[LLVMdev] MI-Sched temporarily enabled on x86.

[Andrew Trick]

I'm briefly enabling the MachineScheduler pass for x86 tonight to collect information on any failures that may show up.

To properly enable the new scheduler, this hook is implemented in X86Subtarget:

  bool enableMachineScheduler() const LLVM_OVERRIDE { return true; }

The MachineScheduler pass itself can be enabled/disabled with -enable-misched=true/false. But most of the codegen changes result rather from moving to source-order SD scheduling and subsequent register coalescing decisions. This makes it extremely challenging to keep the unit tests functioning during the transition. So you'll see unit test churn when I flip the switch.

Later, sometime in July, I'll plan to make the permanent change for X86, with all unit tests switched over to source order scheduling. (I have some compile-time improvements to make first). Changing the default for ARM will follow shortly thereafter.

-Andy