wasip2: Improve "command" startup flow

CMakeLists.txt

@@ -148,6 +148,7 @@ if (NOT (WASI STREQUAL "p1"))
 endif()
 include(check-symbols)
 include(clang-format)
+include(wasm-tools)
 
 # =============================================================================
 # Generic top-level build flags/settings

cmake/ba-download.cmake

@@ -27,13 +27,14 @@ function(ba_download target repo version)
 
   if (target STREQUAL wasmtime)
     set(fmt tar.xz)
-  elseif ((target STREQUAL wasm-component-ld) AND (os STREQUAL windows))
+  elseif ((os STREQUAL windows) AND
+      ((target STREQUAL wasm-component-ld) OR (target STREQUAL wasm-tools)))
     set(fmt zip)
   else()
     set(fmt tar.gz)
   endif()
 
-  if (target STREQUAL wit-bindgen)
+  if (target STREQUAL wit-bindgen OR target STREQUAL wasm-tools)
     set(tag v${version})
   else()
     set(tag ${version})

cmake/wasm-tools.cmake

@@ -0,0 +1,15 @@
+# Favor `wasm-tools` on the system
+find_program(WASM_TOOLS_EXECUTABLE NAMES wasm-tools)
+if (NOT WASM_TOOLS_EXECUTABLE)
+  include(ba-download)
+  ba_download(
+    wasm-tools
+    "https://github.com/bytecodealliance/wasm-tools"
+    "1.244.0"
+  )
+  ExternalProject_Get_Property(wasm-tools SOURCE_DIR)
+  set(wasm_tools "${SOURCE_DIR}/wasm-tools")
+else()
+  add_custom_target(wasm-tools)
+  set(wasm_tools ${WASM_TOOLS_EXECUTABLE})
+endif()

libc-bottom-half/CMakeLists.txt

@@ -180,31 +180,94 @@ foreach(obj bottom-half-shared bottom-half-static)
   )
 endforeach()
 
-add_custom_target(sysroot-startup-objects)
-add_dependencies(sysroot sysroot-startup-objects)
 
 # =============================================================================
 # startup objects
 #
-foreach(file crt/crt1.c
-             crt/crt1-command.c
+# This is the logic for building `crt1{,-reactor,-command}.o` objects which
+# Clang will link by default based on compiler flags. These are compiled into
+# "object libraries" with CMake and then they're copied into the final location
+# with adjustments based on WASI versions.
+foreach(file crt/crt1-command.c
              crt/crt1-reactor.c)
-  # get the filename without the directory and extension
-  cmake_path(GET file STEM filename)
+  cmake_path(GET file STEM stem)
+  add_library(${stem} OBJECT ${file})
+  clang_format_target(${stem})
+  target_link_libraries(${stem} PRIVATE musl-top-half-interface)
+  set_pic(${stem})
+  target_compile_options(${stem} PRIVATE -fvisibility=default)
+endforeach()
 
-  # create a custom target for each file
-  add_library(${filename}.o OBJECT ${file})
-  target_link_libraries(${filename}.o PUBLIC musl-top-half-interface)
-  set_pic(${filename}.o)
-  target_compile_options(${filename}.o PRIVATE -fvisibility=default)
+# crt1-reactor.o is a straight copy of what CMake produces
+add_custom_command(
+  OUTPUT ${SYSROOT_LIB}/crt1-reactor.o
+  COMMAND ${CMAKE_COMMAND} -E copy $<TARGET_OBJECTS:crt1-reactor> ${SYSROOT_LIB}/crt1-reactor.o
+  DEPENDS crt1-reactor $<TARGET_OBJECTS:crt1-reactor>
+)
 
