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a=ALHzrhD1WqeLMmyw1pUA:9 a=yp-Zx5YoBjEA:10 a=4AvGVvso5qoc5vnZhopw:22 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)