Re: Pahole, BTF, and vector types

[Nick Alcock <nick.alcock@oracle.com>]

On 14 May 2025, Alan Maguire told this:

> On 13/05/2025 23:39, Bruce McCulloch wrote:
>> Hello Alan, Arnaldo, All,
>> 
>> Recently the Linux Toolchain team at oracle had reason to start emitting
>> types
>> with the vector_size() attribute as arrays when compiling with -gctf/-
>> gbtf. This
>> is a change from the previous behavior, which was to emit these as unknown
>> types. This emission as an array is consistent with BTF generated in
>> both clang
>> and pahole. We have some ideas on how to implement the change in GCC,
>> but were
>> wondering if anyone involved with dwarves had any insight into why
>> vectors are
>> emitted in this way. More specifically, there were concerns surrounding the
>> practical differences between vectors and arrays in the realm of memory
>> (e.g.
>> Vectors can be passed as args by value, while arrays cannot). More
>> information
>> about this can be found on the DWARF std issues list [1].
>> 
>> Any input on this subject would be much appreciated.
>> 
>> Bruce
>> 
>> [1] https://dwarfstd.org/issues/230413.1.html
>> 
>
>
> As far as I can see in dwarf_loader.c we do record that an array type is
> a vector, noting that struct array_type specifies true for its
> "is_vector" field if the DIE has a DW_AT_GNU_vector attribute, but this
> information is not used during BTF encoding; they are as you say just
> treated like normal arrays.
>
> So we do have the information available to potentially do something
> different for vectors at BTF encoding time. Did you have a specific
> representation in mind? Thanks!

We were thinking of simply turning on the kind_flag in the
BTF_KIND_ARRAY for such arrays-that-are-actually-vectors: it's not used
for anything else as yet, and that means we don't need to add something
to the vlen (which would of course be a compatibility break, though BTF
seems to care relatively little about such things). Adding a new meaning
for kind_flag 1 ("is a vector") is relatively compatibility-concern
free.

(I'm assuming that being a vector is a one-bit decision: vectors don't
seem to have any other properties beyond those arrays have in the DWARF
representation nor in GCC nor clang, that I can see.)

-- 
NULL && (void)