-  # add a custom command to compile the file
-  set(dst ${SYSROOT_LIB}/${filename}.o)
+if (WASI STREQUAL "p1")
+  # wasip1: crt1-command.o is a straight copy of what CMake produces
   add_custom_command(
-    OUTPUT ${dst}
-    COMMAND ${CMAKE_COMMAND} -E copy $<TARGET_OBJECTS:${filename}.o> ${dst}
-    DEPENDS ${filename}.o
+    OUTPUT ${SYSROOT_LIB}/crt1-command.o
+    COMMAND ${CMAKE_COMMAND} -E copy $<TARGET_OBJECTS:crt1-command> ${SYSROOT_LIB}/crt1-command.o
+    DEPENDS crt1-command $<TARGET_OBJECTS:crt1-command>
   )
-  add_custom_target(sysroot-startup-${filename}.o DEPENDS ${dst})
-  add_dependencies(sysroot-startup-objects sysroot-startup-${filename}.o)
-endforeach()
+elseif (WASI STREQUAL "p2")
+  # wasip2: crt1-command.o is modified from what CMake produces to
+  # additionally have a custom section representing the type information needed
+  # for its contained export. This is the `wasi:cli/run` interface, for example.
+  #
+  # TODO: ideally this wouldn't need `wasm-tools` as that's an extra build tool
+  # needed here and it's best to slim down dependencies as much as possible.
+  # This additionally requires downloading/vendoring WITs which isn't great.
+  # Ideally the type information embedded here would be referenced dierctly in
+  # `crt1-command.c` itself in the source. That would likely require some
+  # combination of `__asm__` and `#embed` but I at least couldn't figure out how
+  # to get that working. I'm also not aware of a way to, in Clang, define a
+  # global in C that goes into a custom section in the output object. Another
+  # possible route would be to use the `--relocatable` option of `wasm-ld,` but
+  # it looks like that loses `__attribute__((export_name("...")))` information
+  # which this object file relies on. As a workaround `wasm-tools` is used for
+  # now. If you, dear reader, know of how to do this in the C source itself
+  # please feel free to open an issue or a PR and maintainers can work with you
+  # on getting that integrated.
+  add_custom_command(
+    OUTPUT ${SYSROOT_LIB}/crt1-command.o
+    COMMAND
+      ${wasm_tools} component embed
+        ${wasip2_wit_dir}
+        $<TARGET_OBJECTS:crt1-command>
+        --world wasi:cli/command@0.2.0
+        -o ${SYSROOT_LIB}/crt1-command.o
+    DEPENDS crt1-command wasip2-wits $<TARGET_OBJECTS:crt1-command> wasm-tools
+  )
+elseif (WASI STREQUAL "p3")
+  add_custom_command(
+    OUTPUT ${SYSROOT_LIB}/crt1-command.o
+    COMMAND
+      ${wasm_tools} component embed
+        ${wasip3_wit_dir}
+        $<TARGET_OBJECTS:crt1-command>
+        --world wasi:cli/command@0.3.0-rc-2025-09-16
+        -o ${SYSROOT_LIB}/crt1-command.o
+    DEPENDS crt1-command wasip3-wits $<TARGET_OBJECTS:crt1-command> wasm-tools
+  )
+else()
+  message(FATAL_ERROR "Unknown WASI version: ${WASI}")
+endif()
+
+# Provide a plain crt1.o for toolchain compatibility, identical to
+# `crt1-command.c`
+add_custom_command(
+  OUTPUT ${SYSROOT_LIB}/crt1.o
+  COMMAND ${CMAKE_COMMAND} -E copy ${SYSROOT_LIB}/crt1-command.o ${SYSROOT_LIB}/crt1.o
+  DEPENDS ${SYSROOT_LIB}/crt1-command.o
+)
+
+add_custom_target(sysroot-startup-objects
+  DEPENDS
+    ${SYSROOT_LIB}/crt1-reactor.o
+    ${SYSROOT_LIB}/crt1-command.o
+    ${SYSROOT_LIB}/crt1.o
+)
+add_dependencies(sysroot sysroot-startup-objects)

libc-bottom-half/crt/crt1-command.c

@@ -9,54 +9,64 @@ extern void __wasm_call_ctors(void);
 extern int __main_void(void);
 extern void __wasm_call_dtors(void);
 
