diff --git a/llvm/include/llvm/MC/MCWinEH.h b/llvm/include/llvm/MC/MCWinEH.h
--- a/llvm/include/llvm/MC/MCWinEH.h
+++ b/llvm/include/llvm/MC/MCWinEH.h
@@ -59,6 +59,7 @@
   struct Epilog {
     std::vector<Instruction> Instructions;
     unsigned Condition;
+    MCSymbol *End;
   };
   MapVector<MCSymbol *, Epilog> EpilogMap;
 
diff --git a/llvm/lib/MC/MCWin64EH.cpp b/llvm/lib/MC/MCWin64EH.cpp
--- a/llvm/lib/MC/MCWin64EH.cpp
+++ b/llvm/lib/MC/MCWin64EH.cpp
@@ -1197,7 +1197,8 @@
   return Count;
 }
 
-static uint32_t ARMCountOfInstructionBytes(ArrayRef<WinEH::Instruction> Insns) {
+static uint32_t ARMCountOfInstructionBytes(ArrayRef<WinEH::Instruction> Insns,
+                                           bool *HasCustom = nullptr) {
   uint32_t Count = 0;
   for (const auto &I : Insns) {
     switch (static_cast<Win64EH::UnwindOpcodes>(I.Operation)) {
@@ -1247,12 +1248,39 @@
       // We can't reason about what instructions this maps to; return a
       // phony number to make sure we don't accidentally do epilog packing.
       Count += 1000;
+      if (HasCustom)
+        *HasCustom = true;
       break;
     }
   }
   return Count;
 }
 
+static void checkARMInstructions(MCStreamer &Streamer,
+                                 ArrayRef<WinEH::Instruction> Insns,
+                                 const MCSymbol *Begin, const MCSymbol *End,
+                                 StringRef Name, StringRef Type) {
+  if (!End)
+    return;
+  Optional<int64_t> MaybeDistance =
+      GetOptionalAbsDifference(Streamer, End, Begin);
+  if (!MaybeDistance)
+    return;
+  uint32_t Distance = (uint32_t)*MaybeDistance;
+  bool HasCustom = false;
+  uint32_t InstructionBytes = ARMCountOfInstructionBytes(Insns, &HasCustom);
+  if (HasCustom)
+    return;
+  if (Distance != InstructionBytes) {
+    Streamer.getContext().reportError(
+        SMLoc(), "Incorrect size for " + Name + " " + Type + ": " +
+                     Twine(Distance) +
+                     " bytes of instructions in range, but .seh directives "
+                     "corresponding to " +
+                     Twine(InstructionBytes) + " bytes\n");
+  }
+}
+
 // Unwind opcode encodings and restrictions are documented at
 // https://docs.microsoft.com/en-us/cpp/build/arm-exception-handling
 static void ARMEmitUnwindCode(MCStreamer &streamer,
@@ -1951,6 +1979,17 @@
   streamer.emitLabel(Label);
   info->Symbol = Label;
 
+  if (info->PrologEnd && !info->Fragment)
+    checkARMInstructions(streamer, info->Instructions, info->Begin,
+                         info->PrologEnd, info->Function->getName(),
+                         "prologue");
+  for (auto &I : info->EpilogMap) {
+    MCSymbol *EpilogStart = I.first;
+    auto &Epilog = I.second;
+    checkARMInstructions(streamer, Epilog.Instructions, EpilogStart, Epilog.End,
+                         info->Function->getName(), "epilogue");
+  }
+
   Optional<int64_t> RawFuncLength;
   const MCExpr *FuncLengthExpr = nullptr;
   if (!info->FuncletOrFuncEnd) {
diff --git a/llvm/lib/MC/WinCOFFObjectWriter.cpp b/llvm/lib/MC/WinCOFFObjectWriter.cpp
--- a/llvm/lib/MC/WinCOFFObjectWriter.cpp
+++ b/llvm/lib/MC/WinCOFFObjectWriter.cpp
@@ -958,7 +958,7 @@
   for (const auto &Section : Asm) {
     COFFSection *Sec = SectionMap[&Section];
 
-    if (Sec->Number == -1)
+    if (!Sec || Sec->Number == -1)
       continue;
 
     Sec->Header.SizeOfRawData = Layout.getSectionAddressSize(&Section);
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp
@@ -259,6 +259,8 @@
   InEpilogCFI = false;
   WinEH::Instruction Inst = WinEH::Instruction(UnwindCode, nullptr, -1, 0);
   CurFrame->EpilogMap[CurrentEpilog].Instructions.push_back(Inst);
+  MCSymbol *Label = S.emitCFILabel();
+  CurFrame->EpilogMap[CurrentEpilog].End = Label;
   CurrentEpilog = nullptr;
 }
 
diff --git a/llvm/test/MC/ARM/seh-checks.s b/llvm/test/MC/ARM/seh-checks.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/ARM/seh-checks.s
@@ -0,0 +1,51 @@
+// This test checks error reporting for mismatched prolog/epilog lengths
+
+// RUN: not llvm-mc -triple thumbv7-pc-win32 -filetype=obj -o /dev/null %s 2>&1 | FileCheck %s
+
+// CHECK-NOT: func1
+// CHECK: error: Incorrect size for func2 epilogue: 6 bytes of instructions in range, but .seh directives corresponding to 4 bytes
+// CHECK: error: Incorrect size for func3 prologue: 4 bytes of instructions in range, but .seh directives corresponding to 2 bytes
+
+        .text
+        .syntax unified
+
+        .seh_proc func1
+func1:
+        // Instruction with indeterminate length
+        b other
+        .seh_endprologue
+        nop
+        .seh_startepilogue
+        // The p2align causes the length of the epilogue to be unknown, so
+        // we can't report errors about the mismatch here.
+        .p2align 1
+        pop {r4-r7-lr}
+        .seh_save_regs {r4-r7,lr}
+        bx lr
+        .seh_nop
+        .seh_endepilogue
+        .seh_endproc
+
+        .seh_proc func2
+func2:
+        .seh_endprologue
+        nop
+        .seh_startepilogue
+        // As we're popping into lr instead of directly into pc, this pop
+        // becomes a wide instruction.
+        pop {r4-r7,lr}
+        // The directive things we're making a narrow instruction, which
+        // is wrong.
+        .seh_save_regs {r4-r7,lr}
+        bx lr
+        .seh_nop
+        .seh_endepilogue
+        .seh_endproc
+
+        .seh_proc func3
+func3:
+        nop.w
+        .seh_nop
+        .seh_endprologue
+        nop
+        .seh_endproc