diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -253,6 +253,7 @@
 
   // Trap lowers to wasm unreachable
   setOperationAction(ISD::TRAP, MVT::Other, Legal);
+  setOperationAction(ISD::DEBUGTRAP, MVT::Other, Legal);
 
   // Exception handling intrinsics
   setOperationAction(ISD::INTRINSIC_WO_CHAIN, MVT::Other, Custom);
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td b/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
--- a/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
@@ -85,8 +85,8 @@
 } // Uses = [VALUE_STACK], Defs = [VALUE_STACK]
 
 
-let isTerminator = 1, hasCtrlDep = 1, isBarrier = 1 in {
-
+let hasCtrlDep = 1, isBarrier = 1 in {
+let isTerminator = 1 in {
 let isReturn = 1 in {
 
 defm RETURN : I<(outs), (ins variable_ops), (outs), (ins),
@@ -99,8 +99,21 @@
 
 } // isReturn = 1
 
+let isTrap = 1 in
 defm UNREACHABLE : NRI<(outs), (ins), [(trap)], "unreachable", 0x00>;
-} // isTerminator = 1, hasCtrlDep = 1, isBarrier = 1
+
+} // isTerminator = 1
+
+// debugtrap explicitly returns despite trapping because it is supposed to just
+// get the attention of the debugger. Unfortunately, because UNREACHABLE is a
+// terminator, lowering debugtrap to UNREACHABLE can create an invalid
+// MachineBasicBlock when there is additional code after it. Lower it to this
+// non-terminator version instead.
+// TODO: Actually execute the debugger statement when running on the Web
+let isTrap = 1 in
+defm DEBUG_UNREACHABLE : NRI<(outs), (ins), [(debugtrap)], "unreachable", 0x00>;
+
+} // hasCtrlDep = 1, isBarrier = 1
 
 //===----------------------------------------------------------------------===//
 // Exception handling instructions
diff --git a/llvm/test/CodeGen/WebAssembly/debugtrap.ll b/llvm/test/CodeGen/WebAssembly/debugtrap.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/WebAssembly/debugtrap.ll
@@ -0,0 +1,60 @@
+; RUN: llc < %s -asm-verbose=false -verify-machineinstrs | FileCheck %s
+
+; Test lowering of __builtin_debugtrap in cases where lowering it via
+; the normal UNREACHABLE instruction would yield invalid
+; MachineFunctions.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32"
+
+declare void @llvm.debugtrap()
+
+; CHECK-LABEL: foo:
+; CHECK-NEXT: .functype       foo (i32) -> ()
+; CHECK-NEXT: .LBB0_1:
+; CHECK-NEXT: loop
+; CHECK-NEXT: unreachable
+; CHECK-NEXT: i32.const       0
+; CHECK-NEXT: br_if           0
+; CHECK-NEXT: end_loop
+; CHECK-NEXT: end_function
+define void @foo(i32 %g) {
+entry:
+  br label %for.body
+
+for.body:
+  call void @llvm.debugtrap()
+  %exitcond = icmp eq i32 undef, %g
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:
+  ret void
+}
+
+; CHECK-LABEL: middle_of_block:
+; CHECK-NEXT: .functype       middle_of_block (i32, i32) -> (i32)
+; CHECK-NEXT: unreachable
+; CHECK-NEXT: local.get       0
+; CHECK-NEXT: local.get       1
+; CHECK-NEXT: i32.add
+; CHECK-NEXT: end_function
+define i32 @middle_of_block(i32 %x, i32 %y) {
+  %r = add i32 %x, %y
+  call void @llvm.debugtrap()
+  ret i32 %r
+}
+
+; CHECK-LABEL: really_middle_of_block:
+; CHECK-NEXT: .functype       really_middle_of_block () -> (i32)
+; CHECK-NEXT: call    bar
+; CHECK-NEXT: drop
+; CHECK-NEXT: unreachable
+; CHECK-NEXT: call    bar
+; CHECK-NEXT: end_function
+declare i32 @bar()
+define i32 @really_middle_of_block() {
+  %x = call i32 @bar()
+  call void @llvm.debugtrap()
+  %r = call i32 @bar()
+  ret i32 %r
+}