diff --git a/clang/docs/ClangCommandLineReference.rst b/clang/docs/ClangCommandLineReference.rst
--- a/clang/docs/ClangCommandLineReference.rst
+++ b/clang/docs/ClangCommandLineReference.rst
@@ -3035,6 +3035,12 @@
 
 .. option:: -munimplemented-simd128, -mno-unimplemented-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
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -151,6 +151,10 @@
                            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>">,
@@ -2416,6 +2420,9 @@
 def mno_tail_call : Flag<["-"], "mno-tail-call">, Group<m_wasm_Features_Group>;
 def mreference_types : Flag<["-"], "mreference-types">, Group<m_wasm_Features_Group>;
 def mno_reference_types : Flag<["-"], "mno-reference-types">, Group<m_wasm_Features_Group>;
+def mexec_model_EQ : Joined<["-"], "mexec-model=">, Group<m_wasm_Features_Driver_Group>,
+                     Values<"command,reactor">,
+                     HelpText<"Execution model (WebAssembly only)">;
 
 def mamdgpu_debugger_abi : Joined<["-"], "mamdgpu-debugger-abi=">,
   Flags<[HelpHidden]>,
diff --git a/clang/lib/Driver/ToolChains/WebAssembly.cpp b/clang/lib/Driver/ToolChains/WebAssembly.cpp
--- a/clang/lib/Driver/ToolChains/WebAssembly.cpp
+++ b/clang/lib/Driver/ToolChains/WebAssembly.cpp
@@ -69,8 +69,26 @@
   Args.AddAllArgs(CmdArgs, options::OPT_u);
   ToolChain.AddFilePathLibArgs(Args, CmdArgs);
 
+  const char *Crt1 = "crt1.o";
+  const char *Entry = NULL;
+  if (const Arg *A = Args.getLastArg(options::OPT_mexec_model_EQ)) {
+    StringRef CM = A->getValue();
+    if (CM == "command") {
+      // Use default values.
+    } else if (CM == "reactor") {
+      Crt1 = "crt1-reactor.o";
+      Entry = "_initialize";
+    } else {
+      ToolChain.getDriver().Diag(diag::err_drv_invalid_argument_to_option)
+          << CM << A->getOption().getName();
+    }
+  }
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles))
-    CmdArgs.push_back(Args.MakeArgString(ToolChain.GetFilePath("crt1.o")));
+    CmdArgs.push_back(Args.MakeArgString(ToolChain.GetFilePath(Crt1)));
+  if (Entry) {
+    CmdArgs.push_back(Args.MakeArgString("--entry"));
+    CmdArgs.push_back(Args.MakeArgString(Entry));
+  }
 
   AddLinkerInputs(ToolChain, Inputs, Args, CmdArgs, JA);
 
diff --git a/clang/test/Driver/wasm-toolchain.c b/clang/test/Driver/wasm-toolchain.c
--- a/clang/test/Driver/wasm-toolchain.c
+++ b/clang/test/Driver/wasm-toolchain.c
@@ -107,3 +107,14 @@
 // RUN: %clang %s -### -fsanitize=address -target wasm32-unknown-emscripten 2>&1 | FileCheck -check-prefix=CHECK-ASAN-EMSCRIPTEN %s
 // CHECK-ASAN-EMSCRIPTEN: "-fsanitize=address"
 // CHECK-ASAN-EMSCRIPTEN: "-fsanitize-address-globals-dead-stripping"
+
+// Basic exec-model tests.
+
+// RUN: %clang %s -### -no-canonical-prefixes -target wasm32-unknown-unknown -mexec-model=command 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-COMMAND %s
+// CHECK-COMMAND: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// CHECK-COMMAND: wasm-ld{{.*}}" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
+
+// RUN: %clang %s -### -no-canonical-prefixes -target wasm32-unknown-unknown -mexec-model=reactor 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-REACTOR %s
+// CHECK-REACTOR: wasm-ld{{.*}}" {{.*}} "--entry" "_initialize" {{.*}}