Index: clang/docs/ClangCommandLineReference.rst
===================================================================
--- clang/docs/ClangCommandLineReference.rst
+++ clang/docs/ClangCommandLineReference.rst
@@ -2599,6 +2599,12 @@
 
 .. option:: -msimd128, -mno-simd128
 
+.. option:: -mexec-model=<arg>
+
+Select between "command" and "reactor" executable models. Commands have a main
+function which scopes the lifetime of the program. Reactors are activated and
+remain active until explicitly terminated.
+
 X86
 ---
 .. option:: -m3dnow, -mno-3dnow
Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -148,6 +148,10 @@
                            Group<m_Group>, DocName<"PowerPC">;
 def m_wasm_Features_Group : OptionGroup<"<wasm features group>">,
                             Group<m_Group>, DocName<"WebAssembly">;
+// The features added by this group will not be added to target features.
+// These are explicitly handled.
+def m_wasm_Features_Driver_Group : OptionGroup<"<wasm driver features group>">,
+                                   Group<m_Group>, DocName<"WebAssembly Driver">;
 def m_x86_Features_Group : OptionGroup<"<x86 features group>">,
                            Group<m_Group>, Flags<[CoreOption]>, DocName<"X86">;
 def m_riscv_Features_Group : OptionGroup<"<riscv features group>">,
@@ -2189,6 +2193,9 @@
 def mno_multivalue : Flag<["-"], "mno-multivalue">, Group<m_wasm_Features_Group>;
 def mtail_call : Flag<["-"], "mtail-call">, Group<m_wasm_Features_Group>;
 def mno_tail_call : Flag<["-"], "mno-tail-call">, Group<m_wasm_Features_Group>;
+def mexec_model_EQ : Joined<["-"], "mexec-model=">, Group<m_wasm_Features_Driver_Group>,
+                     Values<"command,reactor">,
+                     HelpText<"Executable model (WebAssembly only)">;
 
 def mamdgpu_debugger_abi : Joined<["-"], "mamdgpu-debugger-abi=">,
   Flags<[HelpHidden]>,
Index: clang/lib/Driver/ToolChains/WebAssembly.cpp
===================================================================
--- clang/lib/Driver/ToolChains/WebAssembly.cpp
+++ clang/lib/Driver/ToolChains/WebAssembly.cpp
@@ -67,8 +67,23 @@
   Args.AddAllArgs(CmdArgs, options::OPT_u);
   ToolChain.AddFilePathLibArgs(Args, CmdArgs);
 
-  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles))
-    CmdArgs.push_back(Args.MakeArgString(ToolChain.GetFilePath("crt1.o")));
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
+    const char *CrtO = "crt1.o";
+    const char *Entry = NULL;
+    if (const Arg *A = Args.getLastArg(options::OPT_mexec_model_EQ)) {
+      CrtO = llvm::StringSwitch<const char *>(A->getValue())
+                 .Case("reactor", "reactor-crt1.o")
+                 .Default(CrtO);
+      Entry = llvm::StringSwitch<const char *>(A->getValue())
+                 .Case("reactor", "__wasi_unstable_reactor_start")
+                 .Default(CrtO);
+    }
+    CmdArgs.push_back(Args.MakeArgString(ToolChain.GetFilePath(CrtO)));
+    if (Entry) {
+      CmdArgs.push_back(Args.MakeArgString("--entry"));
+      CmdArgs.push_back(Args.MakeArgString(Entry));
+    }
+  }
 
   AddLinkerInputs(ToolChain, Inputs, Args, CmdArgs, JA);