WASIp3 Cooperative Threading #799

alexcrichton · 2026-06-12T16:15:49Z

This I think is ok for an initial implementation, but I think we're going to want to eventually ensure that calls to __wasm_get_stack_pointer go directly to the builtin intrinsic without needing a wrapper in the middle. Otherwise there's basically no way to create a module that directly performs the call because even with LTO these won't get inlined.

-Original file line number
+Diff line change
@@ Expand Up / @@ -26,6 +26,15 @@ if (CMAKE_HOST_WIN32) @@
       set(CMAKE_INCLUDE_SYSTEM_FLAG_C "-isystem ")
     endif()
+    set(TARGET_TRIPLE "wasm32-wasip1" CACHE STRING "WASI target to test")
+    set(CMAKE_C_COMPILER_TARGET ${TARGET_TRIPLE})
+    # We're building libc, and the compiler being used may not have a wasm sysroot
+    # for libc, so skip this check. Otherwise the `project` call below attempts a
+    # link to see if the compiler is working and if that fails it doesn't mean it
+    # doesn't work. Let the build keep going to see what happens.
+    set(CMAKE_C_COMPILER_WORKS 1)
     project(wasi-libc LANGUAGES C ASM)
     if(NOT CMAKE_C_COMPILER_ID MATCHES Clang)
@@ Expand All / @@ -36,7 +45,6 @@ message(STATUS "Found executable for `nm`: ${CMAKE_NM}") @@
     message(STATUS "Found executable for `ar`: ${CMAKE_AR}")
     message(STATUS "Found executable for `ranlib`: ${CMAKE_RANLIB}")
-    set(TARGET_TRIPLE "wasm32-wasip1" CACHE STRING "WASI target to test")
     # Note: thin LTO here is just for experimentation. It has known issues:
     # - https://github.com/llvm/llvm-project/issues/91700
     # - https://github.com/llvm/llvm-project/issues/91711
@@ Expand All @@
     option(CHECK_SYMBOLS "Whether or not to check the exported symbols of libc.a" OFF)
     set(WASI_SDK_VERSION "" CACHE STRING "Version information for wasi-sdk to embed in headers")
     option(ENABLE_WERROR "Whether to compile with `-Werror`" OFF)
+    option(ENABLE_COOP_THREADS "Whether to compile with cooperative threads support" OFF)
     if(TARGET_TRIPLE MATCHES "-threads$")
       set(WASI p1)
@@ Expand Down Expand Up / @@ -86,10 +95,10 @@ else() @@
     endif()
     if(TARGET_TRIPLE MATCHES "-threads$")
-      set(THREADS ON)
       add_compile_options(-mthread-model posix -pthread -ftls-model=local-exec -matomics)
+    elseif(ENABLE_COOP_THREADS)
+      add_compile_options(-mthread-model posix -pthread -ftls-model=local-exec)
     else()
-      set(THREADS OFF)
       add_compile_options(-mthread-model single)
     endif()
@@ Expand Down Expand Up / @@ -209,6 +218,28 @@ if (ENABLE_WERROR) @@
       add_compile_options(-Werror)
     endif()
+    # =============================================================================
+    # Detection of a libcall thread context for wasip3
+    include(CheckSourceCompiles)
+    set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
+    check_source_compiles(C [[
+      #ifndef __wasm_libcall_thread_context__
+      #error "missing define"
+      #endif
+    ]] HAVE_WASM_LIBCALL_THREAD_CONTEXT)
+    if(ENABLE_COOP_THREADS)
+      if(NOT WASI STREQUAL "p3")
+        message(FATAL_ERROR "Cooperative threads are only supported on WASIp3")
+      endif()
+      if (NOT HAVE_WASM_LIBCALL_THREAD_CONTEXT)
+        message(FATAL_ERROR "Cooperative threads require clang 23 or later")
+      endif()
+      set(__wasi_cooperative_threads__ ON)
+    else()
+    endif()
     # =============================================================================
     # Helper functions for adding libraries.
@@ Expand Down Expand Up / @@ -259,7 +290,7 @@ function(add_internal_shared_library name) @@
       add_library(${name} SHARED EXCLUDE_FROM_ALL ${ARGN})
       set_pic(${name})
       target_link_libraries(${name} PUBLIC musl-top-half-interface)
-      add_dependencies(${name} sysroot-c builtins)
+      add_dependencies(${name} sysroot-c builtins sysroot-empty-pthread)
       target_compile_options(${name} PRIVATE -fvisibility=default)
       set_target_properties(${name} PROPERTIES NO_SONAME 1)
       clang_format_target(${name})
@@ Expand Down @@

