[LLVMdev] Pre-Allocation Schedulers in LLVM (original) (raw)
Ghassan Shobaki ghassan_shobaki at yahoo.com
Sat Sep 17 10:07:07 PDT 2011
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Hi,
I am currently writing a paper documenting a research project that we have done on pre-allocation instruction scheduling to balance ILP and register pressure. In the paper we compare the pre-allocation scheduler that we have developed to LLVM's default schedulers for two targets: x86-64 and x86-32. We would like to include in our paper some brief descriptions of the two LLVM schedulers that we are comparing against and some information about the machine model that they are scheduling for. So, it would be great if you could confirm or correct the following information and answer my questions below:
The default scheduler for the x86-32 target is the bottom-up register-pressure reduction (BURR) scheduler, while for the x86-64 target it is the ILP Scheduler. According to the brief documentation in the source file ScheduleDAGRRList, the BURR is a register pressure reduction scheduler, while the ILP is a register-pressure aware scheduler that tries to balance ILP and register pressure.
My questions are:
-Are there any references (such as published research) that describe each/any of these scheduling algorithms?
By examining the source code, it appears that neither scheduler has a machine model describing the functional units and the mapping of instructions to functional units on the target x86 machine. Is that right?
Based on the test cases that I have analyzed, it looks that the BURR scheduler sets all latencies to 1, which essentially eliminates any scheduling for ILP and makes scheduling for register pressure reduction the only objective of this scheduler. Can you please confirm or correct this?
Again based on analyzing test cases, it appears that the ILP scheduler sets the latencies of DIV and SQRT (both INT and FP) to 10, while the latencies of all other instructions are set to 10. Can you please confirm or correct this observation?
Apparently, the developers of the ILP scheduler assumed that this rough latency model would be sufficient to do ILP scheduling on the x86 target, because the x86 hardware has a good dynamic scheduler. Our testing, however, shows that this is the case for most but not all programs. For one particular benchmark with a high-degree of ILP, using more precise latency info significantly improved performance. Will the LLVM developers be interested in adding more precise latency info for the x86 target?
Thank you in advance!
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