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Align thread entry point stack

_beginthreadex does not align the stack on 16-byte boundary as expected
by gcc.

On x86 targets, the force_align_arg_pointer attribute may be applied to
individual function definitions, generating an alternate prologue and
epilogue that realigns the run-time stack if necessary. This supports
mixing legacy codes that run with a 4-byte aligned stack with modern
codes that keep a 16-byte stack for SSE compatibility.
https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html

Change-Id: Ie4e4ab32948c238fa87054d5664189972ca6708e
Signed-off-by: Aleksey Vasenev <[email protected]>

--- a/vp8/common/threading.h

+++ b/vp8/common/threading.h

@@ -24,7 +24,13 @@

 /* Win32 */

 #include <process.h>

 #include <windows.h>

+#if defined(__GNUC__) && \

+    (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 2))

+#define THREAD_FUNCTION \

+  __attribute__((force_align_arg_pointer)) unsigned int __stdcall

+#else

 #define THREAD_FUNCTION unsigned int __stdcall

+#endif

 #define THREAD_FUNCTION_RETURN DWORD

 #define THREAD_SPECIFIC_INDEX DWORD

 #define pthread_t HANDLE

--- a/vpx_util/vpx_thread.h

+++ b/vpx_util/vpx_thread.h

@@ -58,7 +58,12 @@

 // simplistic pthread emulation layer

 // _beginthreadex requires __stdcall

+#if defined(__GNUC__) && \

+    (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 2))

+#define THREADFN __attribute__((force_align_arg_pointer)) unsigned int __stdcall

+#else

 #define THREADFN unsigned int __stdcall

+#endif

 #define THREAD_RETURN(val) (unsigned int)((DWORD_PTR)val)

 #if _WIN32_WINNT >= 0x0501  // Windows XP or greater