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

Michael Eager eager at eagercon.com
Wed Mar 9 09:57:21 PST 2016


On 03/09/2016 09:33 AM, Jakub Jelinek wrote:
> On Wed, Mar 09, 2016 at 08:36:44AM -0800, Michael Eager wrote:
>> How is the "float" type described?
>>
>> 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".
>
> I don't think this can really work.  A particular variable (etc., whatever
> has type) can have just one type, while whether it lives in a larger
> container or not really depends on where the value lives.
> The same variable could live in one .debug_loc range in memory
> (where it certainly can't pretend it lives in a larger container, because it
> does not, it is 32-bit there), or when in some other range it lives in
> a general purpose register (which can be 32-bit or 64-bit and either fits in
> there exactly, or is passed in the low bits of that), or in %xmm* register
> (128-bit container, but, actually sometimes 256-bit and other times 512-bit
> container).  I'd say LLDB is just broken to complain about this.  The ABI
> specifies what it means when a float lives in memory, or in GPRs, or in
> XMM/YMM/ZMM registers.

My comment was based on the DWARF in Adrian's email, which describes
a 32-bit floating point variable allocated to a 128-bit register.  Adrian's
DWARF doesn't describe a 32-bit variable in memory which is loaded into a
larger register during the course of expression evaluation, which would
be described by a location list.

A 32-variable stored in a 32-bit memory location may have a location list
which specifies that the value may be found in particular registers (either
larger or smaller) during it's lifetime.  That would be a different description
in DWARF.


-- 
Michael Eager	 eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077


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