clang on windows support

config/external/lgpllibs/moz.build

@@ -11,3 +11,9 @@
 
 GeckoSharedLibrary('lgpllibs', linkage=None)
 SHARED_LIBRARY_NAME = 'lgpllibs'
+
+if CONFIG['CLANG_CL']:
+    # SoundTouch uses clang's OpenMP runtime. Link its import library into the
+    # containing LGPL DLL and place the runtime beside the browser binaries.
+    OS_LIBS += ['C:/clang/lib/libomp']
+    FINAL_TARGET_FILES += ['C:/clang/bin/libomp.dll']

config/external/nss/Makefile.in

@@ -262,6 +262,13 @@ ifeq (WINNT,$(OS_TARGET))
 NSS_XCFLAGS += $(filter -arch:%,$(CFLAGS))
 endif
 
+# freebl's x86 AES/GCM sources use AES-NI and PCLMUL intrinsics. The nested
+# NSS build's per-object flags are not propagated through this wrapper when
+# using clang-cl, so pass them explicitly.
+ifeq (1,$(CLANG_CL))
+NSS_XCFLAGS += -maes -mpclmul
+endif
+
 # Enable short header experiment. Firefox only.
 NSS_XCFLAGS += -DNSS_ENABLE_TLS13_SHORT_HEADERS
 
@@ -466,4 +473,3 @@ endif # MOZ_FOLD_LIBS
 # Work around NSS build system race condition creating certdata.c in
 # security/nss/lib/ckfw/builtins. See bug #836220.
 libs-nss/lib$(if $(MOZ_FOLD_LIBS),/ckfw): $(call mkdir_deps,$(DEPTH)/security/nss/lib/ckfw/builtins)
-

db/sqlite3/src/sqlite3.c

@@ -27219,6 +27219,11 @@ static malloc_zone_t* _sqliteZone_;
 #if HAVE_MALLOC_H && HAVE_MALLOC_USABLE_SIZE
 #  define SQLITE_USE_MALLOC_H 1
 #  define SQLITE_USE_MALLOC_USABLE_SIZE 1
+#  if defined(_WIN32)
+     /* Mozilla's jemalloc wrapper provides this symbol, but the Windows
+      * malloc.h does not declare it for clang-cl. */
+     extern size_t malloc_usable_size(void *);
+#  endif
 /*
 ** The MSVCRT has malloc_usable_size(), but it is called _msize().  The
 ** use of _msize() is automatic, but can be disabled by compiling with

dom/plugins/ipc/PluginInstanceChild.cpp

@@ -3694,12 +3694,12 @@ PluginInstanceChild::UpdateWindowAttributes(bool aForceSetWindow)
         WINDOWPOS winpos = {
             0, 0,
             mWindow.x, mWindow.y,
-            mWindow.width, mWindow.height,
+            static_cast<int>(mWindow.width), static_cast<int>(mWindow.height),
             0
         };
         NPEvent pluginEvent = {
             WM_WINDOWPOSCHANGED, 0,
-            (LPARAM) &winpos
+            reinterpret_cast<uintptr_t>(&winpos)
         };
         mPluginIface->event(&mData, &pluginEvent);
     }

dom/workers/ServiceWorkerPrivate.cpp

@@ -1355,7 +1355,7 @@ public:
     rv = httpChannel->GetReferrerPolicy(&referrerPolicy);
     NS_ENSURE_SUCCESS(rv, rv);
     switch (referrerPolicy) {
-      case nsIHttpChannel::REFERRER_POLICY_UNSET:
+    case static_cast<uint32_t>(nsIHttpChannel::REFERRER_POLICY_UNSET):
       mReferrerPolicy = ReferrerPolicy::_empty;
       break;
     case nsIHttpChannel::REFERRER_POLICY_NO_REFERRER:

gfx/cairo/cairo/src/cairo-d2d-surface.cpp

@@ -2774,7 +2774,7 @@ _cairo_d2d_copy_surface(cairo_d2d_surface_t *dst,
 	cairo_rectangle_int_t transformed_rect = { area_to_copy.x + translation->x,
 						   area_to_copy.y + translation->y,
 						   area_to_copy.width, area_to_copy.height };
-	cairo_rectangle_int_t surface_rect = { 0, 0, srcDesc.Width, srcDesc.Height };
+	cairo_rectangle_int_t surface_rect = { 0, 0, static_cast<int>(srcDesc.Width), static_cast<int>(srcDesc.Height) };
 
 
 	if (!_cairo_rectangle_contains(&surface_rect, &transformed_rect)) {
@@ -2989,8 +2989,8 @@ _cairo_d2d_try_fastblit(cairo_d2d_surface_t *dst,
 
