On Mon, 19 Jan 2015, Maxim Kuvyrkov wrote: > The underlying problem is that the order in which elements of ready_list are > compared matters to the final result. This is because rank_for_schedule > sorting heuristic establishes a partial order on the set of instructions, > and it can happen that with 3 instructions A, B and C: A>B, B>C, C>A. In > this situation the order in which qsort compares the elements affects the > final result, it can be either ABC or BCA or CAB. > > Presence or absence of DEBUG_INSNs in the ready list can change the > comparison order, and cause slightly different instruction schedules.
Could you please provide a bit more detail here? Looking at rank_for_schedule, I see that it considers insn pairs when at least one of them is DEBUG_INSN, first. Debug insns are always moved to front, sorted within themselves by luid, and not considered in the rest of rank_for_schedule. I don't see how rank_for_schedule with DEBUG_INSN can arrive at a A < B < C < A situation. (btw, nitpicking, a "partial order" is when incomparable pairs are possible (i.e. none of A == B, A < B, B < A is true), a "total order" is when all pairs are comparable, and when you can have A < B < C < A, it's not a mathematical order relation) Thanks. Alexander
