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

[via llvm-commits]

================
@@ -0,0 +1,249 @@
+#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 &get_branch_with_dest(MachineBasicBlock &branching_MBB,
+                                                 MachineBasicBlock &dest_MBB) {
+  auto& TII = *branching_MBB.getParent()->getSubtarget<GCNSubtarget>().getInstrInfo();
+  for (MachineInstr &branch_MI : reverse(branching_MBB.instrs()))
+    if (branch_MI.isBranch() && TII.getBranchDestBlock(branch_MI) == &dest_MBB)
+      return branch_MI;
+
+  llvm_unreachable("Don't call this if there's no branch to the destination.");
+}
+
+static inline void move_ins_before_phis(MachineInstr &MI) {
+  MachineBasicBlock& MBB = *MI.getParent();
+  MachineFunction& MF = *MBB.getParent();
+  auto &TII = *MF.getSubtarget<GCNSubtarget>().getInstrInfo();
+  auto& MRI = MF.getRegInfo();
+
+  bool phi_seen = false;
+  MachineBasicBlock::iterator first_phi;
+  for (first_phi = MBB.begin(); first_phi != MBB.end(); first_phi++)
+    if (first_phi->getOpcode() == AMDGPU::PHI) {
+      phi_seen = true;
+      break;
+    }
+  
+  if (!phi_seen) {
+    MI.removeFromParent();
+    MBB.insert(MBB.begin(), &MI);
+  } else {
+    auto phi = BuildMI(MBB, first_phi, MI.getDebugLoc(), TII.get(AMDGPU::PHI),
+            MI.getOperand(0).getReg());
+    for (auto *pred_MBB : MBB.predecessors()) {
+      Register cloned_reg = MRI.cloneVirtualRegister(MI.getOperand(0).getReg());
+      MachineInstr& branch_MI = get_branch_with_dest(*pred_MBB,MBB);
+      MachineInstr *cloned_MI = MF.CloneMachineInstr(&MI);
+      cloned_MI->getOperand(0).setReg(cloned_reg);
+      phi.addReg(cloned_reg).addMBB(pred_MBB);
+      pred_MBB->insertAfterBundle(branch_MI.getIterator(), cloned_MI);
----------------
alex-t wrote:

You need to make sure that there is no Machine Verifier between SILowerControlFlow and SIRestoreNormalEpilog
Also, check if unusual placement instructions after the branch does not break SILowerControlFlow logic itself

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