Fixed invalid cast and unaligned memory access

Although acceptible to Intel CPUs, it's still undefined behaviour according to the C++ standard.

It can be replaced with memcpy, which makes the code simpler, and it generates the same assembly code with gcc and clang with -O2 (verified with godbolt).

Also expanded the test to include other little endian CPUs by testing for __LITTLE_ENDIAN__.

modules/core/src/persistence.cpp

@@ -295,16 +295,20 @@ int decodeSimpleFormat( const char* dt )
 
 }
 
-#if defined __i386__ || defined(_M_IX86) || defined __x86_64__ || defined(_M_X64)
-#define CV_UNALIGNED_LITTLE_ENDIAN_MEM_ACCESS 1
+#if defined __i386__ || defined(_M_IX86) || defined __x86_64__ || defined(_M_X64) || \
+    (defined (__LITTLE_ENDIAN__) && __LITTLE_ENDIAN__)
+#define CV_LITTLE_ENDIAN_MEM_ACCESS 1
 #else
-#define CV_UNALIGNED_LITTLE_ENDIAN_MEM_ACCESS 0
+#define CV_LITTLE_ENDIAN_MEM_ACCESS 0
 #endif
 
 static inline int readInt(const uchar* p)
 {
-#if CV_UNALIGNED_LITTLE_ENDIAN_MEM_ACCESS
-    return *(const int*)p;
+    // On little endian CPUs, both branches produce the same result. On big endian, only the else branch does.
+#if CV_LITTLE_ENDIAN_MEM_ACCESS
+    int val;
+    memcpy(&val, p, sizeof(val));
+    return val;
 #else
     int val = (int)(p[0] | (p[1] << 8) | (p[2] << 16) | (p[3] << 24));
     return val;
@@ -313,8 +317,11 @@ static inline int readInt(const uchar* p)
 
 static inline double readReal(const uchar* p)
 {
-#if CV_UNALIGNED_LITTLE_ENDIAN_MEM_ACCESS
-    return *(const double*)p;
+    // On little endian CPUs, both branches produce the same result. On big endian, only the else branch does.
+#if CV_LITTLE_ENDIAN_MEM_ACCESS
+    double val;
+    memcpy(&val, p, sizeof(val));
+    return val;
 #else
     unsigned val0 = (unsigned)(p[0] | (p[1] << 8) | (p[2] << 16) | (p[3] << 24));
     unsigned val1 = (unsigned)(p[4] | (p[5] << 8) | (p[6] << 16) | (p[7] << 24));
@@ -326,9 +333,9 @@ static inline double readReal(const uchar* p)
 
 static inline void writeInt(uchar* p, int ival)
 {
-#if CV_UNALIGNED_LITTLE_ENDIAN_MEM_ACCESS
-    int* ip = (int*)p;
-    *ip = ival;
+    // On little endian CPUs, both branches produce the same result. On big endian, only the else branch does.
+#if CV_LITTLE_ENDIAN_MEM_ACCESS
+    memcpy(p, &ival, sizeof(ival));
 #else
     p[0] = (uchar)ival;
     p[1] = (uchar)(ival >> 8);
@@ -339,9 +346,9 @@ static inline void writeInt(uchar* p, int ival)
 
 static inline void writeReal(uchar* p, double fval)
 {
-#if CV_UNALIGNED_LITTLE_ENDIAN_MEM_ACCESS
-    double* fp = (double*)p;
-    *fp = fval;
+    // On little endian CPUs, both branches produce the same result. On big endian, only the else branch does.
+#if CV_LITTLE_ENDIAN_MEM_ACCESS
+    memcpy(p, &fval, sizeof(fval));
 #else
     Cv64suf v;
     v.f = fval;