From: Rafael J. Wysocki
> Sent: 01 April 2021 14:50
...
> So what exactly is wrong with using "packed"? It is way easier to
> understand for a casual reader of the code.
Because it is usually wrong!
If I have:
struct foo {
u64 val;
} __packed;
And then have:
u64 bar(struct foo *foo)
{
return foo->val;
}
The on some cpu the compiler has to generate the equivalent of:
u8 *x = (void *)&foo->val;
return x[0] | x[1] << 8 | x[2] << 16 | x[3] << 24 | x[4] << 32 | x[5]
<< 40 | x[6] << 48 | x[7] << 56;
If you can guarantee that the structure is 32bit aligned
then it can generate the simpler:
u32 *x = (void *)&foo->val;
return x[0] | x[1] << 32;
(Yes I've missed out the 64-bit casts)
This is why you should almost never use __packed.
There are historic structures with 64 bit items on 4 byte boundaries
(and 32 bit values on 2 byte boundaries).
Typically most of the fields are shorter so can be read directly
(although they might need a byte-swapping load).
David
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