On 15/05/2019 12:13, David Laight wrote:
From: Robin Murphy
Sent: 15 May 2019 11:58
To: David Laight; 'Will Deacon'
Cc: Zhangshaokun; Ard Biesheuvel; linux-arm-ker...@lists.infradead.org;
netdev@vger.kernel.org;
ilias.apalodi...@linaro.org; huanglingyan (A); steve.cap...@arm.com
Subject: Re: [PATCH] arm64: do_csum: implement accelerated scalar version
On 15/05/2019 11:15, David Laight wrote:
...
ptr = (u64 *)(buff - offset);
shift = offset * 8;
/*
* Head: zero out any excess leading bytes. Shifting back by the same
* amount should be at least as fast as any other way of handling the
* odd/even alignment, and means we can ignore it until the very end.
*/
data = *ptr++;
#ifdef __LITTLE_ENDIAN
data = (data >> shift) << shift;
#else
data = (data << shift) >> shift;
#endif
I suspect that
#ifdef __LITTLE_ENDIAN
data &= ~0ull << shift;
#else
data &= ~0ull >> shift;
#endif
is likely to be better.
Out of interest, better in which respects? For the A64 ISA at least,
that would take 3 instructions plus an additional scratch register, e.g.:
MOV x2, #~0
LSL x2, x2, x1
AND x0, x0, x1
[That should have been "AND x0, x1, x2", obviously...]
(alternatively "AND x0, x0, x1 LSL x2" to save 4 bytes of code, but that
will typically take as many cycles if not more than just pipelining the
two 'simple' ALU instructions)
Whereas the original is just two shift instruction in-place.
LSR x0, x0, x1
LSL x0, x0, x1
If the operation were repeated, the constant generation could certainly
be amortised over multiple subsequent ANDs for a net win, but that isn't
the case here.
On a superscaler processor you reduce the register dependency
chain by one instruction.
The original code is pretty much a single dependency chain so
you are likely to be able to generate the mask 'for free'.
Gotcha, although 'free' still means additional I$ and register rename
footprint, vs. (typically) just 1 extra cycle to forward an ALU result.
It's an interesting consideration, but in our case there are almost
certainly far more little in-order cores out in the wild than big OoO
ones, and the double-shift will always be objectively better for those.
Thanks,
Robin.
