I want to replace the lowest byte in an integer. On x86 this is exactly mov al, [mem]
but I can't seem to get compilers to output this. Am I missing an obvious code pattern that is recognized, am I misunderstanding something, or is this simply a missed optimization?
unsigned insert_1(const unsigned* a, const unsigned char* b)
{
return (*a & ~255) | *b;
}
unsigned insert_2(const unsigned* a, const unsigned char* b)
{
return *a >> 8 << 8 | *b;
}
GCC actually uses al
but just for zeroing.
mov eax, DWORD PTR [rdi]
movzx edx, BYTE PTR [rsi]
xor al, al
or eax, edx
ret
Clang compiles both practically verbatim
mov ecx, -256
and ecx, dword ptr [rdi]
movzx eax, byte ptr [rsi]
or eax, ecx
ret
On x86 this is exactly
mov al, [mem]
but I can't seem to get compilers to output this.
Try this one, arithmetic-free:
unsigned insert_4(const unsigned* a, const unsigned char* b)
{
unsigned int t = *a;
unsigned char *tcp = (unsigned char *) & t;
tcp[0] = *b;
return t;
}
insert_4(unsigned int const*, unsigned char const*):
mov eax, DWORD PTR [rdi]
mov al, BYTE PTR [rsi]
ret
A bit screwy, I know but the compilers are good at removing indirection and address taken for local variables (took a couple of tries though..).
An alternative using union:
unsigned insert_5(const unsigned* a, const unsigned char* b)
{
union {
unsigned int ui;
unsigned char uc;
} u;
u.ui = *a;
u.uc = *b;
return u.ui;
}
Note, these solutions are endian-specific, though it seems like what you're looking for, and, as needed can be adjusted for the other endian.