Why is movl preferred to movb when translating a C downcast from unsigned int to unsigned char?

Question

Considering a pared-down example of down-casting unsigned to unsigned char,

void unsigned_to_unsigned_char(unsigned *sp, unsigned char *dp)
{
  *dp = (unsigned char)*sp;
}

The above C code is translated to assembly code with gcc -Og -S as

movl    (%rdi), %eax
movb    %al, (%rsi)

For what reason is the C-to-assembly translation not as below?

movb    (%rdi), %al
movb    %al, (%rsi)

Is it because this is incorrect, or because movl is more conventional, or shorter in encoding, than is movb?

Answer 1

Writing to an 8 bit x86 register possibly incurs an extra merge µop when the new low byte is merged with the old high bytes of the corresponding 32/64 bit register. This can also cause an unexpected data dependency on the previous value of the register.

For this reason, it is generally a good idea to only write to 32/64 bit variants of general purpose registers on x86.