diff --git a/llvm/lib/CodeGen/MachineOutliner.cpp b/llvm/lib/CodeGen/MachineOutliner.cpp
index a64295cf82064d..ef6069255dfb7b 100644
--- a/llvm/lib/CodeGen/MachineOutliner.cpp
+++ b/llvm/lib/CodeGen/MachineOutliner.cpp
@@ -941,13 +941,12 @@ void MachineOutliner::populateMapper(InstructionMapper &Mapper, Module &M,
                                      MachineModuleInfo &MMI) {
   // Build instruction mappings for each function in the module. Start by
   // iterating over each Function in M.
+  LLVM_DEBUG(dbgs() << "*** Populating mapper ***\n");
   for (Function &F : M) {
+    LLVM_DEBUG(dbgs() << "MAPPING FUNCTION: " << F.getName() << "\n");
 
     if (F.hasFnAttribute("nooutline")) {
-      LLVM_DEBUG({
-        dbgs() << "... Skipping function with nooutline attribute: "
-               << F.getName() << "\n";
-      });
+      LLVM_DEBUG(dbgs() << "SKIP: Function has nooutline attribute\n");
       continue;
     }
 
@@ -957,36 +956,51 @@ void MachineOutliner::populateMapper(InstructionMapper &Mapper, Module &M,
 
     // If it doesn't, then there's nothing to outline from. Move to the next
     // Function.
-    if (!MF)
+    if (!MF) {
+      LLVM_DEBUG(dbgs() << "SKIP: Function does not have a MachineFunction\n");
       continue;
+    }
 
     const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
-
-    if (!RunOnAllFunctions && !TII->shouldOutlineFromFunctionByDefault(*MF))
+    if (!RunOnAllFunctions && !TII->shouldOutlineFromFunctionByDefault(*MF)) {
+      LLVM_DEBUG(dbgs() << "SKIP: Target does not want to outline from "
+                           "function by default\n");
       continue;
+    }
 
     // We have a MachineFunction. Ask the target if it's suitable for outlining.
     // If it isn't, then move on to the next Function in the module.
-    if (!TII->isFunctionSafeToOutlineFrom(*MF, OutlineFromLinkOnceODRs))
+    if (!TII->isFunctionSafeToOutlineFrom(*MF, OutlineFromLinkOnceODRs)) {
+      LLVM_DEBUG(dbgs() << "SKIP: " << MF->getName()
+                        << ": unsafe to outline from\n");
       continue;
+    }
 
     // We have a function suitable for outlining. Iterate over every
     // MachineBasicBlock in MF and try to map its instructions to a list of
     // unsigned integers.
+    const unsigned MinMBBSize = 2;
+
     for (MachineBasicBlock &MBB : *MF) {
+      LLVM_DEBUG(dbgs() << "  MAPPING MBB: '" << MBB.getName() << "'\n");
       // If there isn't anything in MBB, then there's no point in outlining from
       // it.
       // If there are fewer than 2 instructions in the MBB, then it can't ever
       // contain something worth outlining.
       // FIXME: This should be based off of the maximum size in B of an outlined
       // call versus the size in B of the MBB.
-      if (MBB.size() < 2)
+      if (MBB.size() < MinMBBSize) {
+        LLVM_DEBUG(dbgs() << "    SKIP: MBB size less than minimum size of "
+                          << MinMBBSize << "\n");
         continue;
+      }
 
       // Check if MBB could be the target of an indirect branch. If it is, then
       // we don't want to outline from it.
-      if (MBB.hasAddressTaken())
+      if (MBB.hasAddressTaken()) {
+        LLVM_DEBUG(dbgs() << "    SKIP: MBB's address is taken\n");
         continue;
+      }
 