-Original file line number
+Diff line change
@@ Expand Up / @@ -22,10 +22,12 @@ add_custom_command( @@
         ${CMAKE_CURRENT_LIST_DIR}/scripts/run-check-symbols.cmake
     )
-    if(THREADS)
-      set(expected "${CMAKE_SOURCE_DIR}/expected/wasm32-wasip1-threads")
-    elseif(WASI STREQUAL "p1")
-      set(expected "${CMAKE_SOURCE_DIR}/expected/wasm32-wasip1")
+    if(WASI STREQUAL "p1")
+      if(TARGET_TRIPLE MATCHES "threads")
+        set(expected "${CMAKE_SOURCE_DIR}/expected/wasm32-wasip1-threads")
+      else()
+        set(expected "${CMAKE_SOURCE_DIR}/expected/wasm32-wasip1")
+      endif()
     elseif(WASI STREQUAL "p2")
       set(expected "${CMAKE_SOURCE_DIR}/expected/wasm32-wasip2")
     elseif(WASI STREQUAL "p3")
@@ Expand Down @@

-Original file line number
+Diff line change
@@ Expand Up / @@ -21,7 +21,8 @@ set(formatted_sources) @@
     function(clang_format_file file)
       if (file MATCHES "__generated" OR
-          file MATCHES "wasip.\.c$")  # Skip auto-generated files
+          file MATCHES "wasip.\.c$" OR # Skip auto-generated files
+          file MATCHES "\.(s|S)$")     # Skip assembly files
         return()
       endif()
       cmake_path(ABSOLUTE_PATH file BASE_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} OUTPUT_VARIABLE src)
@@ Expand All / @@ -30,7 +31,7 @@ function(clang_format_file file) @@
       # excludes upstream projects like cloudlibc/musl, malloc implementations, etc.
       if (NOT EXISTS ${src}
         OR ${src} MATCHES "cloudlibc"
-        OR ${src} MATCHES "libc-top-half"
+        OR (${src} MATCHES "libc-top-half" AND NOT ${src} MATCHES "libc-top-half/musl/src/thread/coop-threads")
         OR ${src} MATCHES "fts/musl-fts"
         OR ${src} MATCHES "dlmalloc"
         OR ${src} MATCHES "emmalloc"
@@ Expand Down @@

-Original file line number
+Diff line change
@@ Expand Up / @@ -10,7 +10,7 @@ if (NOT WASM_COMPONENT_LD_EXECUTABLE) @@
       ba_download(
         wasm-component-ld
         "https://github.com/bytecodealliance/wasm-component-ld"
-        "v0.5.21"
+        "v0.5.25"
       )
       ExternalProject_Get_Property(wasm-component-ld SOURCE_DIR)
       set(WASM_COMPONENT_LD_EXECUTABLE "${SOURCE_DIR}/wasm-component-ld")
@@ Expand Down @@