 	// Areas outside of the surface do not matter.
 	cairo_rectangle_int_t surface_rect = { 0, 0,
-					       dst->rt->GetPixelSize().width,
+					       static_cast<int>(dst->rt->GetPixelSize().width),
-					       dst->rt->GetPixelSize().height };
+					       static_cast<int>(dst->rt->GetPixelSize().height) };
 	cairo_region_intersect_rectangle(region, &surface_rect);
     }
 

media/ffvpx/libavcodec/moz.build

@@ -162,6 +162,13 @@ SOURCES += [
     'xiph.c'
 ]
 
+# dummy_funcs.c intentionally provides tentative definitions for FFmpeg
+# objects that are disabled by the generated configuration. Clang's default
+# -fno-common turns those placeholders into duplicate definitions when an
+# enabled codec supplies the real object.
+if CONFIG['CLANG_CL']:
+    SOURCES['dummy_funcs.c'].flags += ['-fcommon']
+
 SYMBOLS_FILE = 'avcodec.symbols'
 NO_VISIBILITY_FLAGS = True
 

media/ffvpx/libavutil/atomic_win32.h

@@ -41,7 +41,8 @@ static inline void atomic_int_set_win32(volatile int *ptr, int val)
 #define avpriv_atomic_int_add_and_fetch atomic_int_add_and_fetch_win32
 static inline int atomic_int_add_and_fetch_win32(volatile int *ptr, int inc)
 {
-    return inc + InterlockedExchangeAdd(ptr, inc);
+    /* Windows int and LONG are both 32-bit, but have distinct C types. */
+    return inc + InterlockedExchangeAdd((volatile LONG *)ptr, inc);
 }
 
 #define avpriv_atomic_ptr_cas atomic_ptr_cas_win32

media/libdav1d/src/src/cpu.c

@@ -85,10 +85,24 @@ COLD int dav1d_num_logical_processors(Dav1dContext *const c) {
 #ifdef _WIN32
 #if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
     GROUP_AFFINITY affinity;
-    if (GetThreadGroupAffinity(GetCurrentThread(), &affinity)) {
+    DWORD_PTR mask = 0, system_mask;
-        int num_processors = 1;
+    HMODULE kernel32 = GetModuleHandleW(L"kernel32.dll");
-        while (affinity.Mask &= affinity.Mask - 1)
+    typedef BOOL (WINAPI *get_thread_group_affinity_fn)(HANDLE, PGROUP_AFFINITY);
+    get_thread_group_affinity_fn get_thread_group_affinity = kernel32 ?
+        (get_thread_group_affinity_fn)GetProcAddress(kernel32, "GetThreadGroupAffinity") : NULL;
+
+    /* Processor groups are only available on Windows 7 and later. */
+    if (get_thread_group_affinity &&
+        get_thread_group_affinity(GetCurrentThread(), &affinity))
+        mask = affinity.Mask;
+    else if (!GetProcessAffinityMask(GetCurrentProcess(), &mask, &system_mask))
+        mask = 0;
+
+    if (mask) {
+        int num_processors = 0;
+        do {
             num_processors++;
+        } while (mask &= mask - 1);
         return num_processors;
     }
 #else

media/libdav1d/src/src/thread.h

@@ -33,8 +33,8 @@
 #include <limits.h>
 #include <windows.h>
 
-#define PTHREAD_MUTEX_INITIALIZER SRWLOCK_INIT
+#define PTHREAD_MUTEX_INITIALIZER { 0 }
-#define PTHREAD_ONCE_INIT INIT_ONCE_STATIC_INIT
+#define PTHREAD_ONCE_INIT 0
 
 typedef struct {
     HANDLE h;
@@ -46,9 +46,16 @@ typedef struct {
     unsigned stack_size;
 } pthread_attr_t;
 