       // MBB is suitable for outlining. Map it to a list of unsigneds.
       Mapper.convertToUnsignedVec(MBB, *TII);
diff --git a/llvm/test/CodeGen/AArch64/machine-outliner-mapper-debug-output.mir b/llvm/test/CodeGen/AArch64/machine-outliner-mapper-debug-output.mir
new file mode 100644
index 00000000000000..826157e68d75c7
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/machine-outliner-mapper-debug-output.mir
@@ -0,0 +1,58 @@
+# RUN: llc -mtriple=aarch64 -debug-only=machine-outliner -run-pass=machine-outliner -verify-machineinstrs %s -o /dev/null 2>&1 | FileCheck %s
+# REQUIRES: asserts
+
+# CHECK-LABEL: *** Populating mapper ***
+# CHECK-NEXT: MAPPING FUNCTION: block_too_small
+# CHECK-NEXT:   MAPPING MBB: ''
+# CHECK-NEXT:     SKIP: MBB size less than minimum size of 2
+# CHECK-NEXT: MAPPING FUNCTION: no_outline
+# CHECK-NEXT: SKIP: Function has nooutline attribute
+# CHECK-NEXT: MAPPING FUNCTION: redzone
+# CHECK-NEXT: SKIP: redzone: unsafe to outline from
+# CHECK-NEXT: MAPPING FUNCTION: no_mf
+# CHECK-NEXT: SKIP: Function does not have a MachineFunction
+# CHECK-NEXT: MAPPING FUNCTION: block_addr_fn
+# CHECK-NEXT:   MAPPING MBB: 'label'
+# CHECK-NEXT:     SKIP: MBB's address is taken
+
+--- |
+  define void @block_too_small() noredzone { unreachable }
+  define void @no_outline() noredzone "nooutline" { unreachable }
+  define void @redzone() { unreachable }
+  declare void @no_mf()
+  define void @block_addr_fn() noredzone {
+  entry:
+    br label %label
+  label:
+    call void @block_addr_fn(ptr blockaddress(@block_addr_fn, %label))
+    ret void
+  }
+...
+---
+name:            no_outline
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $lr, $w8
+    RET undef $lr
+...
+---
+name:            block_too_small
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $lr, $w8
+    RET undef $lr
+...
+---
+name:            block_addr_fn
+tracksRegLiveness: true
+body:             |
+  bb.0.label (ir-block-address-taken %ir-block.label):
+    liveins: $w0, $lr, $w8
+    $w0 = ORRWri $wzr, 1
+    $w0 = ORRWri $wzr, 1
+    $w0 = ORRWri $wzr, 4
+    RET undef $lr
+...
+---
diff --git a/llvm/test/CodeGen/AArch64/machine-outliner-nooutline-attribute.mir b/llvm/test/CodeGen/AArch64/machine-outliner-nooutline-attribute.mir
deleted file mode 100644
index 477a16c8413300..00000000000000
--- a/llvm/test/CodeGen/AArch64/machine-outliner-nooutline-attribute.mir
+++ /dev/null
@@ -1,42 +0,0 @@
-# RUN: llc -mtriple=aarch64 -debug-only=machine-outliner -run-pass=machine-outliner -verify-machineinstrs %s -o - 2>&1 | FileCheck %s
-# REQUIRES: asserts
-
-# CHECK: ... Skipping function with nooutline attribute: no_outline
-# CHECK-NOT: ... Skipping function with nooutline attribute: baz
-# CHECK-NOT: OUTLINED
-
---- |
-  define void @no_outline() #0 { unreachable }
-  define void @baz() { unreachable }
-  attributes #0 = { noredzone "nooutline" }
-...
----
-
-name:            no_outline
-tracksRegLiveness: true
-body:             |
-  bb.0:
-    liveins: $w0, $lr, $w8
-    $sp = frame-setup SUBXri $sp, 32, 0
-    $fp = frame-setup ADDXri $sp, 16, 0
-  bb.1:
-    BL @baz, implicit-def dead $lr, implicit $sp
-    $w17 = ORRWri $wzr, 1
-    $w17 = ORRWri $wzr, 1
-    $w0 = ORRWri $wzr, 4
-    BL @baz, implicit-def dead $lr, implicit $sp
-    $w17 = ORRWri $wzr, 1
-    $w17 = ORRWri $wzr, 1
-    $w0 = ORRWri $wzr, 3
-    BL @baz, implicit-def dead $lr, implicit $sp
-    $w17 = ORRWri $wzr, 1
-    $w17 = ORRWri $wzr, 1
-    $w0 = ORRWri $wzr, 2
-    BL @baz, implicit-def dead $lr, implicit $sp
-    $w17 = ORRWri $wzr, 1
-    $w17 = ORRWri $wzr, 1
-    $w0 = ORRWri $wzr, 1
-  bb.2:
-    $fp, $lr = LDPXi $sp, 2
-    RET undef $lr
-