[llvm] [AMDGPU] Introduce GCNPostGenericScheduler to model MFMA-VALU coexecution (PR #184084)

[Frederik Harwath via llvm-commits]

================
@@ -3244,6 +3244,41 @@ static bool hasIGLPInstrs(ScheduleDAGInstrs *DAG) {
   });
 }
 
+void GCNPostGenericScheduler::initPolicy(MachineBasicBlock::iterator Begin,
+                                         MachineBasicBlock::iterator End,
+                                         unsigned NumRegionInstrs) {
+  LastTopScheduledIsMFMA = false;
+  LastBottomScheduledIsMFMA = false;
+  PostGenericScheduler::initPolicy(Begin, End, NumRegionInstrs);
+}
+
+void GCNPostGenericScheduler::schedNode(SUnit *SU, bool IsTopNode) {
+  // A non-MFMA VALU scheduled immediately after an MFMA requires a nop in
+  // between. Account for this by advancing the top boundary by one cycle
+  // before the VALU is scheduled.
+  if (SU->isInstr()) {
+    MachineInstr *MI = SU->getInstr();
+    bool IsNonMFMAVALU = SIInstrInfo::isVALU(*MI) && !SIInstrInfo::isMFMA(*MI);
+    if (IsNonMFMAVALU) {
+      if (IsTopNode && isLastTopScheduledIsMFMA()) {
+        Top.bumpCycle(Top.getCurrCycle() + 1);
+        LastTopScheduledIsMFMA = false;
+      }
+      if (!IsTopNode && isLastBottomScheduledIsMFMA()) {
+        Bot.bumpCycle(Bot.getCurrCycle() + 1);
+        LastBottomScheduledIsMFMA = false;
+      }
+    } else if (SIInstrInfo::isMFMA(*MI)) {
+      if (IsTopNode)
+        LastTopScheduledIsMFMA = true;
+      if (!IsTopNode)
+        LastBottomScheduledIsMFMA = true;
+    }
+  }
----------------
frederik-h wrote:

```suggestion
  if (SU->isInstr() && SIInstrInfo::isVALU(*SU->getInstr())) {
    bool IsMFMA = SIInstrInfo::isMFMA(*SU->getInstr());
    auto Bump = [IsMFMA](SchedBoundary &Boundary, bool &LastIsMFMA) {
      if (LastIsMFMA && !IsMFMA)
        Boundary.bumpCycle(Boundary.getCurrCycle() + 1);
      LastIsMFMA = IsMFMA;
    };

    if (IsTopNode)
      Bump(Top, LastTopScheduledIsMFMA);
    else
      Bump(Bot, LastBottomScheduledIsMFMA);
  }
 ```
   

https://github.com/llvm/llvm-project/pull/184084