On Fri, Dec 09 2016, Adrian Prantl wrote: > Here's my take on this.
Thanks! >> On Dec 9, 2016, at 11:11 AM, Andreas Arnez <ar...@linux.vnet.ibm.com> wrote: >> >> Although I've already created public comments for (most of) this, >> Michael Eager suggested that I post my questions regarding DWARF pieces >> on this list (again). >> >> All of these questions are related to the definition of DW_OP_piece and >> DW_OP_bit_piece: >> >> * Is "DW_OP_piece(n)" equivalent to "DW_OP_bit_piece(8*n, 0)"? > > I would think the answer to this is yes, and if it isn't I would > really like to see the counterexample added to the Appendix. Right, I think clarification would be appropriate either way. Does anybody vote for "no"? >> * What is the correct placement rule for register pieces? ABI-defined >> or lsb0-ordered? If lsb0-ordered, how exactly is that order defined >> for non-integer registers? > > "If the location is a register, the offset is from the least > significant bit end of the register." > Does this text from the latest draft answer the question? > If not, can you give an example where it isn't? The definition of DW_OP_piece differs, though: "[...] the placement of the piece within that register is defined by the ABI." >> * Shall DW_OP_bit_piece ignore the offset for stack- and implicit >> values? > > "If the location is any implicit value or stack value, the > DW_OP_bit_piece operation describes a sequence of bits using the least > significant bits of that value." > > No. Really? IMHO the wording implies that the least significant bit always belongs to the piece. >> * What is the "least significant bit" of an implicit value? And is it >> intentional that the placement rule differs between implicit- and >> memory values? > > I don't know. > >> >> * Some DWARF producers describe a register pair just with a single >> DW_OP_reg<n>. Can a register piece reach into the next register like >> that as well? > > You mean for a pair of r1,r2 they would emit only DW_OP_reg1? I don't > think this use is supported by the DWARF specification. OK. Are there other opinions? >> * Is "DW_OP_reg*" followed by "DW_OP_piece(size of object type)" always >> equivalent to omitting the piece operation? > > If sizeof(obj) == sizeof(reg) it is obviously. > If sizeof(obj) > sizeof(reg) the expression is malformed because there > are undefined bits in the object that should have been represented > with an empty DW_OP_piece. GCC may describe a long double struct member on x86 as DW_OP_regx: 33 (st0); DW_OP_piece: 16 where st0 is only 10 bytes wide. Is that invalid then? > If sizeof(obj) < sizeof(reg) the expression is overspecified, but > still sound, so I would say yes. So you'd say the placement rules for DW_OP_reg* and DW_OP_piece shall be compatible, both defined by the ABI -- not necessarily lsb0-ordered? And the bytes referred to by a "DW_OP_reg*"-location may depend on the object's size, but not its type, right? Note that this wouldn't work well with cases like the "SPU preferred slots"; see https://sourceware.org/ml/gdb/2016-01/msg00013.html, section 2.2. -- Andreas _______________________________________________ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
