I tried a CU with an int variable instead of float, and enough other stuff to keep it from being optimized away completely. gcc produced DW_OP_reg3, while Clang produced "DW_OP_reg3; DW_OP_piece 4" for the same variable.
DW_OP_piece is really meant for assembling partial values in different places, not subdividing a register. --paulr > -----Original Message----- > From: Dwarf-Discuss [mailto:dwarf-discuss-boun...@lists.dwarfstd.org] On > Behalf Of Adrian Prantl > Sent: Friday, March 04, 2016 1:13 PM > To: dwarf-discuss@lists.dwarfstd.org > Subject: [Dwarf-Discuss] Representing the location of smaller values in > vector registers > > 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? > > -- adrian > _______________________________________________ > Dwarf-Discuss mailing list > Dwarf-Discuss@lists.dwarfstd.org > http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org _______________________________________________ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
