On Thu, Apr 3, 2014 at 2:13 AM, Jan Kara <j...@suse.cz> wrote:
> OK, but have you checked the generated code is actually any better? This
> is something I'd expect a compiler might be able to optimize anyway. And the
> original code looks more readable to me.
Hi, Jan,
I've disassemble the code on my x86_64 box
(it's inline though, I just look at its call site),
and found that this patch DOES make it more efficient.
Orig asm snippt with
patch asm snippt
============ ================
mov %edx,%ecx mov %rdx,%r9
xor %r8d,%ecx xor %r8d,%r8d
test $0x80,%cl and $0x380,%r9d
jne 14c5 <blk_rq_merge_ok+0x15> test $0x380,%ecx
and $0x3,%ch sete %r8b
jne 14c5 <blk_rq_merge_ok+0x15> cmp %r8,%r9
je 14b5 <blk_rq_merge_ok+0x15>
This saves a branch.
Furthermore, I found that gcc is smart enough to try to optimize the
code, so if we do
like this, it will generate the most optimal and smallest code :
static inline bool blk_check_merge_flags(unsigned int flags1,
¦unsigned int flags2)
{
return ((flags1 ^ flags2) &
(REQ_DISCARD | REQ_SECURE | REQ_WRITE_SAME))
== 0;
}
this gives out :
mov %edx,%r8d
xor %ecx,%r8d
and $0x380,%r8d
jne 14a5 <blk_rq_merge_ok+0x15>
But yes, it compromises readibility.
Regards,
Jianyu Zhan
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