[llvm] Move SI Lower Control Flow Up (PR #159557)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,261 @@
+#pragma once
+
+#include "GCNSubtarget.h"
+#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/Support/ErrorHandling.h"
+
+#include "SIInstrInfo.h"
+
+#include <cassert>
+#include <unordered_set>
+
+using namespace llvm;
+
+using std::unordered_set;
+using std::vector;
+
+static inline MachineInstr &getBranchWithDest(MachineBasicBlock &BranchingMBB,
+                                              MachineBasicBlock &DestMBB) {
+  auto &TII =
+      *BranchingMBB.getParent()->getSubtarget<GCNSubtarget>().getInstrInfo();
+  for (MachineInstr &BranchMI : reverse(BranchingMBB.instrs()))
+    if (BranchMI.isBranch() && TII.getBranchDestBlock(BranchMI) == &DestMBB)
+      return BranchMI;
+
+  llvm_unreachable("Don't call this if there's no branch to the destination.");
+}
+
+static inline void moveInsBeforePhis(MachineInstr &MI) {
+  MachineBasicBlock &MBB = *MI.getParent();
+  MachineFunction &MF = *MBB.getParent();
+  auto &TII = *MF.getSubtarget<GCNSubtarget>().getInstrInfo();
+  auto &MRI = MF.getRegInfo();
+
+  bool PhiSeen = false;
+  MachineBasicBlock::iterator FirstPhi;
+  for (FirstPhi = MBB.begin(); FirstPhi != MBB.end(); FirstPhi++)
+    if (FirstPhi->getOpcode() == AMDGPU::PHI) {
+      PhiSeen = true;
+      break;
+    }
+
+  if (!PhiSeen) {
+    MI.removeFromParent();
+    MBB.insert(MBB.begin(), &MI);
+  } else {
+    auto Phi = BuildMI(MBB, FirstPhi, MI.getDebugLoc(), TII.get(AMDGPU::PHI),
+                       MI.getOperand(0).getReg());
+    for (auto *PredMBB : MBB.predecessors()) {
+      Register ClonedReg = MRI.cloneVirtualRegister(MI.getOperand(0).getReg());
+      MachineInstr &BranchMI = getBranchWithDest(*PredMBB, MBB);
+      MachineInstr *ClonedMI = MF.CloneMachineInstr(&MI);
+      ClonedMI->getOperand(0).setReg(ClonedReg);
+      Phi.addReg(ClonedReg).addMBB(PredMBB);
+      PredMBB->insertAfterBundle(BranchMI.getIterator(), ClonedMI);
+      ClonedMI->bundleWithPred();
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arsenm wrote:

Didn't we discuss that you should not be trying to use bundles for this? 

https://github.com/llvm/llvm-project/pull/159557