Hi All: I'm studying instruction scheduling of gcc. there are some words about default_lantency in gcc internal document , which I don't understand. the words are : ---------- There is an important difference between the old description and the automaton based pipeline description. The latency time is used for all dependencies when we use the old description. In the automaton based pipeline description, the given latency time is only used for true dependencies. The cost of anti-dependencies is always zero and the cost of output dependencies is the difference between latency times of the producing and consuming insns (if the diference isnegative, the cost is considered to be zero). ---------- I am puzzled about why cost of anti-dependencies is zero. does it have any reason or advantage? Further more, the mips target defined macro TARGET_SCHED_ADJUST_COST as mips_adjust_cost, which assumes output-dependencies have no cost. Now think about a tiny basic block like: insn1 : set1 r1, r2 insn2 : set2 r1, r3 insn3 : set3 r4, r1 ----- the forward dependencies are like: output-dep : <insn1, insn2> true-dep : <insn1, insn3>, <insn2, insn3> ------- because the cost of output-dep is zero on mips, function insn_cost1 returns 0. finally costs of insn computed by function priority will be: cost(insn3) = defalut_latency(insn3) cost(insn2) = default_latency(insn2) + cost(insn3) cost(insn1) = default_latency(insn1) + cost(insn3) ------- Now the scheduling order of insn1 and insn2 is depends on default_lantency(insn1) > default_latency(insn2) or not. there must be something wrong in my understanding, pelase help and any tip will be appreciated. -- Best Regards.
