diff --git a/lib/ClangImporter/ClangImporter.cpp b/lib/ClangImporter/ClangImporter.cpp
index 7942bcd75521c..d699c50b057ef 100644
--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -600,6 +600,10 @@ importer::getNormalInvocationArguments(
       });
     }
 
+    if (triple.isOSWASI()) {
+      invocationArgStrs.insert(invocationArgStrs.end(), {"-D_WASI_EMULATED_MMAN"});
+    }
+
     if (triple.isOSWindows()) {
       switch (triple.getArch()) {
       default: llvm_unreachable("unsupported Windows architecture");