x86_64 gcc optimizations

main.c

#define UNPACK32(buf) ((buf)[0] << 24 | (buf)[1] << 16 | (buf)[2] << 8 | (buf)[3])
int main() {
	int i;
	return UNPACK32(&i);
}

O0.s

main:
.LFB0:
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	movl	-4(%rbp), %eax
	movl	%eax, %edx
	sall	$24, %edx
	leaq	-4(%rbp), %rax
	addq	$4, %rax
	movl	(%rax), %eax
	sall	$16, %eax
	orl	%eax, %edx
	leaq	-4(%rbp), %rax
	addq	$8, %rax
	movl	(%rax), %eax
	sall	$8, %eax
	orl	%eax, %edx
	leaq	-4(%rbp), %rax
	addq	$12, %rax
	movl	(%rax), %eax
	orl	%edx, %eax
	popq	%rbp
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc

O1.s

main:
.LFB0:
	.cfi_startproc
	movl	-4(%rsp), %edx
	sall	$24, %edx
	movl	(%rsp), %eax
	sall	$16, %eax
	orl	%edx, %eax
	orl	8(%rsp), %eax
	movl	4(%rsp), %edx
	sall	$8, %edx
	orl	%edx, %eax
	ret
	.cfi_endproc

O2.s

main:
.LFB0:
	.cfi_startproc
	movl	-4(%rsp), %eax
	movl	(%rsp), %edx
	sall	$24, %eax
	sall	$16, %edx
	orl	%edx, %eax
	movl	4(%rsp), %edx
	orl	8(%rsp), %eax
	sall	$8, %edx
	orl	%edx, %eax
	ret
	.cfi_endproc