Index: llvm/test/tools/llvm-reduce/mir/reduce-blocks-g_br.mir
===================================================================
--- llvm/test/tools/llvm-reduce/mir/reduce-blocks-g_br.mir
+++ llvm/test/tools/llvm-reduce/mir/reduce-blocks-g_br.mir
@@ -12,17 +12,20 @@
 # RESULT0-NEXT: %1:_(s32) = G_CONSTANT i32 0
 # RESULT0-NEXT: %2:_(s32) = G_ICMP intpred(ugt), %0(s32), %1
 # RESULT0-NEXT: S_NOP 0
-# RESULT0-NEXT: G_BRCOND %2(s32), %bb.[[UNDEF_BB:[0-9]+]]
+# RESULT0-NEXT: %3:_(s1) = G_IMPLICIT_DEF
 # RESULT0-NEXT: G_BR %bb.1
 
 # RESULT0: bb.1:
 # RESULT0-NEXT: S_NOP 1
-# RESULT0-NEXT: G_BRCOND %2(s32), %bb.[[UNDEF_BB]]
-# RESULT0-NEXT: G_BR %bb.[[UNDEF_BB]]
+# RESULT0-NEXT: G_BR %bb.[[UNDEF_BB:[0-9]+]]
 
-# RESULT0: bb.4:
+# RESULT0-NOT: bb.
+
+# RESULT0: bb.5:
 # RESULT0-NEXT: S_ENDPGM 0
 
+# RESULT0-NOT: bb.
+
 # RESULT0: bb.[[UNDEF_BB]]:
 
 ---
@@ -50,9 +53,15 @@
     G_BR %bb.3
 
   bb.3:
+    ; Conditional fallthrough
     S_NOP 3
+    %4:_(s1) = G_IMPLICIT_DEF
+    G_BRCOND %4, %bb.5
 
   bb.4:
+    S_NOP 3
+
+  bb.5:
     S_ENDPGM 0
 
 ...
Index: llvm/test/tools/llvm-reduce/mir/reduce-blocks-g_brjt.mir
===================================================================
--- llvm/test/tools/llvm-reduce/mir/reduce-blocks-g_brjt.mir
+++ llvm/test/tools/llvm-reduce/mir/reduce-blocks-g_brjt.mir
@@ -24,7 +24,7 @@
 # RESULT0-NEXT: %10:_(s1) = G_IMPLICIT_DEF
 # RESULT0-NEXT: %5:_(s32) = G_IMPLICIT_DEF
 # RESULT0-NEXT: %17:_(s32) = G_IMPLICIT_DEF
-# RESULT0-NEXT: G_BRCOND %10(s1), %bb.[[UNDEF_BB:[0-9]+]]
+# RESULT0-NEXT: G_BR %bb.1
 
 # RESULT0: bb.1.entry:
 # RESULT0-NEXT: S_NOP 1
@@ -35,7 +35,7 @@
 # RESULT0-NEXT: %15:_(s32) = G_IMPLICIT_DEF
 # RESULT0-NEXT: G_BRJT %12(p0), %jump-table.0, %7(s64)
 
-# RESULT0: bb.[[UNDEF_BB]]:
+# RESULT0-NOT: bb.
 
 
 # RESULT1: bb.0.entry:
Index: llvm/tools/llvm-reduce/deltas/ReduceBlocksMIR.cpp
===================================================================
--- llvm/tools/llvm-reduce/deltas/ReduceBlocksMIR.cpp
+++ llvm/tools/llvm-reduce/deltas/ReduceBlocksMIR.cpp
@@ -26,6 +26,46 @@
 
 using BlockSet = DenseSet<MachineBasicBlock *>;
 
+// Analyze the successors for a block using GlobalISel's generic branch
+// instructions, G_BR and G_BRCOND.
+static bool analyzeBranchGeneric(const MachineBasicBlock &MBB,
+                                 MachineBasicBlock *&TrueMBB,
+                                 MachineBasicBlock *&FalseMBB,
+                                 SmallVectorImpl<MachineOperand> &Cond) {
+  MachineBasicBlock::const_iterator I = MBB.getFirstTerminator();
+  if (I == MBB.end())
+    return false;
+
+  if (I->getOpcode() == TargetOpcode::G_BR) {
+    TrueMBB = I->getOperand(0).getMBB();
+    return false;
+  }
+
+  if (I->getOpcode() != TargetOpcode::G_BRCOND)
+    return true;
+
+  Cond.push_back(I->getOperand(0));
+  TrueMBB = I->getOperand(1).getMBB();
+  ++I;
+  if (I == MBB.end())
+    return false;
+
+  if (I->getOpcode() != TargetOpcode::G_BR)
+    return true;
+
+  FalseMBB = I->getOperand(0).getMBB();
+  return false;
+}
+
+static void removeBranchGeneric(MachineBasicBlock &MBB) {
+  for (MachineInstr &MI : make_early_inc_range(MBB.terminators())) {
+    assert(MI.getOpcode() == TargetOpcode::G_BR ||
+           MI.getOpcode() == TargetOpcode::G_BRCOND);
+    if (MI.isBranch())
+      MI.eraseFromParent();
+  }
+}
+
 static bool isPseudoUndefBlock(const MachineBasicBlock &MBB) {
   return MBB.empty() && &MBB == &MBB.getParent()->back() && MBB.succ_empty();
 }
@@ -82,10 +122,24 @@
   if (&MBB != EntryBB && BBsToDelete.count(&MBB))
     return;
 
+  enum BranchType {
+    UnknownBranching,
+    TargetBranching,
+    GenericBranching
+  } BranchType = UnknownBranching;
+
   MachineBasicBlock *TBB = nullptr;
   MachineBasicBlock *FBB = nullptr;
   SmallVector<MachineOperand, 2> Cond;
+
   if (TII.analyzeBranch(MBB, TBB, FBB, Cond)) {
+    TBB = FBB = nullptr;
+    if (!analyzeBranchGeneric(MBB, TBB, FBB, Cond))
+      BranchType = GenericBranching;
+  } else
+    BranchType = TargetBranching;
+
+  if (BranchType == UnknownBranching) {
     // If we don't understand this branch, replace deleted block references with
     // the dummy undef block.
     bool UsedUndefBlock = false;
@@ -109,7 +163,10 @@
   }
 
   // If we can analyze this branch, insert simplified terminators.
-  TII.removeBranch(MBB);
+  if (BranchType == GenericBranching)
+    removeBranchGeneric(MBB);
+  else
+    TII.removeBranch(MBB);
 
   MachineBasicBlock *NewSucc;
   if (MBB.succ_empty()) {
@@ -124,7 +181,12 @@
     NewSucc = *MBB.succ_begin();
   }
 
-  TII.insertUnconditionalBranch(MBB, NewSucc, DebugLoc());
+  if (BranchType == GenericBranching) {
+    BuildMI(&MBB, DebugLoc(), TII.get(TargetOpcode::G_BR))
+      .addMBB(NewSucc);
+  } else {
+    TII.insertUnconditionalBranch(MBB, NewSucc, DebugLoc());
+  }
 }
 
 static unsigned getImpDefOpcode(const MachineRegisterInfo &MRI, Register Reg) {