Index: llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
===================================================================
--- llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
+++ llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
@@ -499,6 +499,15 @@
   // This pseudo-feature tells the linker whether shared memory would be safe
   EmitFeature("shared-mem");
 
+  // This is an "architecture", not a "feature", but we emit it as such for
+  // the benefit of tools like Binaryen and consistency with other producers.
+  // FIXME: Subtarget is null here, so can't Subtarget->hasAddr64() ?
+  if (M.getDataLayout().getMaxPointerSize() == 8) {
+    // Can't use EmitFeature since "wasm-feature-memory64" is not a module
+    // flag.
+    EmittedFeatures.push_back({wasm::WASM_FEATURE_PREFIX_USED, "memory64"});
+  }
+
   if (EmittedFeatures.size() == 0)
     return;
 
Index: llvm/test/CodeGen/WebAssembly/memory64-feature.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/WebAssembly/memory64-feature.ll
@@ -0,0 +1,15 @@
+; RUN: llc < %s | FileCheck %s
+
+; Test that wasm64 is properly emitted into the target features section
+
+target triple = "wasm64-unknown-unknown"
+
+define void @foo() {
+  ret void
+}
+
+; CHECK-LABEL: .custom_section.target_features
+; CHECK-NEXT: .int8 1
+; CHECK-NEXT: .int8 43
+; CHECK-NEXT: .int8 8
+; CHECK-NEXT: .ascii "memory64"
Index: llvm/test/MC/WebAssembly/debug-info64.ll
===================================================================
--- llvm/test/MC/WebAssembly/debug-info64.ll
+++ llvm/test/MC/WebAssembly/debug-info64.ll
@@ -129,6 +129,12 @@
 ; CHECK-NEXT:     Offset: 1073
 ; CHECK-NEXT:     Name: producers
 ; CHECK-NEXT:   }
+; CHECK-NEXT:   Section {
+; CHECK-NEXT:     Type: CUSTOM (0x0)
+; CHECK-NEXT:     Size: 11
+; CHECK-NEXT:     Offset: 1166
+; CHECK-NEXT:     Name: target_features
+; CHECK-NEXT:   }
 ; CHECK-NEXT: ]
 ; CHECK-NEXT: Relocations [
 ; CHECK-NEXT:   Section (7) DATA {