Re: [PATCH 2/2 resend] libata-sff: avoid byte swapping in ata_sff_data_xfer()

[Sergei Shtylyov]

Hello, I wrote:

> > > Handling of the trailing byte in ata_sff_data_xfer() is suboptimal 
> > > bacause:

> > > - it always initializes the padding buffer to 0 which is not really 
> > > needed in
> > >   both the read and write cases;

> > > - it has to use memcpy() to transfer a single byte from/to the 
> > > padding buffer;

> > Have you looked at the assembly, before deciding it is suboptiomal?

>    I'm estimating the code itself, not what the compiler can do to fix 
> it. :-)

> > gcc optimizes tiny arrays and structures quite well, and is well 
> > capable of seeing one path where the initialization is clobbered 
> > without a single read, and another code path where it is used.

>    The initialier just shouldn't have been there in the first place, 
> clobbered or not. And let's looks at what gcc gave me:

[...]

> > As for memcpy, for small and/or constant values that is quite often a 
> > compiler builtin.  It is rarely useful, these days, to convert a 
> > memcpy() to a hand-rolled 

> version of same.

>    Here memcpy() just shouldn't have appeared in the first place. But 
> indeed, gcc did optimize it away.

   In fact, we could do without both memcpy and io*15_rep() I think:

 	if (unlikely(buflen & 0x01)) {
		u16 pad;

		/* Point buf to the tail of buffer */
		buf += buflen - 1;

		/*
		 * Copy from/to pad's LSB only (host order),
		 * dropping its MSB or zero-extending it...
		 */
 		if (rw == READ) {
			pad = ioread16(data_addr);
			*buf = (unsigned char)pad;
 		} else {
			pad = *buf;
			iowrite16(pad, data_addr);
		}
	}

   It should work -- that easy... although io{read|write}16() will still 
byte-swap.

> >     Jeff

MBR, Sergei
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/