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
(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.
Robin.
