ICU-22838 Add WebAssembly/WASI cross-compilation support

* Add config/mh-wasi and update configure script.

  WebAssembly/WASI does not support threads and dynamic linking, so
  the new mh-wasi file disables those features in ICU.

* Teach putilimp.h not to define U_TZSET, U_TIMEZONE, and U_TZNAME for WASI

  WASI does not define timezone-related functionalities, and wasi-libc
  does not provide tzset, timezone, and tzname. This change teaches
  putilimp.h not to define U_TZSET, U_TIMEZONE, and U_TZNAME for WASI.

* Add `U_HAVE_ATOMICS` and support platforms without atomics like WASI

  This change adds a new macro `U_HAVE_ATOMICS` to ICU. This macro is
  always defined to 1 except for platforms that do not support atomic
  operations like WASI. Due to the lack of threading support in WASI, the
  wasi-sdk does not provide `std::atomic`, `std::mutex`, and related
  types, so the new macro is used to disable the use of these types.

icu4c/source/acinclude.m4

@@ -85,6 +85,7 @@ powerpc*-apple-darwin*)	icu_cv_host_frag=mh-darwin-ppc ;;
 *-dec-osf*) icu_cv_host_frag=mh-alpha-osf ;;
 *-*-nto*)	icu_cv_host_frag=mh-qnx ;;
 *-ncr-*)	icu_cv_host_frag=mh-mpras ;;
+*-wasi*)	icu_cv_host_frag=mh-wasi ;;
 *) 		icu_cv_host_frag=mh-unknown ;;
 esac
 		]

icu4c/source/common/putilimp.h

@@ -103,6 +103,8 @@ typedef size_t uintptr_t;
 #endif
 #elif U_PLATFORM == U_PF_OS400
    /* not defined */
+#elif defined(__wasi__)
+   /* not defined */
 #else
 #   define U_TZSET tzset
 #endif
@@ -128,6 +130,8 @@ typedef size_t uintptr_t;
    /* not defined */
 #elif U_PLATFORM == U_PF_IPHONE
    /* not defined */
+#elif defined(__wasi__)
+   /* not defined */
 #else
 #   define U_TIMEZONE timezone
 #endif
@@ -141,6 +145,8 @@ typedef size_t uintptr_t;
 #endif
 #elif U_PLATFORM == U_PF_OS400
    /* not defined */
+#elif defined(__wasi__)
+   /* not defined */
 #else
 #   define U_TZNAME tzname
 #endif

icu4c/source/common/umutex.cpp

@@ -43,6 +43,7 @@ U_NAMESPACE_BEGIN
  *
  *************************************************************************************************/
 
+#if U_HAVE_ATOMICS
 namespace {
 std::mutex *initMutex;
 std::condition_variable *initCondition;
@@ -55,9 +56,11 @@ std::once_flag initFlag;
 std::once_flag *pInitFlag = &initFlag;
 
 }  // Anonymous namespace
+#endif
 
 U_CDECL_BEGIN
 static UBool U_CALLCONV umtx_cleanup() {
+#if U_HAVE_ATOMICS
     initMutex->~mutex();
     initCondition->~condition_variable();
     UMutex::cleanup();
@@ -66,17 +69,21 @@ static UBool U_CALLCONV umtx_cleanup() {
     // Do not use this trick anywhere else in ICU; use umtx_initOnce, not std::call_once().
     pInitFlag->~once_flag();
     pInitFlag = new(&initFlag) std::once_flag();
+#endif
     return true;
 }
 
 static void U_CALLCONV umtx_init() {
+#if U_HAVE_ATOMICS
     initMutex = STATIC_NEW(std::mutex);
     initCondition = STATIC_NEW(std::condition_variable);
     ucln_common_registerCleanup(UCLN_COMMON_MUTEX, umtx_cleanup);
+#endif
 }
 U_CDECL_END
 
 
+#if U_HAVE_ATOMICS
 std::mutex *UMutex::getMutex() {
     std::mutex *retPtr = fMutex.load(std::memory_order_acquire);
     if (retPtr == nullptr) {
@@ -93,14 +100,17 @@ std::mutex *UMutex::getMutex() {
     U_ASSERT(retPtr != nullptr);
     return retPtr;
 }
+#endif
 
 UMutex *UMutex::gListHead = nullptr;
 
 void UMutex::cleanup() {
     UMutex *next = nullptr;
     for (UMutex *m = gListHead; m != nullptr; m = next) {
+#if U_HAVE_ATOMICS
         (*m->fMutex).~mutex();
         m->fMutex = nullptr;
+#endif
         next = m->fListLink;
         m->fListLink = nullptr;
     }
@@ -110,20 +120,24 @@ void UMutex::cleanup() {
 
 U_CAPI void  U_EXPORT2
 umtx_lock(UMutex *mutex) {
+#if U_HAVE_ATOMICS
     if (mutex == nullptr) {
         mutex = &globalMutex;
     }
     mutex->lock();
+#endif
 }
 
 
 U_CAPI void  U_EXPORT2
 umtx_unlock(UMutex* mutex)
 {
+#if U_HAVE_ATOMICS
     if (mutex == nullptr) {
         mutex = &globalMutex;
     }
     mutex->unlock();
+#endif
 }
 
