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:
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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