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
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