Messages in this thread | | | Date | Thu, 3 Apr 2014 14:45:42 +0200 | From | Jan Kara <> | Subject | Re: [PATCH] blkdev: use an efficient way to check merge flags |
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On Thu 03-04-14 16:00:44, Zhan Jianyu wrote: > 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. OK, I'd expect gcc is more clever ;). Thanks for the comparison. Anyway if that function is performance sensitive, we can use your optimization. Just add a comment there that we want to check whether the three flags are the same in both flags and that checking your way generates better code.
Honza -- Jan Kara <jack@suse.cz> SUSE Labs, CR -- 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/
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