-typedef SRWLOCK pthread_mutex_t;
+typedef struct {
-typedef CONDITION_VARIABLE pthread_cond_t;
+    volatile LONG state;
-typedef INIT_ONCE pthread_once_t;
+    CRITICAL_SECTION cs;
+} pthread_mutex_t;
+struct dav1d_cond_waiter;
+typedef struct {
+    CRITICAL_SECTION cs;
+    struct dav1d_cond_waiter *head;
+} pthread_cond_t;
+typedef volatile LONG pthread_once_t;
 
 void dav1d_init_thread(void);
 void dav1d_set_thread_name(const wchar_t *name);
@@ -84,50 +91,51 @@ static inline int pthread_attr_setstacksize(pthread_attr_t *const attr,
 static inline int pthread_mutex_init(pthread_mutex_t *const mutex,
                                      const void *const attr)
 {
-    InitializeSRWLock(mutex);
+    mutex->state = 0;
     return 0;
 }
 
 static inline int pthread_mutex_destroy(pthread_mutex_t *const mutex) {
+    if (mutex->state == 2) DeleteCriticalSection(&mutex->cs);
     return 0;
 }
 
 static inline int pthread_mutex_lock(pthread_mutex_t *const mutex) {
-    AcquireSRWLockExclusive(mutex);
+    if (InterlockedCompareExchange(&mutex->state, 1, 0) == 0) {
+        InitializeCriticalSection(&mutex->cs);
+        InterlockedExchange(&mutex->state, 2);
+    } else {
+        while (InterlockedCompareExchange(&mutex->state, 2, 2) != 2)
+            Sleep(1);
+    }
+    EnterCriticalSection(&mutex->cs);
     return 0;
 }
 
 static inline int pthread_mutex_unlock(pthread_mutex_t *const mutex) {
-    ReleaseSRWLockExclusive(mutex);
+    LeaveCriticalSection(&mutex->cs);
     return 0;
 }
 
 static inline int pthread_cond_init(pthread_cond_t *const cond,
                                     const void *const attr)
 {
-    InitializeConditionVariable(cond);
+    InitializeCriticalSection(&cond->cs);
+    cond->head = NULL;
     return 0;
 }
 
 static inline int pthread_cond_destroy(pthread_cond_t *const cond) {
+    DeleteCriticalSection(&cond->cs);
     return 0;
 }
 
-static inline int pthread_cond_wait(pthread_cond_t *const cond,
+#define pthread_cond_wait dav1d_pthread_cond_wait
-                                    pthread_mutex_t *const mutex)
+#define pthread_cond_signal dav1d_pthread_cond_signal
-{
+#define pthread_cond_broadcast dav1d_pthread_cond_broadcast
-    return !SleepConditionVariableSRW(cond, mutex, INFINITE, 0);
+int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);
-}
+int pthread_cond_signal(pthread_cond_t *cond);
-
+int pthread_cond_broadcast(pthread_cond_t *cond);
-static inline int pthread_cond_signal(pthread_cond_t *const cond) {
-    WakeConditionVariable(cond);
-    return 0;
-}
-
-static inline int pthread_cond_broadcast(pthread_cond_t *const cond) {
-    WakeAllConditionVariable(cond);
-    return 0;
-}
 
 #else
 

media/libdav1d/src/src/win32/thread.c

@@ -85,15 +85,73 @@ COLD int dav1d_pthread_join(pthread_t *const thread, void **const res) {
 COLD int dav1d_pthread_once(pthread_once_t *const once_control,
                             void (*const init_routine)(void))
 {
-    BOOL pending = FALSE;
+    if (InterlockedCompareExchange(once_control, 1, 0) == 0) {
-
-    if (InitOnceBeginInitialize(once_control, 0, &pending, NULL) != TRUE)
-        return 1;
-
-    if (pending == TRUE)
         init_routine();
+        InterlockedExchange(once_control, 2);
+    } else {
+        while (InterlockedCompareExchange(once_control, 2, 2) != 2)
+            Sleep(1);
+    }
+    return 0;
+}
 
