Strange assembly from array 0-initialization

Question

Inspired by the question Difference in initalizing and zeroing an array in c/c++ ?, I decided to actually examine the assembly of, in my case, an optimized release build for

Answer 1

Some quick testing indicates that Microsoft's x86 compiler generates different assembly if the initializer list is empty, compared to when it contains a zero. Maybe their ARM compiler does too. What happens if you do this?

byte a[10] = { };

Here's the assembly listing I got (with options /EHsc /FAs /O2 on Visual Studio 2008). Note that including a zero in the initializer list causes the compiler to use unaligned memory accesses to initialize the array, while the empty initializer list version and the memset() version both use aligned memory accesses:

; unsigned char a[10] = { };

xor eax, eax
mov DWORD PTR _a$[esp+40], eax
mov DWORD PTR _a$[esp+44], eax
mov WORD PTR _a$[esp+48], ax

; unsigned char b[10] = { 0 };

mov BYTE PTR _b$[esp+40], al
mov DWORD PTR _b$[esp+41], eax
mov DWORD PTR _b$[esp+45], eax
mov BYTE PTR _b$[esp+49], al

; unsigned char c[10];
; memset(c, 0, sizeof(c));

mov DWORD PTR _c$[esp+40], eax
mov DWORD PTR _c$[esp+44], eax
mov WORD PTR _c$[esp+48], ax