Index: llvm/trunk/include/llvm/IR/RuntimeLibcalls.def
===================================================================
--- llvm/trunk/include/llvm/IR/RuntimeLibcalls.def
+++ llvm/trunk/include/llvm/IR/RuntimeLibcalls.def
@@ -539,6 +539,9 @@
 // Deoptimization
 HANDLE_LIBCALL(DEOPTIMIZE, "__llvm_deoptimize")
 
+// Return address
+HANDLE_LIBCALL(RETURN_ADDRESS, nullptr)
+
 HANDLE_LIBCALL(UNKNOWN_LIBCALL, nullptr)
 
 #undef HANDLE_LIBCALL
Index: llvm/trunk/lib/Target/WebAssembly/WebAssemblyISelLowering.h
===================================================================
--- llvm/trunk/lib/Target/WebAssembly/WebAssemblyISelLowering.h
+++ llvm/trunk/lib/Target/WebAssembly/WebAssemblyISelLowering.h
@@ -88,6 +88,7 @@
   // Custom lowering hooks.
   SDValue LowerOperation(SDValue Op, SelectionDAG &DAG) const override;
   SDValue LowerFrameIndex(SDValue Op, SelectionDAG &DAG) const;
+  SDValue LowerRETURNADDR(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerFRAMEADDR(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerGlobalAddress(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerExternalSymbol(SDValue Op, SelectionDAG &DAG) const;
Index: llvm/trunk/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
===================================================================
--- llvm/trunk/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ llvm/trunk/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -273,6 +273,11 @@
   setLibcallName(RTLIB::FPEXT_F16_F32, "__extendhfsf2");
   setLibcallName(RTLIB::FPROUND_F32_F16, "__truncsfhf2");
 
+  // Define the emscripten name for return address helper.
+  // TODO: when implementing other WASM backends, make this generic or only do
+  // this on emscripten depending on what they end up doing.
+  setLibcallName(RTLIB::RETURN_ADDRESS, "emscripten_return_address");
+
   // Always convert switches to br_tables unless there is only one case, which
   // is equivalent to a simple branch. This reduces code size for wasm, and we
   // defer possible jump table optimizations to the VM.
@@ -919,9 +924,8 @@
   case ISD::BRIND:
     fail(DL, DAG, "WebAssembly hasn't implemented computed gotos");
     return SDValue();
-  case ISD::RETURNADDR: // Probably nothing meaningful can be returned here.
-    fail(DL, DAG, "WebAssembly hasn't implemented __builtin_return_address");
-    return SDValue();
+  case ISD::RETURNADDR:
+    return LowerRETURNADDR(Op, DAG);
   case ISD::FRAMEADDR:
     return LowerFRAMEADDR(Op, DAG);
   case ISD::CopyToReg:
@@ -978,6 +982,26 @@
   return DAG.getTargetFrameIndex(FI, Op.getValueType());
 }
 
+SDValue WebAssemblyTargetLowering::LowerRETURNADDR(SDValue Op,
+                                                   SelectionDAG &DAG) const {
+  SDLoc DL(Op);
+
+  if (!Subtarget->getTargetTriple().isOSEmscripten()) {
+    fail(DL, DAG,
+         "Non-Emscripten WebAssembly hasn't implemented "
+         "__builtin_return_address");
+    return SDValue();
+  }
+
+  if (verifyReturnAddressArgumentIsConstant(Op, DAG))
+    return SDValue();
+
+  unsigned Depth = cast<ConstantSDNode>(Op.getOperand(0))->getZExtValue();
+  return makeLibCall(DAG, RTLIB::RETURN_ADDRESS, Op.getValueType(),
+                     {DAG.getConstant(Depth, DL, MVT::i32)}, false, DL)
+      .first;
+}
+
 SDValue WebAssemblyTargetLowering::LowerFRAMEADDR(SDValue Op,
                                                   SelectionDAG &DAG) const {
   // Non-zero depths are not supported by WebAssembly currently. Use the
Index: llvm/trunk/lib/Target/WebAssembly/WebAssemblyRuntimeLibcallSignatures.cpp
===================================================================
--- llvm/trunk/lib/Target/WebAssembly/WebAssemblyRuntimeLibcallSignatures.cpp
+++ llvm/trunk/lib/Target/WebAssembly/WebAssemblyRuntimeLibcallSignatures.cpp
@@ -325,6 +325,9 @@
     // __stack_chk_fail
     Table[RTLIB::STACKPROTECTOR_CHECK_FAIL] = func;
 
+    // Return address handling
+    Table[RTLIB::RETURN_ADDRESS] = i32_func_i32;
+
     // Element-wise Atomic memory
     // TODO: Fix these when we implement atomic support
     Table[RTLIB::MEMCPY_ELEMENT_UNORDERED_ATOMIC_1] = unsupported;
@@ -499,6 +502,8 @@
     // consistent with the f64 and f128 names.
     Map["__extendhfsf2"] = RTLIB::FPEXT_F16_F32;
     Map["__truncsfhf2"] = RTLIB::FPROUND_F32_F16;
+
+    Map["emscripten_return_address"] = RTLIB::RETURN_ADDRESS;
   }
 };
 
Index: llvm/trunk/test/CodeGen/WebAssembly/return-address-emscripten.ll
===================================================================
--- llvm/trunk/test/CodeGen/WebAssembly/return-address-emscripten.ll
+++ llvm/trunk/test/CodeGen/WebAssembly/return-address-emscripten.ll
@@ -0,0 +1,19 @@
+; RUN: llc < %s -asm-verbose=false | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-emscripten"
+
+; This tests the implementation of __builtin_return_address on emscripten
+
+; CHECK-LABEL: test_returnaddress:
+; CHECK-NEXT: .functype test_returnaddress () -> (i32){{$}}
+; CHECK-NEXT: {{^}} i32.const 0{{$}}
+; CHECK-NEXT: {{^}} i32.call emscripten_return_address{{$}}
+; CHECK-NEXT: {{^}} end_function{{$}}
+define i8* @test_returnaddress() {
+  %r = call i8* @llvm.returnaddress(i32 0)
+  ret i8* %r
+}
+
+; LLVM represents __builtin_return_address as call to this function in IR.
+declare i8* @llvm.returnaddress(i32 immarg)
Index: llvm/trunk/test/CodeGen/WebAssembly/return-address-unknown.ll
===================================================================
--- llvm/trunk/test/CodeGen/WebAssembly/return-address-unknown.ll
+++ llvm/trunk/test/CodeGen/WebAssembly/return-address-unknown.ll
@@ -0,0 +1,16 @@
+; RUN: not llc < %s -asm-verbose=false 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+; This tests the implementation of __builtin_return_address on the unknown OS.
+; Since this is not implemented, it should fail.
+
+; CHECK: Non-Emscripten WebAssembly hasn't implemented __builtin_return_address
+define i8* @test_returnaddress() {
+  %r = call i8* @llvm.returnaddress(i32 0)
+  ret i8* %r
+}
+
+; LLVM represents __builtin_return_address as call to this function in IR.
+declare i8* @llvm.returnaddress(i32 immarg)