-    return !InitOnceComplete(once_control, 0, NULL);
+/* Each waiter owns an event. Queue operations and signalling share a lock,
+ * so a new waiter cannot consume an earlier waiter's notification. */
+struct dav1d_cond_waiter {
+    HANDLE event;
+    struct dav1d_cond_waiter *next;
+};
+
+int dav1d_pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex) {
+    struct dav1d_cond_waiter waiter, **link;
+    DWORD result;
+    waiter.event = CreateEventW(NULL, FALSE, FALSE, NULL);
+    if (!waiter.event) return 1;
+    EnterCriticalSection(&cond->cs);
+    waiter.next = NULL;
+    for (link = &cond->head; *link; link = &(*link)->next) {}
+    *link = &waiter;
+    pthread_mutex_unlock(mutex);
+    LeaveCriticalSection(&cond->cs);
+
+    result = WaitForSingleObject(waiter.event, INFINITE);
+    EnterCriticalSection(&cond->cs);
+    /* Also unlink on wait failure before the stack record goes away. */
+    for (link = &cond->head; *link; link = &(*link)->next) {
+        if (*link == &waiter) {
+            *link = waiter.next;
+            break;
+        }
+    }
+    CloseHandle(waiter.event);
+    LeaveCriticalSection(&cond->cs);
+    pthread_mutex_lock(mutex);
+    return result != WAIT_OBJECT_0;
+}
+
+int dav1d_pthread_cond_signal(pthread_cond_t *cond) {
+    int result = 0;
+    EnterCriticalSection(&cond->cs);
+    if (cond->head) {
+        if (SetEvent(cond->head->event)) cond->head = cond->head->next;
+        else result = 1;
+    }
+    LeaveCriticalSection(&cond->cs);
+    return result;
+}
+
+int dav1d_pthread_cond_broadcast(pthread_cond_t *cond) {
+    int result = 0;
+    EnterCriticalSection(&cond->cs);
+    while (cond->head) {
+        if (!SetEvent(cond->head->event)) {
+            result = 1;
+            break;
+        }
+        cond->head = cond->head->next;
+    }
+    LeaveCriticalSection(&cond->cs);
+    return result;
 }
 
 #endif

media/libsoundtouch/src/moz.build

@@ -47,6 +47,9 @@ if CONFIG['CC_TYPE'] == 'gcc':
     OS_LIBS += ['gomp']
     if CONFIG['OS_ARCH'] == 'WINNT':
         OS_LIBS += ['dl']
+elif CONFIG['CLANG_CL']:
+    # clang-cl lowers OpenMP pragmas to the LLVM OpenMP runtime.
+    CXXFLAGS += ['-fopenmp']
 
 DEFINES['BUILDING_SOUNDTOUCH'] = 1
 

media/libstagefright/moz.build

@@ -39,11 +39,12 @@ if CONFIG['OS_TARGET'] != 'Android':
     DEFINES['FAKE_LOG_DEVICE'] = True
     SOURCES += [
         'system/core/liblog/fake_log_device.c',
+        # io.h conflicts with the access stub in logd_write.c on Windows.
+        'system/core/liblog/logprint.c',
     ]
     UNIFIED_SOURCES += [
         'system/core/libcutils/strdup16to8.c',
         'system/core/liblog/logd_write.c',
-        'system/core/liblog/logprint.c',
     ]
 
 EXPORTS.mp4_demuxer += [

media/libstagefright/system/core/liblog/logprint.c

@@ -26,6 +26,12 @@
 #include <assert.h>
 #include <arpa/inet.h>
 
+#ifdef _WIN32
+#include <io.h>
+#else
+#include <unistd.h>
+#endif
+
 #include <log/logd.h>
 #include <log/logprint.h>
 
@@ -945,7 +951,11 @@ int android_log_printLogLine(
         return -1;
 
     do {
+#ifdef _WIN32
+        ret = _write(fd, outBuffer, totalLen);
+#else
         ret = write(fd, outBuffer, totalLen);
+#endif
     } while (ret < 0 && errno == EINTR);
 
     if (ret < 0) {

modules/zlib/src/moz.build

@@ -31,6 +31,15 @@ if CONFIG['INTEL_ARCHITECTURE']:
         'inflate_simd.c',
         'x86.c',
     ]
+    # Clang (including clang-cl) and GCC require explicit ISA flags for
+    # intrinsics. MSVC accepts these intrinsics without additional flags.
+    # Keep advanced instructions confined to the runtime-dispatched sources.
+    if CONFIG['GNU_CC'] or CONFIG['CLANG_CL']:
+        SOURCES['adler32_simd.c'].flags += ['-mssse3']
+        for source in ['crc32_simd.c', 'crc_folding.c']:
+            SOURCES[source].flags += ['-msse4.2', '-mpclmul']
+        for source in ['fill_window_sse.c', 'inffast_chunk.c', 'inflate_simd.c']:
+            SOURCES[source].flags += ['-msse2']
 else:
     SOURCES += [
         'simd_stub.c',

modules/zlib/src/x86.c

@@ -65,8 +65,8 @@ static void _x86_check_features(void)
 #include <windows.h>
 #include <stdint.h>
 
