> On Mar 9, 2016, at 8:36 AM, Michael Eager <ea...@eagercon.com> wrote:
>
> On 03/04/2016 01:12 PM, Adrian Prantl wrote:
>> I have a best-practice kind of question:
>>
>> The x86_64 System V ABI passes floating point values in the xmm0, xmm1, ...
>> 128-bit SSE vector registers. I’m wondering what the correct DWARF v4
>> (lacking the DWARF 5 type conversions) expression for a 32-bit float in xmm0
>> is.
>>
>> Given the following program:
>>
>> float return_float() __attribute__((noinline)) { return 3.14f; }
>>
>> int main(int argc, char** argv) {
>> float f = return_float();
>> return (int)f;
>> }
>>
>> Clang (-O1) currently produces the following DWARF for the variable f:
>>
>> DW_TAG_variable
>> DW_AT_location( reg17 ) // xmm0 = 17
>> DW_AT_name( "f" )
>> DW_AT_decl_file( "test.c" )
>> DW_AT_decl_line( 8 )
>> DW_AT_type( {0x0000004a} ( float ) )
>>
>> Which causes LLDB to complain that the size of register xmm0 (128 Bits) is
>> different from the size of f (32 Bits). I wonder who is wrong? Is LLDB being
>> too strict, or should clang actually emit something like DW_OP_reg 17 (xmm0)
>> DW_OP_piece 4 to make it abundantly clear that only the lower 32 bits of the
>> vector register are interesting?
>
> 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:
Shouldn’t the container be orthogonal to the base type? How would this work
with location lists (the same variable might be once in a 128-bit container,
then spilled onto the stack, or a constant)?
-- adrian
>
> 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".
>
>
> --
> Michael Eager ea...@eagercon.com
> 1960 Park Blvd., Palo Alto, CA 94306 650-325-8077
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