Fix libpinyin crash on ARMv7

Fix the alignment trap in get_unigram_frequency(). Fix also other
places where this same trap could happen (depending on compiler
and surrounding code).

The trap happened when the ARM GCC generated a SIMD instruction
(specifically VLDR) to load 32 bits in a single instruction, and
when the memory address was not aligned to 32 bits. VLDR traps if
the address is not aligned.

GCC generated the instruction because of the cast to uint32 from
the address. The fix is to allocate a uint32 variable in stack and
use memcpy to copy the data to that variable. This way we ensure
that appropriate instructions are generated.

**Links**

About the issue with GCC:
https://trust-in-soft.com/blog/2020/04/06/gcc-always-assumes-aligned-pointer-accesses/

How Linux does it:
https://elixir.bootlin.com/linux/v5.10.155/source/include/linux/unaligned/memmove.h#L13

ARM documentation:
https://documentation-service.arm.com/static/5f8dc043f86e16515cdbbc92?token=
    See 'A3.2.1 Unaligned data access'

1 files changed, 43 insertions, 0 deletions

diff --git a/src/include/unaligned_memory.h b/src/include/unaligned_memory.h
new file mode 100644
index 0000000..27b2f19
--- /dev/null
+++ b/src/include/unaligned_memory.h
@@ -0,0 +1,43 @@
+#ifndef UNALIGNED_MEMORY_H
+#define UNALIGNED_MEMORY_H
+
+#include <cstring>
+
+/**
+ * UnalignedMemory: Safe unaligned memory access.
+ * 
+ * Some instruction sets, or some instructions in some instruction sets
+ * require that memory access is aligned to a specific boundary. These
+ * instructions may trap on unaligned access.
+ * 
+ * This class provides methods to load and store values at unaligned
+ * addresses. It ensures that the compiler doesn't generate instructions
+ * that could trap on the unaligned memory access.
+ */
+
+namespace pinyin{
+    template <typename T>
+    class UnalignedMemory{
+    public:
+        /**
+         * Read a value from a possibly unaligned memory address.
+         * 
+         */
+        static T load(const void * src) {
+            T value;
+            memcpy(&value, src, sizeof(T));
+            return value;
+        }
+
+        /**
+         * Store a value into a possibly unaligned memory address.
+         * 
+         */
+        static void store(T value, void * dest) {
+            memcpy(dest, &value, sizeof(T));
+        }
+    };
+};
+
+
+#endif