On Mon, Mar 10, 2014 at 7:33 PM, lin zuojian <manjian2...@gmail.com> wrote: > Hi, > I just ask for opinions.I think many GCC developers do familiar with > the opponent.If I ask in the LLVM mailing list, I have to worry > about If they are familiar with GCC, too(what's sched2 pass?).
I suspect you will have the same problem on both lists. The internal details of the scheduling system are not likely to be widely known honestly. To provide a very brief summary of what is going on in LLVM to the best of my knowledge (although I am not one of the experts on this area): The DAG (or more fully, the SelectionDAG) is not really relevant to scheduling any more[1]. It is just a mechanism used for legalization and combining the target-specific representation prior to producing very low level "MI" or Machine Instructions. That is, it is entirely an instruction selection tool, not a a scheduling tool. The scheduling takes place amongst the machine instructions either before or after register allocation (depending on the target) and with a wide variety of heuristics. My understanding is that it is attempting to solve the same fundamental scheduling problems as GCC's infrastructure (ILP and register pressure). The infrastructure is the bulk of it though, and that is likely entirely specific to the representation and innards of the respective compilers. Given that LLVM's machine level scheduler is significantly younger than GCC's, I would expect it to be less well tuned and have less complete and/or accurate modeling for some targets. -Chandler [1] Historically, we did both instruction selection and scheduling using the DAG, but the scheduling has moved into the MI layer specifically to address register pressure and ILP concerns that were hard/impossible to handle at a higher level. There is some hope that the DAG goes away completely and is replaced with some simpler selection and legalization framework, but that hasn't yet emerged.
