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

Adrian Prantl aprantl at apple.com
Fri Mar 4 13:12:39 PST 2016

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_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?

-- adrian

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