[Dwarf-discuss] ISSUE: vector types. V2

Keith Walker Keith.Walker@arm.com
Thu Apr 6 08:35:23 GMT 2023


Comment about DW_AT_variable_vector_width below.


> -----Original Message-----
> From: Dwarf-discuss <dwarf-discuss-
> bounces+keith.walker=arm.com@lists.dwarfstd.org> On Behalf Of Ben
> Woodard via Dwarf-discuss
> Sent: Thursday, April 6, 2023 3:17 AM
> To: dwarf-discuss@lists.dwarfstd.org
> Subject: [Dwarf-discuss] ISSUE: vector types. V2
> This is version 2 of my vector types submission.
> Differences from V1:
> - Made the submission about vector types rather than vector registers.
> - Substituted Pedro's much better introduction for my own with minor edits.
> - Removed the modifications to the DWARF Stack. The AMD people like Pedro
> and Tony have been working on vector registers and vector types for a long
> time and they do not know of a good reason why we need the capability to a
> vector register on the DWARF stack. So I'm dropping that part. Furthermore, I
> could not find any evidence in code that uses the vector types that they
> needed to put vector types on the DWARF stack for locations.
> - Removed the variable vector length attribute. This was not existing behavior.
> I'm fairly certain something like that is going to be necessary but it doesn't
> need to be in this submission.
> ----------------------------------------------------------------------------------------------
> ------
> Vector types
> Some languages support vector data types, which are not possible to
> represent today in standard DWARF.  A vector is an array of values.
> These can be allocated to a SIMD vector register, if available, either
> permanently or temporarily, and operations on vectors make use of SIMD
> instructions, again if available.
> For example, as an extension to C and C++, GCC supports defining vector data
> types as described here:
>    https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html
> In this C/C++ extension, vector types are similar to arrays, and you can index
> and initialize them similarly, but they have some important differences.  For
> example:
> - C arrays automatically decay to pointers.  Vector types do not.
> - Vector types can be passed by value to functions, and likewise
>    functions can return vector types by value. Neither of which can be
>    done with C arrays.
> - Vector types can be used with a subset of normal C operations: +, -,
>    *, /, unary minus, ^, |, &, ~, %.  For example, addition is defined as
>    the addition of the corresponding elements of the operands.
> A debugger that supports C/C++ expression evaluation will want to be able to
> support these vector operations on vector objects too.
> Vector types appear on function prototypes, so they have their own mangling
> scheme in the Itanium ABI.
> Other vendors have similar C/C++ extensions.  For example, Motorola’s Altivec
> C/C++ Language Extensions, which predates GCC's extensions.
> To distinguish these vector types from regular C arrays, GCC's DWARF
> describes a vector type as an array with the DW_AT_GNU_vector
> attribute.  Clang also supports the GCC vector extensions, and describes the
> vector types in DWARF using the same attribute as GCC.
> Support for this DWARF extension has been implemented in GDB for well over
> a decade.
> Other languages have support for vector types, with similar ABI and/or API
> implications, and so DW_AT_GNU_vector is also used for languages beyond
> C/C++ today.
> This proposal standardizes the existing behavior.
> -------------------------------------------------
> In Section 2.2 Attribute Types, DW_AT_vector and
> DW_AT_variable_vector_width shall be added to Table 2.2

[Keith Walker] DW_AT_variable_vector_width is stated to be added to the table but then there is no definition what this attribute means or to what tag entries it should be applied.

There should also be mention on which entries the added attributes should be added in Appendix A (may be implicit from the current proposal, but the proposal should be explicit)

> --------------------------------------------------------------------
>      DW_AT_vector                | A hardware vector type
> --------------------------------------------------------------------
> The hyperlink in the "Identifies or Specifies" column shall point to the
> paragraph added to Section 5.5 below for DW_AT_vector.
> In Section 5.5 Array Type Entries, replace first paragraph of non-normative
> text with:
> --------------------------------------------------------------------
>      [non-normative] Many languages share the concept of an “array,”
>      which is a table of components of identical type. Furthermore,
>      many architectures contain vector types which mirror the
>      language concept of an array.
> --------------------------------------------------------------------
> Insert the following paragraph between the first paragraph of normative text
> describing DW_TAG_array_type and the second paragraph dealing with
> multidimensional ordering.
> --------------------------------------------------------------------
>      An array type that refers to a vector type, shall be denoted with
>      DW_AT_vector. The the width of the type shall be specified as an
>      array dimension and the type contained within the register must be
>      a DW_TAG_base_type entry.
> --------------------------------------------------------------------
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