 
@@ -143,18 +157,22 @@ umtx_unlock(UMutex* mutex)
 //
 U_COMMON_API UBool U_EXPORT2
 umtx_initImplPreInit(UInitOnce &uio) {
+#if U_HAVE_ATOMICS
     std::call_once(*pInitFlag, umtx_init);
     std::unique_lock<std::mutex> lock(*initMutex);
+#endif
     if (umtx_loadAcquire(uio.fState) == 0) {
         umtx_storeRelease(uio.fState, 1);
         return true;      // Caller will next call the init function.
     } else {
+#if U_HAVE_ATOMICS
         while (umtx_loadAcquire(uio.fState) == 1) {
             // Another thread is currently running the initialization.
             // Wait until it completes.
             initCondition->wait(lock);
         }
         U_ASSERT(uio.fState == 2);
+#endif
         return false;
     }
 }
@@ -168,11 +186,13 @@ umtx_initImplPreInit(UInitOnce &uio) {
 
 U_COMMON_API void U_EXPORT2
 umtx_initImplPostInit(UInitOnce &uio) {
+#if U_HAVE_ATOMICS
     {
         std::unique_lock<std::mutex> lock(*initMutex);
         umtx_storeRelease(uio.fState, 2);
     }
     initCondition->notify_all();
+#endif
 }
 
 U_NAMESPACE_END

icu4c/source/common/umutex.h

@@ -20,9 +20,6 @@
 #ifndef UMUTEX_H
 #define UMUTEX_H
 
-#include <atomic>
-#include <condition_variable>
-#include <mutex>
 #include <type_traits>
 
 #include "unicode/utypes.h"
@@ -37,6 +34,12 @@
 #error U_USER_ATOMICS and U_USER_MUTEX_H are not supported
 #endif
 
+#if U_HAVE_ATOMICS
+
+#include <atomic>
+#include <condition_variable>
+#include <mutex>
+
 // Export an explicit template instantiation of std::atomic<int32_t>. 
 // When building DLLs for Windows this is required as it is used as a data member of the exported SharedObject class.
 // See digitlst.h, pluralaffix.h, datefmt.h, and others for similar examples.
@@ -61,6 +64,7 @@ template struct std::atomic<std::mutex *>;
 #endif
 #endif
 
+#endif
 
 U_NAMESPACE_BEGIN
 
@@ -70,6 +74,8 @@ U_NAMESPACE_BEGIN
  *
  ****************************************************************************/
 
+#if U_HAVE_ATOMICS
+
 typedef std::atomic<int32_t> u_atomic_int32_t;
 
 inline int32_t umtx_loadAcquire(u_atomic_int32_t &var) {
@@ -88,6 +94,29 @@ inline int32_t umtx_atomic_dec(u_atomic_int32_t *var) {
     return var->fetch_sub(1) - 1;
 }
 
+#else
+
+// No atomic operations available. Use a simple int32_t instead.
+
+typedef int32_t u_atomic_int32_t;
+
+inline int32_t umtx_loadAcquire(u_atomic_int32_t &var) {
+    return var;
+}
+
+inline void umtx_storeRelease(u_atomic_int32_t &var, int32_t val) {
+    var = val;
+}
+
+inline int32_t umtx_atomic_inc(u_atomic_int32_t *var) {
+    return ++(*var);
+}
+
+inline int32_t umtx_atomic_dec(u_atomic_int32_t *var) {
+    return --(*var);
+}
+
+#endif
 
 /*************************************************************************************************
  *
@@ -227,17 +256,25 @@ class U_COMMON_API UMutex {
 
     // requirements for C++ BasicLockable, allows UMutex to work with std::lock_guard
     void lock() {
+#if U_HAVE_ATOMICS
         std::mutex *m = fMutex.load(std::memory_order_acquire);
         if (m == nullptr) { m = getMutex(); }
         m->lock();
+#endif
+    }
+    void unlock() {
+#if U_HAVE_ATOMICS
+        fMutex.load(std::memory_order_relaxed)->unlock();
+#endif
     }
-    void unlock() { fMutex.load(std::memory_order_relaxed)->unlock(); }
 
     static void cleanup();
 
 private:
+#if U_HAVE_ATOMICS
     alignas(std::mutex) char fStorage[sizeof(std::mutex)] {};
     std::atomic<std::mutex *> fMutex { nullptr };
+#endif
 
     /** All initialized UMutexes are kept in a linked list, so that they can be found,
      * and the underlying std::mutex destructed, by u_cleanup().
@@ -249,7 +286,9 @@ class U_COMMON_API UMutex {
      * Initial fast check is inline, in lock().  The returned value may never
      * be nullptr.
      */
+#if U_HAVE_ATOMICS
     std::mutex *getMutex();
+#endif
 };
 
 

icu4c/source/common/unicode/platform.h

@@ -313,6 +313,20 @@
 #   define U_PLATFORM_IS_DARWIN_BASED 0
 #endif
 
+/**
+ * \def U_HAVE_ATOMICS
+ * Defines whether the platform supports atomic operations.
+ * @internal
+ */
+#ifdef U_HAVE_ATOMICS
+    /* Use the predefined value. */
+#elif defined(__wasi__) && !defined(_REENTRANT)
+    /* WASI does not support atomics when wasi-threads feature is not enabled */
+#   define U_HAVE_ATOMICS 0
+#else
+#   define U_HAVE_ATOMICS 1
+#endif
+
 /*===========================================================================*/
 /** @{ Compiler and environment features                                     */
 /*===========================================================================*/