OSX implementation of rtl_AllocateAligned

This was SVN commit r6206.

source/lib/sysdep/rtl/gcc/gcc.cpp

@@ -1,5 +1,12 @@
 #include "precompiled.h"
+
 #include "lib/sysdep/rtl.h"
+#include "lib/bits.h"
+
+// Linux has posix_memalign, AFAIK our other gcc platforms do not have it.
+#define HAVE_POSIX_MEMALIGN OS_LINUX
+
+#if HAVE_POSIX_MEMALIGN
 
 void* rtl_AllocateAligned(size_t size, size_t alignment)
 {
@@ -16,3 +23,39 @@ void rtl_FreeAligned(void* alignedPointer)
 {
 	free(alignedPointer);
 }
+
+#else // Fallback aligned allocation using malloc
+
+void* rtl_AllocateAligned(size_t size, size_t align)
+{
+	// This ensures we have enough extra space to store the original pointer,
+	// and produce an aligned buffer, assuming the platform malloc ensures at
+	// least sizeof(void*) alignment.
+	if (align < 2*sizeof(void*))
+		align = 2*sizeof(void*);
+	
+	void* const malloc_ptr = malloc(size + align);
+	if (!malloc_ptr)
+		return NULL;
+	
+	// Round malloc_ptr up to the next aligned address, leaving some unused
+	// space before the pointer we'll return. The minimum alignment above
+	// ensures we'll have at least sizeof(void*) extra space.
+	void* const aligned_ptr =
+		(void *)(round_down(uintptr_t(malloc_ptr), uintptr_t(align)) + align);
+
+	// Just make sure we did the right thing with all the alignment hacks above.
+	debug_assert(((void**)aligned_ptr) - 1 >= malloc_ptr);
+
+	// Store the original pointer which will have to be sent to free().
+	((void **)aligned_ptr)[-1] = malloc_ptr;
+
+	return aligned_ptr;
+}
+
+void rtl_FreeAligned(void* alignedPointer)
+{
+	free(((void**)alignedPointer)[-1]);
+}
+
+#endif