-static volatile int32_t once_control = 0;
+static volatile LONG once_control = 0;
-static int fake_pthread_once(volatile int32_t *once_control,
+static int fake_pthread_once(volatile LONG *once_control,
                              void (*init_routine)(void));
 
 void x86_check_features(void)
@@ -75,7 +75,7 @@ void x86_check_features(void)
 }
 
 /* Copied from "perftools_pthread_once" in tcmalloc */
-static int fake_pthread_once(volatile int32_t *once_control,
+static int fake_pthread_once(volatile LONG *once_control,
                              void (*init_routine)(void)) {
     // Try for a fast path first. Note: this should be an acquire semantics read
     // It is on x86 and x64, where Windows runs.

mozconfigs/windows/uxp-win64-clang

@@ -10,7 +10,6 @@ export CLANG=/c/clang/bin
 export CC="clang-cl.exe"
 export MT="llvm-mt.exe"
 export LD="lld-link.exe"
-export LIB="lib.exe"
 
 # Force PATH so UXP thinks only Clang is there
 export PATH="$CLANG:$PATH"
@@ -24,8 +23,8 @@ mk_add_options MOZ_MAKE_FLAGS="-j6"
 ac_add_options --host=x86_64-pc-mingw32
 ac_add_options --target=x86_64-pc-mingw32
 
-# Build only Hydra
+# Build only Dactyloidae
-ac_add_options --enable-application=basilisk
+ac_add_options --enable-application=browser
 ac_add_options --disable-artifact-builds
 
 # Updater stuffs
@@ -43,7 +42,7 @@ ac_add_options --enable-devtools
 #ac_add_options --enable-install-strip
 ac_add_options --enable-jemalloc
 ac_add_options --enable-js-lto
-ac_add_options --enable-optimize="-O2 -Ob2ity -openmp -arch:AVX -w0 -w"
+ac_add_options --enable-optimize="-O2 -Ob2ity -arch:SSE2 -w0 -w"
 ac_add_options --enable-phoenix-extensions
 ac_add_options --enable-release
 ac_add_options --enable-shared-js

nsprpub/pr/src/io/prsocket.c

@@ -321,8 +321,8 @@ static PRStatus PR_CALLBACK SocketConnectContinue(
         } else {
 #if defined(_WIN64)
             if (fd->secret->overlappedActive) {
-                PRInt32 rvSent;
+                DWORD rvSent;
-                if (GetOverlappedResult(osfd, &fd->secret->ol, &rvSent, FALSE) == FALSE) {
+                if (GetOverlappedResult((HANDLE)osfd, &fd->secret->ol, &rvSent, FALSE) == FALSE) {
                     err = WSAGetLastError();
                     PR_LOG(_pr_io_lm, PR_LOG_MIN,
                            ("SocketConnectContinue GetOverlappedResult failed %d\n", err));
@@ -353,8 +353,8 @@ static PRStatus PR_CALLBACK SocketConnectContinue(
      * To get result we need to use GetOverlappedResult. */
     if (fd->secret->overlappedActive) {
         PR_ASSERT(fd->secret->nonblocking);
-        PRInt32 rvSent;
+        DWORD rvSent;
-        if (GetOverlappedResult(osfd, &fd->secret->ol, &rvSent, FALSE) == TRUE) {
+        if (GetOverlappedResult((HANDLE)osfd, &fd->secret->ol, &rvSent, FALSE) == TRUE) {
             fd->secret->overlappedActive = PR_FALSE;
             PR_LOG(_pr_io_lm, PR_LOG_MIN,
                    ("SocketConnectContinue GetOverlappedResult succeeded\n"));

nsprpub/pr/src/md/windows/ntmisc.c

@@ -769,10 +769,13 @@ PRStatus _PR_WaitWindowsProcess(PRProcess *process,
         return PR_FAILURE;
     }
     PR_ASSERT(dwRetVal == WAIT_OBJECT_0);
-    if (exitCode != NULL &&
+    if (exitCode != NULL) {
-        GetExitCodeProcess(process->md.handle, exitCode) == FALSE) {
+        DWORD dwExitCode;
-        PR_SetError(PR_UNKNOWN_ERROR, GetLastError());
+        if (GetExitCodeProcess(process->md.handle, &dwExitCode) == FALSE) {
-        return PR_FAILURE;
+            PR_SetError(PR_UNKNOWN_ERROR, GetLastError());
+            return PR_FAILURE;
+        }
+        *exitCode = (PRInt32)dwExitCode;
     }
     CloseHandle(process->md.handle);
     PR_DELETE(process);

