[MachineScheduler] Optional scheduling of single-MI regions (#129704)

Following 15e295d the machine scheduler no longer filters-out single-MI
regions when emitting regions to schedule. While this has no functional
impact at the moment, it generally has a negative compile-time impact
(see #128739).

Since all targets but AMDGPU do not care for this behavior, this
introduces an off-by-default flag to `ScheduleDAGInstrs` to control
whether such regions are going to be scheduled, effectively reverting
15e295d for all targets but AMDGPU (currently the only target enabling
this flag).

1 files changed, 4 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/MachineScheduler.cpp b/llvm/lib/CodeGen/MachineScheduler.cpp
index 0c0c616..2a790a2 100644
--- a/llvm/lib/CodeGen/MachineScheduler.cpp
+++ b/llvm/lib/CodeGen/MachineScheduler.cpp
@@ -769,6 +769,7 @@ void MachineSchedulerBase::scheduleRegions(ScheduleDAGInstrs &Scheduler,
 
     MBBRegionsVector MBBRegions;
     getSchedRegions(&*MBB, MBBRegions, Scheduler.doMBBSchedRegionsTopDown());
+    bool ScheduleSingleMI = Scheduler.shouldScheduleSingleMIRegions();
     for (const SchedRegion &R : MBBRegions) {
       MachineBasicBlock::iterator I = R.RegionBegin;
       MachineBasicBlock::iterator RegionEnd = R.RegionEnd;
@@ -778,11 +779,9 @@ void MachineSchedulerBase::scheduleRegions(ScheduleDAGInstrs &Scheduler,
       // it. Perhaps it still needs to be bundled.
       Scheduler.enterRegion(&*MBB, I, RegionEnd, NumRegionInstrs);
 
-      // Skip empty scheduling regions but include single-MI regions; we want
-      // those to be scheduled so that backends which move MIs across regions
-      // during scheduling can reason about and schedule those regions
-      // correctly.
-      if (I == RegionEnd) {
+      // Skip empty scheduling regions and, conditionally, regions with a single
+      // MI.
+      if (I == RegionEnd || (!ScheduleSingleMI && I == std::prev(RegionEnd))) {
         // Close the current region. Bundle the terminator if needed.
         // This invalidates 'RegionEnd' and 'I'.
         Scheduler.exitRegion();