-__attribute__((export_name("_start")))
-void _start(void) {
-    // Commands should only be called once per instance. This simple check
-    // ensures that the `_start` function isn't started more than once.
-    //
-    // We use `volatile` here to prevent the store to `started` from being
-    // sunk past any subsequent code, and to prevent any compiler from
-    // optimizing based on the knowledge that `_start` is the program
-    // entrypoint.
+#if defined(__wasip1__)
+__attribute__((export_name("_start"))) void _start(void)
+#elif defined(__wasip2__)
+// Note that this is manually doing what `wit-bindgen` might otherwise be
+// doing. Given the special nature of this symbol this skip the typical
+// `wit-bindgen` rigamarole and the signature of this function is simple enough
+// that this shouldn't be too problematic (in theory).
+__attribute__((export_name("wasi:cli/run@0.2.0#run"))) int _start(void)
+#elif defined(__wasip3__)
+__attribute__((export_name("wasi:cli/run@0.3.0-rc-2025-09-16#run"))) int
+_start(void)
+#else
+#error "Unsupported WASI version"
+#endif
+{
+  // Commands should only be called once per instance. This simple check
+  // ensures that the `_start` function isn't started more than once.
+  //
+  // We use `volatile` here to prevent the store to `started` from being
+  // sunk past any subsequent code, and to prevent any compiler from
+  // optimizing based on the knowledge that `_start` is the program
+  // entrypoint.
 #ifdef _REENTRANT
-    static volatile _Atomic int started = 0;
-    int expected = 0;
-    if (!atomic_compare_exchange_strong(&started, &expected, 1)) {
-        __builtin_trap();
-    }
+  static volatile _Atomic int started = 0;
+  int expected = 0;
+  if (!atomic_compare_exchange_strong(&started, &expected, 1)) {
+    __builtin_trap();
+  }
 #else
-    static volatile int started = 0;
-    if (started != 0) {
-        __builtin_trap();
-    }
-    started = 1;
+  static volatile int started = 0;
+  if (started != 0) {
+    __builtin_trap();
+  }
+  started = 1;
 #endif
 
-    __wasi_init_tp();
+  __wasi_init_tp();
 
-    // The linker synthesizes this to call constructors.
-    __wasm_call_ctors();
+  // The linker synthesizes this to call constructors.
+  __wasm_call_ctors();
 
-    // Call `__main_void` which will either be the application's zero-argument
-    // `__main_void` function or a libc routine which obtains the command-line
-    // arguments and calls `__main_argv_argc`.
-    int r = __main_void();
+  // Call `__main_void` which will either be the application's zero-argument
+  // `__main_void` function or a libc routine which obtains the command-line
+  // arguments and calls `__main_argv_argc`.
+  int r = __main_void();
 
-    // Call atexit functions, destructors, stdio cleanup, etc.
-    __wasm_call_dtors();
+  // Call atexit functions, destructors, stdio cleanup, etc.
+  __wasm_call_dtors();
 
-    // If main exited successfully, just return, otherwise call
-    // `__wasi_proc_exit`.
+  // If main exited successfully, just return, otherwise call
+  // `__wasi_proc_exit`.
 #if defined(__wasip1__)
-    if (r != 0) {
-        __wasi_proc_exit(r);
-    }
+  if (r != 0) {
+    __wasi_proc_exit(r);
+  }
 #elif defined(__wasip2__) || defined(__wasip3__)
-    if (r != 0) {
-        exit_result_void_void_t status = { .is_err = true };
-        exit_exit(&status);
-    }
+  return r != 0;
 #else
-# error "Unsupported WASI version"
+#error "Unsupported WASI version"
 #endif
 }

libc-bottom-half/crt/crt1-reactor.c

@@ -4,23 +4,22 @@
 extern void __wasi_init_tp(void);
 extern void __wasm_call_ctors(void);
 
-__attribute__((export_name("_initialize")))
-void _initialize(void) {
+__attribute__((export_name("_initialize"))) void _initialize(void) {
 #if defined(_REENTRANT)
-    static volatile atomic_int initialized = 0;
-    int expected = 0;
-    if (!atomic_compare_exchange_strong(&initialized, &expected, 1)) {
-        __builtin_trap();
-    }
+  static volatile atomic_int initialized = 0;
+  int expected = 0;
+  if (!atomic_compare_exchange_strong(&initialized, &expected, 1)) {
+    __builtin_trap();
+  }
 #else
-    static volatile int initialized = 0;
-    if (initialized != 0) {
-        __builtin_trap();
-    }
-    initialized = 1;
+  static volatile int initialized = 0;
+  if (initialized != 0) {
+    __builtin_trap();
+  }
+  initialized = 1;
 #endif
-    __wasi_init_tp();
+  __wasi_init_tp();
 
-    // The linker synthesizes this to call constructors.
-    __wasm_call_ctors();
+  // The linker synthesizes this to call constructors.
+  __wasm_call_ctors();
 }