[MISched] Add a hook to override PostRA scheduling policy (#115455)

PostRA scheduling supports different directions now, but we can
only specify it via command line options.

This patch adds a new hook `overridePostRASchedPolicy` for targets
to override PostRA scheduling policy.

Note that some options like tracking register pressure won't take
effect in PostRA scheduling.

Author: wangpc-pp

llvm/include/llvm/CodeGen/TargetSubtargetInfo.h

@@ -232,6 +232,16 @@ class TargetSubtargetInfo : public MCSubtargetInfo {
   virtual void overrideSchedPolicy(MachineSchedPolicy &Policy,
                                    unsigned NumRegionInstrs) const {}
 
+  /// Override generic post-ra scheduling policy within a region.
+  ///
+  /// This is a convenient way for targets that don't provide any custom
+  /// scheduling heuristics (no custom MachineSchedStrategy) to make
+  /// changes to the generic  post-ra scheduling policy.
+  /// Note that some options like tracking register pressure won't take effect
+  /// in post-ra scheduling.
+  virtual void overridePostRASchedPolicy(MachineSchedPolicy &Policy,
+                                         unsigned NumRegionInstrs) const {}
+
   // Perform target-specific adjustments to the latency of a schedule
   // dependency.
   // If a pair of operands is associated with the schedule dependency, DefOpIdx

llvm/lib/CodeGen/MachineScheduler.cpp

@@ -3897,15 +3897,28 @@ void PostGenericScheduler::initialize(ScheduleDAGMI *Dag) {
 void PostGenericScheduler::initPolicy(MachineBasicBlock::iterator Begin,
                                       MachineBasicBlock::iterator End,
                                       unsigned NumRegionInstrs) {
-  if (PostRADirection == MISchedPostRASched::TopDown) {
-    RegionPolicy.OnlyTopDown = true;
-    RegionPolicy.OnlyBottomUp = false;
-  } else if (PostRADirection == MISchedPostRASched::BottomUp) {
-    RegionPolicy.OnlyTopDown = false;
-    RegionPolicy.OnlyBottomUp = true;
-  } else if (PostRADirection == MISchedPostRASched::Bidirectional) {
-    RegionPolicy.OnlyBottomUp = false;
-    RegionPolicy.OnlyTopDown = false;
+  const MachineFunction &MF = *Begin->getMF();
+
+  // Default to top-down because it was implemented first and existing targets
+  // expect that behavior by default.
+  RegionPolicy.OnlyTopDown = true;
+  RegionPolicy.OnlyBottomUp = false;
+
+  // Allow the subtarget to override default policy.
+  MF.getSubtarget().overridePostRASchedPolicy(RegionPolicy, NumRegionInstrs);
+
+  // After subtarget overrides, apply command line options.
+  if (PostRADirection.getNumOccurrences() > 0) {
+    if (PostRADirection == MISchedPostRASched::TopDown) {
+      RegionPolicy.OnlyTopDown = true;
+      RegionPolicy.OnlyBottomUp = false;
+    } else if (PostRADirection == MISchedPostRASched::BottomUp) {
+      RegionPolicy.OnlyTopDown = false;
+      RegionPolicy.OnlyBottomUp = true;
+    } else if (PostRADirection == MISchedPostRASched::Bidirectional) {
+      RegionPolicy.OnlyBottomUp = false;
+      RegionPolicy.OnlyTopDown = false;
+    }
   }
 }