[Dwarf-discuss] ISSUE: vector types. V2

[Ben Woodard]

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.

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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

--------------------------------------------------------------------
     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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