Messages in this thread | | | From | Zhan Jianyu <> | Date | Thu, 3 Apr 2014 16:00:44 +0800 | Subject | Re: [PATCH] blkdev: use an efficient way to check merge flags |
| |
On Thu, Apr 3, 2014 at 2:13 AM, Jan Kara <jack@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |