mi-sched: Force bottom up scheduling for generic targets.

Fast register pressure tracking currently only takes effect during
bottom up scheduling. Forcing this is a bit faster and simpler for
targets that don't have many scheduling constraints and don't need
top-down scheduling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190014 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineScheduler.cpp

@@ -1594,6 +1594,12 @@ private:
   SchedBoundary Top;
   SchedBoundary Bot;
 
+  // Allow the driver to force top-down or bottom-up scheduling. If neither is
+  // true, the scheduler runs in both directions and converges. For generic
+  // targets, we default to bottom-up, because it's simpler and more
+  // compile-time optimizations have been implemented in that direction.
+  bool OnlyBottomUp;
+  bool OnlyTopDown;
 public:
   /// SUnit::NodeQueueId: 0 (none), 1 (top), 2 (bot), 3 (both)
   enum {
@@ -1604,7 +1610,8 @@ public:
 
   ConvergingScheduler(const MachineSchedContext *C):
     Context(C), DAG(0), SchedModel(0), TRI(0),
-    Top(TopQID, "TopQ"), Bot(BotQID, "BotQ") {}
+    Top(TopQID, "TopQ"), Bot(BotQID, "BotQ"),
+    OnlyBottomUp(true), OnlyTopDown(false) {}
 
   virtual bool shouldTrackPressure(unsigned NumRegionInstrs);
 
@@ -1709,6 +1716,19 @@ void ConvergingScheduler::initialize(ScheduleDAGMI *dag) {
   }
   assert((!ForceTopDown || !ForceBottomUp) &&
          "-misched-topdown incompatible with -misched-bottomup");
+
+  // Check -misched-topdown/bottomup can force or unforce scheduling direction.
+  // e.g. -misched-bottomup=false allows scheduling in both directions.
+  if (ForceBottomUp.getNumOccurrences() > 0) {
+    OnlyBottomUp = ForceBottomUp;
+    if (OnlyBottomUp)
+      OnlyTopDown = false;
+  }
+  if (ForceTopDown.getNumOccurrences() > 0) {
+    OnlyTopDown = ForceTopDown;
+    if (OnlyTopDown)
+      OnlyBottomUp = false;
+  }
 }
 
 void ConvergingScheduler::releaseTopNode(SUnit *SU) {
@@ -2674,7 +2694,7 @@ SUnit *ConvergingScheduler::pickNode(bool &IsTopNode) {
   }
   SUnit *SU;
   do {
-    if (ForceTopDown) {
+    if (OnlyTopDown) {
       SU = Top.pickOnlyChoice();
       if (!SU) {
         CandPolicy NoPolicy;
@@ -2686,7 +2706,7 @@ SUnit *ConvergingScheduler::pickNode(bool &IsTopNode) {
       }
       IsTopNode = true;
     }
-    else if (ForceBottomUp) {
+    else if (OnlyBottomUp) {
       SU = Bot.pickOnlyChoice();
       if (!SU) {
         CandPolicy NoPolicy;