-Original file line number
+Diff line change
@@ Expand Up / @@ -13,10 +13,9 @@ __SIG_ERR @@
     __SIG_IGN
     __acquire_ptc
     __aio_close
+    __allocate_tls
     __asctime_r
     __assert_fail
-    __at_quick_exit_lockptr
-    __atexit_lockptr
     __c_dot_utf8
     __c_dot_utf8_locale
     __c_locale
@@ Expand Down Expand Up / @@ -81,6 +80,7 @@ __ftello @@
     __ftello_unlocked
     __funcs_on_exit
     __funcs_on_quick_exit
+    __futexwait
     __futimesat
     __fwritable
     __fwritex
@@ Expand All / @@ -92,6 +92,7 @@ __getopt_msg @@
     __gmtime_r
     __hwcap
     __inet_aton
+    __inhibit_ptc
     __init_ssp
     __init_tp
     __intscan
@@ Expand Down Expand Up / @@ -148,7 +149,6 @@ __libc_free @@
     __libc_malloc
     __loc_is_allocated
     __locale_lock
-    __locale_lockptr
     __localtime_r
     __lock
     __lockfile
@@ Expand Down Expand Up / @@ -219,7 +219,6 @@ __pthread_tsd_size @@
     __putenv
     __qsort_r
     __rand48_step
-    __random_lockptr
     __reallocarray
     __register_locked_file
     __release_ptc
@@ Expand Down Expand Up / @@ -248,7 +247,6 @@ __stdin_used @@
     __stdio_close
     __stdio_exit
     __stdio_exit_needed
-    __stdio_ofl_lockptr
     __stdio_read
     __stdio_seek
     __stdio_write
@@ Expand Down Expand Up / @@ -308,6 +306,7 @@ __unlockfile @@
     __uselocale
     __utc
     __wait
+    __wake
     __wasi_args_get
     __wasi_args_sizes_get
     __wasi_clock_res_get
@@ Expand Down Expand Up / @@ -359,7 +358,6 @@ __wasi_thread_start_C @@
     __wasilibc_access
     __wasilibc_cwd
     __wasilibc_cwd_lock
-    __wasilibc_cwd_unlock
     __wasilibc_deinitialize_environ
     __wasilibc_dttoif
     __wasilibc_enable_futex_busywait_on_current_thread
@@ Expand Down Expand Up / @@ -946,6 +944,9 @@ modff @@
     modfl
     mprotect
     mrand48
+    mtx_destroy
+    mtx_init
+    mtx_timedlock
     munmap
     nan
     nanf
@@ Expand Down @@

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WASIp3 Cooperative Threading #799

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alexcrichton Jun 12, 2026

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-Original file line number
+Diff line change
@@ Expand Up / @@ -179,6 +179,13 @@ if (WASI STREQUAL "p3") @@
       )
     endif()
+    if (HAVE_WASM_LIBCALL_THREAD_CONTEXT)
+      list(APPEND bottom_half_sources
+        sources/__libcall_thread_context.S
+        sources/__wasm_init_task.S
+      )
+    endif()
     # Don't export symbols of generated code in shared libraries, so specifically
     # pass `-fvisibility=hidden`.
     set_source_files_properties(sources/wasip2.c PROPERTIES
@@ Expand Down @@

-Original file line number
+Diff line change
@@ Expand Up / @@ -7,7 +7,15 @@ @@
     #include <sched.h>
     int sched_yield(void) {
-    #ifdef __wasip1__
+    #ifdef __wasi_cooperative_threads__
+      #ifdef __wasip3__
+        wasip3_thread_yield();
+        return 0;
+      #else
+        #error "Unknown WASI version"
+      #endif
+    #elif defined(__wasip1__)
       __wasi_errno_t error = __wasi_sched_yield();
       if (error != 0) {
         errno = error;
@@ Expand Down @@

-Original file line number
+Diff line change
@@ Expand Up @@
         &closed);
       filesystem_stream_u8_drop_writable(writer);
-      // Wait for the subtask to resolve now that the writer half is closed and if
+      // Wait for the future to resolve now that the writer half is closed and if
       // we failed to write bytes (0 bytes written) and the result is an error we
       // can return -1.
       filesystem_result_void_error_code_t result;
@@ Expand Down @@

-Original file line number
+Diff line change
@@ Expand Up / @@ -9,6 +9,14 @@ extern void __wasm_call_ctors(void); @@
     extern int __main_void(void);
     extern void __wasm_call_dtors(void);
+    #ifdef __wasm_libcall_thread_context__
+    // Force __wasm_init_task and __wasm_init_async_task to be linked in for wasip3
+    extern void __wasm_init_task(void);
+    extern void __wasm_init_async_task(void);
+    __attribute__((used)) void *__wasm_init_task_ref = __wasm_init_task;
+    __attribute__((used)) void *__wasm_init_async_task_ref = __wasm_init_async_task;
+    #endif
     #if defined(__wasip1__)
     __attribute__((export_name("_start"))) void _start(void)
     #elif defined(__wasip2__)
@@ Expand Down @@

