[LLVMdev] Enabling MI Scheduler on x86 (was Experimental Evaluation of the Schedulers in LLVM 3.3)

[Andrew Trick]

In my last message, I explained the goals of the generic MI scheduler and current status. This week, I'll see if we can enable MI scheduling by default for x86. I'm not sure which flags you're using to test it now. But by making it default and enabling the corresponding coalescer changes, we can be confident that benchmarking efforts are improving on the same baseline. At that point, I expect bugs to be filed for specific instances of badly scheduled code. Getting a fix committed may not be easy, because we have to show that new heuristics aren't likely to pessimize other code. But at least I'll be able to provide an explanation for why MI isn't currently handling it.

There are other reasons that MI sched should be enabled now on x86 anyway:

(1) The Selection DAG scheduler will be disabled as soon as I can implement a complete replacement. That should eliminate about 10% of codegen (llc) compile time. The Selection DAG scheduler has also long suffered from unnacceptable worst-cast compile time behavior and unresolved defects. We been chipping away at the problems, but some remain: PR15941, PR16365. This is a fundamentally bad place to perform scheduling.

(2) The postRA scheduler will also be eliminated. That will eliminate another 10% of compile time for targets that currently enable it. It also eliminates a maintenance problem because its dependence on kill flags and implicit operands is frightening--these can easily be valid for some targets but not others.

(3) Non-x86 targets have been using MI sched for the past year to achieve important performance and compile time benefits. For quality and maintenance reasons, we should use the same scheduling infrastructure for mainstream targets.

The basic theme here is that we want a single scheduling infrastructure that is efficient enough to enable by default--even if it is typically performance-neutral, can leverage verification across many targets, and can be safely customized by plugging in heuristics.

-Andy