[llvm] [MCP] Move dependencies if they block copy propagation (PR #105562)

[Gábor Spaits via llvm-commits]

================
@@ -1033,23 +1229,39 @@ void MachineCopyPropagation::propagateDefs(MachineInstr &MI) {
     LLVM_DEBUG(dbgs() << "MCP: Replacing " << printReg(MODef.getReg(), TRI)
                       << "\n     with " << printReg(Def, TRI) << "\n     in "
                       << MI << "     from " << *Copy);
+    if (!MoveDependenciesForBetterCopyPropagation) {
+      MODef.setReg(Def);
+      MODef.setIsRenamable(CopyOperands->Destination->isRenamable());
 
-    MODef.setReg(Def);
-    MODef.setIsRenamable(CopyOperands->Destination->isRenamable());
-
-    LLVM_DEBUG(dbgs() << "MCP: After replacement: " << MI << "\n");
-    MaybeDeadCopies.insert(Copy);
-    Changed = true;
-    ++NumCopyBackwardPropagated;
+      LLVM_DEBUG(dbgs() << "MCP: After replacement: " << MI << "\n");
+      MaybeDeadCopies.insert(Copy);
+      Changed = true;
+      ++NumCopyBackwardPropagated;
+    } else if (InstructionsToMove) {
+      for (auto *I : *InstructionsToMove) {
+        MI.getParent()->splice(MI.getIterator(), MI.getParent(), I->getIterator());
+      }
+    }
   }
 }
 
 void MachineCopyPropagation::BackwardCopyPropagateBlock(
-    MachineBasicBlock &MBB) {
+    MachineBasicBlock &MBB, bool MoveDependenciesForBetterCopyPropagation) {
+  ScheduleDAGMCP DG{*(MBB.getParent()), nullptr, false};
----------------
spaits wrote:

I think the `ScheduleDAG` can only operate on basic blocks, it can not handle branching.
in the next line we have:
```cpp
DG.enterRegion(&MBB, MBB.begin(), MBB.end(), MBB.size());
``` 
In this call the following members of the `ScheduleDAG` are set:
```cpp
RegionBegin = begin;
RegionEnd = end;
```
and then in the `buildSchedGraph` call the main loop goes from `RegionEnd` to `RegionBegin`:
```cpp
for (MachineBasicBlock::iterator MII = RegionEnd, MIE = RegionBegin;
       MII != MIE; --MII)
```

So for this reason I think there is a separate `ScheduleDAG` for each MBB. (Basically the MBB is the scheduling region here).

I may be wrong here. What do you think?

https://github.com/llvm/llvm-project/pull/105562