-Original file line number
+Diff line change
@@ Expand Up / @@ -11,6 +11,13 @@ @@
     #cmakedefine __wasip2__
     #cmakedefine __wasip3__
+    // The `__wasi_cooperative_threads__` define is used to test whether cooperative
+    // threads are enabled for this build of wasi-libc. This is detected at
+    // build-time and only works on supported versions of clang.
+    #ifdef __wasm_libcall_thread_context__
+    #cmakedefine __wasi_cooperative_threads__
+    #endif
     #cmakedefine __wasi_sdk_major__ @__wasi_sdk_major__@
     #cmakedefine __wasi_sdk_version__ "@__wasi_sdk_version__@"
@@ Expand Down @@

-Original file line number
+Diff line change
@@ -0,0 +1,73 @@
+    # LLVM emits calls to the following functions when the libcall-thread-context feature is enabled (default on WASIp3)
+    #include <wasi/version.h>
+        .globl      __wasm_get_stack_pointer
+        .type       __wasm_get_stack_pointer,@function
+        .globl      __wasm_set_stack_pointer
+        .type       __wasm_set_stack_pointer,@function
+        .functype __wasm_component_model_builtin_context_get_0 () -> (i32)
+        .import_module __wasm_component_model_builtin_context_get_0, "$root"
+        .import_name __wasm_component_model_builtin_context_get_0, "[context-get-0]"
+        .functype __wasm_component_model_builtin_context_set_0 (i32) -> ()
+        .import_module __wasm_component_model_builtin_context_set_0, "$root"
+        .import_name __wasm_component_model_builtin_context_set_0, "[context-set-0]"
+    __wasm_get_stack_pointer:
+        .functype   __wasm_get_stack_pointer () -> (i32)
+        call __wasm_component_model_builtin_context_get_0
+        end_function
+    __wasm_set_stack_pointer:
+        .functype   __wasm_set_stack_pointer (i32) -> ()
+        local.get 0
+        call __wasm_component_model_builtin_context_set_0
+        end_function
+        .globl      __wasm_get_tls_base
+        .type       __wasm_get_tls_base,@function
+        .globl      __wasm_set_tls_base
+        .type       __wasm_set_tls_base,@function
+    #ifdef __wasi_cooperative_threads__
+        # When coop threads are enabled the tls base lives in this async tasks's
+        # context slot index as 1. Note that this slot isn't valid when the
+        # component-model-threading feature is disabled, however, so it can't be
+        # used.
+        .functype __wasm_component_model_builtin_context_get_1 () -> (i32)
+        .import_module __wasm_component_model_builtin_context_get_1, "$root"
+        .import_name __wasm_component_model_builtin_context_get_1, "[context-get-1]"
+        .functype __wasm_component_model_builtin_context_set_1 (i32) -> ()
+        .import_module __wasm_component_model_builtin_context_set_1, "$root"
+        .import_name __wasm_component_model_builtin_context_set_1, "[context-set-1]"
+    __wasm_get_tls_base:
+        .functype   __wasm_get_tls_base () -> (i32)
+        call __wasm_component_model_builtin_context_get_1
+        end_function
+    __wasm_set_tls_base:
+        .functype   __wasm_set_tls_base (i32) -> ()
+        local.get 0
+        call __wasm_component_model_builtin_context_set_1
+        end_function
+    #else
+        # When coop threads are disabled do something similar to the basic output
+        # which is to have a global that stores the TLS base.
+        .globaltype __wasilibc_tls_base, i32
+    __wasm_get_tls_base:
+        .functype   __wasm_get_tls_base () -> (i32)
+        global.get __wasilibc_tls_base
+        end_function
+    __wasm_set_tls_base:
+        .functype   __wasm_set_tls_base (i32) -> ()
+        local.get 0
+        global.set __wasilibc_tls_base
+        end_function
+        .section .wasm.__wasilibc_tls_base,"",@
+    __wasilibc_tls_base:
+    #endif

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WASIp3 Cooperative Threading #799

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WASIp3 Cooperative Threading #799

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alexcrichton Jun 12, 2026

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