Index: include/llvm/ADT/Triple.h
===================================================================
--- include/llvm/ADT/Triple.h
+++ include/llvm/ADT/Triple.h
@@ -187,7 +187,8 @@
     AMDPAL,     // AMD PAL Runtime
     HermitCore, // HermitCore Unikernel/Multikernel
     Hurd,       // GNU/Hurd
-    LastOSType = Hurd
+    COWS,       // Capability-Oriented WebAssembly Syscalls
+    LastOSType = COWS
   };
   enum EnvironmentType {
     UnknownEnvironment,
@@ -588,6 +589,11 @@
     return getOS() == Triple::Hurd;
   }
 
+  /// Tests whether the OS is COWS.
+  bool isOSCOWS() const {
+    return getOS() == Triple::COWS;
+  }
+
   /// Tests whether the OS uses glibc.
   bool isOSGlibc() const {
     return (getOS() == Triple::Linux || getOS() == Triple::KFreeBSD ||
Index: lib/Support/Triple.cpp
===================================================================
--- lib/Support/Triple.cpp
+++ lib/Support/Triple.cpp
@@ -211,6 +211,7 @@
   case AMDPAL: return "amdpal";
   case HermitCore: return "hermit";
   case Hurd: return "hurd";
+  case COWS: return "cows";
   }
 
   llvm_unreachable("Invalid OSType");
@@ -510,6 +511,7 @@
     .StartsWith("amdpal", Triple::AMDPAL)
     .StartsWith("hermit", Triple::HermitCore)
     .StartsWith("hurd", Triple::Hurd)
+    .StartsWith("cows", Triple::COWS)
     .Default(Triple::UnknownOS);
 }
 
Index: tools/clang/lib/Basic/Targets.cpp
===================================================================
--- tools/clang/lib/Basic/Targets.cpp
+++ tools/clang/lib/Basic/Targets.cpp
@@ -572,18 +572,20 @@
   case llvm::Triple::wasm32:
     if (Triple.getSubArch() != llvm::Triple::NoSubArch ||
         Triple.getVendor() != llvm::Triple::UnknownVendor ||
-        Triple.getOS() != llvm::Triple::UnknownOS ||
-        Triple.getEnvironment() != llvm::Triple::UnknownEnvironment ||
-        !(Triple.isOSBinFormatELF() || Triple.isOSBinFormatWasm()))
+        !Triple.isOSBinFormatWasm())
       return nullptr;
+    if (Triple.getOS() != llvm::Triple::UnknownOS &&
+        Triple.getOS() != llvm::Triple::COWS)
+      return nullptr;
     return new WebAssemblyOSTargetInfo<WebAssembly32TargetInfo>(Triple, Opts);
   case llvm::Triple::wasm64:
     if (Triple.getSubArch() != llvm::Triple::NoSubArch ||
         Triple.getVendor() != llvm::Triple::UnknownVendor ||
-        Triple.getOS() != llvm::Triple::UnknownOS ||
-        Triple.getEnvironment() != llvm::Triple::UnknownEnvironment ||
-        !(Triple.isOSBinFormatELF() || Triple.isOSBinFormatWasm()))
+        !Triple.isOSBinFormatWasm())
       return nullptr;
+    if (Triple.getOS() != llvm::Triple::UnknownOS &&
+        Triple.getOS() != llvm::Triple::COWS)
+      return nullptr;
     return new WebAssemblyOSTargetInfo<WebAssembly64TargetInfo>(Triple, Opts);
 
   case llvm::Triple::renderscript32:
Index: tools/clang/lib/Driver/ToolChains/WebAssembly.cpp
===================================================================
--- tools/clang/lib/Driver/ToolChains/WebAssembly.cpp
+++ tools/clang/lib/Driver/ToolChains/WebAssembly.cpp
@@ -24,6 +24,15 @@
 wasm::Linker::Linker(const ToolChain &TC)
     : GnuTool("wasm::Linker", "lld", TC) {}
 
+/// Following the conventions in https://wiki.debian.org/Multiarch/Tuples,
+/// we remove the vendor field to form the multiarch triple.
+static std::string getMultiarchTriple(const Driver &D,
+                                      const llvm::Triple &TargetTriple,
+                                      StringRef SysRoot) {
+    return TargetTriple.getArchName() + "-" +
+           TargetTriple.getOSAndEnvironmentName();
+}
+
 bool wasm::Linker::isLinkJob() const { return true; }
 
 bool wasm::Linker::hasIntegratedCPP() const { return false; }
@@ -75,7 +84,16 @@
 
   getProgramPaths().push_back(getDriver().getInstalledDir());
 
