unpack_i8x8.c

#include <stdint.h>
#include <string.h>

struct alpha {
    int8_t a;
    int8_t b;
    int8_t c;
    int8_t d;
    int8_t e;
    int8_t f;
    int8_t g;
    int8_t h;
};

struct alpha unpack(uint64_t x)
{
    struct alpha r;
    memcpy(&r, &x, 8);
    return r;
}

struct alpha wrapper(uint64_t y)
{
    return unpack(y);
}

unpack_i8x8_clang.s

unpack:
	mov	rax, rdi
	ret
wrapper:
	mov	rax, rdi
	ret

unpack_i8x8_gcc.s

unpack:
	mov	rax, rdi
	ret
wrapper:
	mov	eax, edi
	xor	ecx, ecx
	mov	esi, edi
	shr	ax, 8
	mov	cl, dil
	shr	esi, 24
	mov	ch, al
	mov	rax, rdi
	movzx	edx, sil
	and	eax, 16711680
	and	rcx, -16711681
	sal	rdx, 24
	movabs	rsi, -4278190081
	or	rcx, rax
	mov	rax, rcx
	movabs	rcx, -1095216660481
	and	rax, rsi
	or	rax, rdx
	movabs	rdx, 1095216660480
	and	rdx, rdi
	and	rax, rcx
	movabs	rcx, -280375465082881
	or	rax, rdx
	movabs	rdx, 280375465082880
	and	rdx, rdi
	and	rax, rcx
	movabs	rcx, -71776119061217281
	or	rax, rdx
	movabs	rdx, 71776119061217280
	and	rdx, rdi
	and	rax, rcx
	shr	rdi, 56
	or	rax, rdx
	sal	rdi, 56
	movabs	rdx, 72057594037927935
	and	rax, rdx
	or	rax, rdi
	ret