[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.

1 files changed, 22 insertions, 9 deletions

diff --git a/llvm/lib/CodeGen/MachineScheduler.cpp b/llvm/lib/CodeGen/MachineScheduler.cpp
index 757f492..d65db91 100644
--- a/llvm/lib/CodeGen/MachineScheduler.cpp
+++ b/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;
+    }
   }
 }