[Dwarf-Discuss] Representing the location of smaller values in vector registers

Cary Coutant ccoutant at gmail.com
Wed Mar 9 14:53:40 PST 2016

> A value which is contained in a register larger than the size of
> the value should be described by a DW_TAG_base_type.  (DWARF 4,
> Sect. 5.1, bottom of page 75).
> For a 32-bit float in a 128-bit container, use something like the following:
>   DW_TAG_base_type
>     DW_AT_name "float"
>     DW_AT_encoding DW_ATE_float
>     DW_AT_byte_size 16
>     DW_AT_bit_size 32
>     DW_AT_data_bit_offset 0
> Note that there is no requirement that the names on base types
> be unique.  There may be more than one with the name "float".

Sorry, Michael, but you're just muddying the waters. The
data_bit_offset attribute is used only when "the value of an object of
the given type does not fully occupy the storage described by a byte
size attribute." That is not the case here -- the original question
was about a value of a type that does not fully occupy a register,
which is a totally different question. We're just not going to expect
(nor is it even physically possible in the DWARF representation) that
the producer invent a different base type for each possible location
where a variable might be stored. Whether it's a single location or a
location list is irrelevant to the question, but the possibility that
there may be a location list is what exposes the impracticality of
your suggestion -- a variable can have only a single type, regardless
of how many location list entries it has.

I think the rest of us agree that the proper answer is that the ABI
defines how a value is stored in a register when the value is smaller
than the register, and that clang is producing good DWARF for this
example. Whether it's a 32-bit float in a 256-bit xmm register or an
8-bit char in a 64-bit general register, we let the ABI define the
representation. In most cases, it's a natural representation that just
falls out from the way the instruction set works, and no one really
needs to give it much thought (e.g., store the value in the low-order
bits of the register, possibly with zero- or sign-extension). In other
cases, like some floating-point register files, a value is actually
converted to a wider type when loaded into a register. At any rate, we
have never attempted to describe those rules in the DWARF, and it
would make little sense to try.

The source of the original uncertainty about representation of a
smaller value in a larger register is probably that the DWARF spec
*does* explicitly state, for DW_OP_piece, if "the piece is located in
a register, but does not occupy the entire register, the placement of
the piece within that register is defined by the ABI." But we make no
such similar claim for simple register location descriptions. That led
Adrian to wonder if he should be using DW_OP_piece. I think it's an
oversight, easily clarified.


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