-  getFilePaths().push_back(getDriver().SysRoot + "/lib");
+  if (Triple.getOS() == Triple::UnknownOS) {
+    // Theoretically an "unknown" OS should mean no standard libraries, however
+    // it could also mean that a custom set of libraries is in use, so just add
+    // /lib to the search path. Disable multiarch in this case, to discourage
+    // paths containing "unknown" from acquiring meanings.
+    getFilePaths().push_back(getDriver().SysRoot + "/lib");
+  } else {
+    const std::string MultiarchTriple = getMultiarchTriple(D, Triple, SysRoot);
+    getFilePaths().push_back(getDriver().SysRoot + "/lib/" + MultiarchTriple);
+  }
 }
 
 bool WebAssembly::IsMathErrnoDefault() const { return false; }
@@ -124,16 +142,27 @@
 
 void WebAssembly::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
                                             ArgStringList &CC1Args) const {
-  if (!DriverArgs.hasArg(options::OPT_nostdinc))
+  if (!DriverArgs.hasArg(options::OPT_nostdinc)) {
+    if (Triple.getOS() != Triple::UnknownOS) {
+      const std::string MultiarchTriple = getMultiarchTriple(D, Triple, SysRoot);
+      addSystemInclude(DriverArgs, CC1Args, getDriver().SysRoot + "/include/" + MultiarchTriple);
+    }
     addSystemInclude(DriverArgs, CC1Args, getDriver().SysRoot + "/include");
+  }
 }
 
 void WebAssembly::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
                                                ArgStringList &CC1Args) const {
   if (!DriverArgs.hasArg(options::OPT_nostdlibinc) &&
-      !DriverArgs.hasArg(options::OPT_nostdincxx))
+      !DriverArgs.hasArg(options::OPT_nostdincxx)) {
+    if (Triple.getOS() != Triple::UnknownOS) {
+      const std::string MultiarchTriple = getMultiarchTriple(D, Triple, SysRoot);
+      addSystemInclude(DriverArgs, CC1Args,
+                       getDriver().SysRoot + "/include/" + MultiarchTriple + "/c++/v1");
+    }
     addSystemInclude(DriverArgs, CC1Args,
                      getDriver().SysRoot + "/include/c++/v1");
+  }
 }
 
 void WebAssembly::AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
Index: tools/clang/test/Driver/wasm-toolchain.c
===================================================================
--- tools/clang/test/Driver/wasm-toolchain.c
+++ tools/clang/test/Driver/wasm-toolchain.c
@@ -10,14 +10,31 @@
 // RUN: %clang %s -### -target wasm32-unknown-unknown -fvisibility=default 2>&1 | FileCheck -check-prefix=FVISIBILITY_DEFAULT %s
 // FVISIBILITY_DEFAULT-NOT: hidden
 
-// A basic C link command-line.
+// A basic C link command-line with unknown OS.
 
 // RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo -fuse-ld=wasm-ld %s 2>&1 | FileCheck -check-prefix=LINK %s
 // LINK: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK: wasm-ld{{.*}}" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
-// A basic C link command-line with optimization.
+// A basic C link command-line with optimization with unknown OS.
 
 // RUN: %clang -### -O2 -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo -fuse-ld=wasm-ld %s 2>&1 | FileCheck -check-prefix=LINK_OPT %s
 // LINK_OPT: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK_OPT: wasm-ld{{.*}}" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
+
+// A basic C link command-line with known OS.
+
+// RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-cows-musl --sysroot=/foo -fuse-ld=wasm-ld %s 2>&1 | FileCheck -check-prefix=LINK %s
+// LINK: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK: wasm-ld{{.*}}" "-L/foo/lib/wasm32-cows-musl" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
+
+// A basic C link command-line with optimization with known OS.
+
+// RUN: %clang -### -O2 -no-canonical-prefixes -target wasm32-unknown-cows-musl --sysroot=/foo -fuse-ld=wasm-ld %s 2>&1 | FileCheck -check-prefix=LINK_OPT %s
+// LINK_OPT: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK_OPT: wasm-ld{{.*}}" "-L/foo/lib/wasm32-cows-musl" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
+
+// A basic C compile command-line with known OS.
+
+// RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-cows-musl --sysroot=/foo %s 2>&1 | FileCheck -check-prefix=COMPILE %s
+// COMPILE: clang{{.*}}" "-cc1" {{.*}} -I /foo/wasm32-cows-musl/include -I /include
Index: tools/clang/test/Driver/wasm-toolchain.cpp
===================================================================
--- tools/clang/test/Driver/wasm-toolchain.cpp
+++ tools/clang/test/Driver/wasm-toolchain.cpp
@@ -10,14 +10,31 @@
 // RUN: %clangxx %s -### -target wasm32-unknown-unknown -fvisibility=default 2>&1 | FileCheck -check-prefix=FVISIBILITY_DEFAULT %s
 // FVISIBILITY_DEFAULT-NOT: hidden
 
-// A basic C++ link command-line.
+// A basic C++ link command-line with unknown OS.
 
-// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo --stdlib=c++ -fuse-ld=wasm-ld %s 2>&1 | FileCheck -check-prefix=LINK %s
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo -fuse-ld=wasm-ld --stdlib=c++ %s 2>&1 | FileCheck -check-prefix=LINK %s
 // LINK: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK: wasm-ld{{.*}}" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
-// A basic C++ link command-line with optimization.
+// A basic C++ link command-line with optimization with unknown OS.
 
 // RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s --stdlib=c++ -fuse-ld=wasm-ld 2>&1 | FileCheck -check-prefix=LINK_OPT %s
 // LINK_OPT: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK_OPT: wasm-ld{{.*}}" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
+
+// A basic C++ link command-line with known OS.
+
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-unknown-cows-musl --sysroot=/foo -fuse-ld=wasm-ld --stdlib=c++ %s 2>&1 | FileCheck -check-prefix=LINK %s
+// LINK: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK: wasm-ld{{.*}}" "-L/foo/lib/wasm32-cows-musl" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
+
+// A basic C++ link command-line with optimization with known OS.
+
+// RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-unknown-cows-musl --sysroot=/foo %s --stdlib=c++ -fuse-ld=wasm-ld 2>&1 | FileCheck -check-prefix=LINK_OPT %s
+// LINK_OPT: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK_OPT: wasm-ld{{.*}}" "-L/foo/lib/wasm32-cows-musl" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
+
+// A basic C++ compile command-line with known OS.
+
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-unknown-cows-musl --sysroot=/foo --stdlib=c++ %s 2>&1 | FileCheck -check-prefix=COMPILE %s
+// COMPILE: clang{{.*}}" "-cc1" {{.*}} -I /foo/wasm32-cows-musl/include -I /include -I /foo/wasm32-cows-musl/include/c++/v1 -I /foo/wasm32/include/c++/v1
Index: unittests/ADT/TripleTest.cpp
===================================================================
--- unittests/ADT/TripleTest.cpp
+++ unittests/ADT/TripleTest.cpp
@@ -253,6 +253,12 @@
   EXPECT_EQ(Triple::UnknownOS, T.getOS());
   EXPECT_EQ(Triple::UnknownEnvironment, T.getEnvironment());
 
+  T = Triple("wasm32-unknown-cows-musl");
+  EXPECT_EQ(Triple::wasm32, T.getArch());
+  EXPECT_EQ(Triple::UnknownVendor, T.getVendor());
+  EXPECT_EQ(Triple::COWS, T.getOS());
+  EXPECT_EQ(Triple::Musl, T.getEnvironment());
+
   T = Triple("wasm64-unknown-unknown");
   EXPECT_EQ(Triple::wasm64, T.getArch());
   EXPECT_EQ(Triple::UnknownVendor, T.getVendor());
@@ -259,6 +265,12 @@
   EXPECT_EQ(Triple::UnknownOS, T.getOS());
   EXPECT_EQ(Triple::UnknownEnvironment, T.getEnvironment());
 
+  T = Triple("wasm64-unknown-cows-musl");
+  EXPECT_EQ(Triple::wasm64, T.getArch());
+  EXPECT_EQ(Triple::UnknownVendor, T.getVendor());
+  EXPECT_EQ(Triple::COWS, T.getOS());
+  EXPECT_EQ(Triple::Musl, T.getEnvironment());
+
   T = Triple("avr-unknown-unknown");
   EXPECT_EQ(Triple::avr, T.getArch());
   EXPECT_EQ(Triple::UnknownVendor, T.getVendor());
@@ -1235,11 +1247,17 @@
 
   EXPECT_EQ(Triple::Wasm, Triple("wasm32-unknown-unknown").getObjectFormat());
   EXPECT_EQ(Triple::Wasm, Triple("wasm64-unknown-unknown").getObjectFormat());
+  EXPECT_EQ(Triple::Wasm, Triple("wasm32-unknown-cows-musl").getObjectFormat());
+  EXPECT_EQ(Triple::Wasm, Triple("wasm64-unknown-cows-musl").getObjectFormat());
 
   EXPECT_EQ(Triple::Wasm,
             Triple("wasm32-unknown-unknown-wasm").getObjectFormat());
   EXPECT_EQ(Triple::Wasm,
             Triple("wasm64-unknown-unknown-wasm").getObjectFormat());
+  EXPECT_EQ(Triple::Wasm,
+            Triple("wasm32-unknown-cows-musl-wasm").getObjectFormat());
+  EXPECT_EQ(Triple::Wasm,
+            Triple("wasm64-unknown-cows-musl-wasm").getObjectFormat());
 
   Triple MSVCNormalized(Triple::normalize("i686-pc-windows-msvc-elf"));
   EXPECT_EQ(Triple::ELF, MSVCNormalized.getObjectFormat());