably just stash that into a ->priv
member in 'struct symbol' and use that in pseudo_to_value().
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