nsprpub/pr/src/md/windows/w95io.c

@@ -266,7 +266,7 @@ _PR_MD_OPEN_FILE(const char *name, PRIntn osflags, int mode)
 PRInt32
 _PR_MD_READ(PRFileDesc *fd, void *buf, PRInt32 len)
 {
-    PRUint32 bytes;
+    DWORD bytes;
     int rv, err;
 
     rv = ReadFile((HANDLE)fd->secret->md.osfd,
@@ -295,7 +295,7 @@ PRInt32
 _PR_MD_WRITE(PRFileDesc *fd, const void *buf, PRInt32 len)
 {
     PROsfd f = fd->secret->md.osfd;
-    PRInt32 bytes;
+    DWORD bytes;
     int rv;
 
     rv = WriteFile((HANDLE)f,

nsprpub/pr/src/md/windows/w95sock.c

@@ -7,6 +7,8 @@
  *
  */
 
+/* Include Winsock 2 before primpl.h brings in windows.h and Winsock 1. */
+#include <winsock2.h>
 #include "primpl.h"
 
 #if defined(_WIN64)
@@ -117,7 +119,7 @@ _MD_CloseSocket(PROsfd osfd)
 PRInt32
 _MD_SocketAvailable(PRFileDesc *fd)
 {
-    PRInt32 result;
+    u_long result;
 
     if (ioctlsocket(fd->secret->md.osfd, FIONREAD, &result) < 0) {
         PR_SetError(PR_BAD_DESCRIPTOR_ERROR, WSAGetLastError());
@@ -363,7 +365,7 @@ static PRStatus PR_CALLBACK _pr_set_connectex(void)
 {
     _pr_win_connectex = NULL;
     SOCKET sock;
-    PRInt32 dwBytes;
+    DWORD dwBytes;
     int rc;
 
     /* Dummy socket needed for WSAIoctl */
@@ -445,7 +447,7 @@ _PR_MD_TCPSENDTO(PRFileDesc *fd, const void *buf, PRInt32 amount, PRIntn flags,
     memset(&fd->secret->ol, 0, sizeof(fd->secret->ol));
     /* ConnectEx return TRUE on a success and FALSE on an error. */
     if (_pr_win_connectex( (SOCKET)osfd, (struct sockaddr *) addr,
-                           addrlen, buf, amount,
+                           addrlen, (PVOID)buf, amount,
                            &rvSent, &fd->secret->ol) == TRUE) {
         /* When ConnectEx is used, all previously set socket options and
          * property are not enabled and to enable them
@@ -494,7 +496,7 @@ _PR_MD_TCPSENDTO(PRFileDesc *fd, const void *buf, PRInt32 amount, PRIntn flags,
             if ( rv < 0 ) {
                 return -1;
             }
-            rv = GetOverlappedResult(osfd, &fd->secret->ol, &rvSent, FALSE);
+            rv = GetOverlappedResult((HANDLE)osfd, &fd->secret->ol, &rvSent, FALSE);
             if ( rv == TRUE ) {
                 return rvSent;
             } else {

old-configure.in

@@ -4773,11 +4773,22 @@ if test "$MOZ_TREE_CAIRO"; then
 
         if test "$COMPILE_ENVIRONMENT"; then
 
-          MOZ_CHECK_HEADER(d3d9.h, MOZ_ENABLE_D3D9_LAYER=1)
+          dnl SDK include paths can change between build shells. Do not cache
+          dnl these checks, or a stale failure disables required D3D exports.
+          unset ac_cv_header_d3d9_h ac_cv_header_d3d10_h
+          AC_MSG_CHECKING([for d3d9.h])
+          AC_TRY_COMPILE([#include <d3d9.h>], ,
+            [MOZ_ENABLE_D3D9_LAYER=1
+             AC_MSG_RESULT(yes)],
+            [AC_MSG_RESULT(no)])
 
           dnl D3D10 Layers depend on D2D Surfaces.
           if test -n "$WIN32_D2D_SURFACE_FEATURE"; then
-            MOZ_CHECK_HEADER(d3d10.h, MOZ_ENABLE_D3D10_LAYER=1)
+            AC_MSG_CHECKING([for d3d10.h])
+            AC_TRY_COMPILE([#include <d3d10.h>], ,
+              [MOZ_ENABLE_D3D10_LAYER=1
+               AC_MSG_RESULT(yes)],
+              [AC_MSG_RESULT(no)])
           fi
         fi
         ;;