From: David Howells
> Sent: Friday, August 18, 2023 12:43 PM
>
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> > Well, that part is trivially fixable, and we should do that anyway for
> > other reasons.
> > ..
> > enum iter_type {
> > /* iter types */
> > - ITER_IOVEC,
> > - ITER_KVEC,
> > - ITER_BVEC,
> > - ITER_XARRAY,
> > - ITER_DISCARD,
> > - ITER_UBUF,
> > + ITER_IOVEC = 1,
> > + ITER_UBUF = 2,
> > + ITER_KVEC = 4,
> > + ITER_BVEC = 8,
> > + ITER_XARRAY = 16,
> > + ITER_DISCARD = 32,
That could be zero - no bits and default.
> > };
>
> It used to be this way, but Al switched it:
>
> 8cd54c1c848031a87820e58d772166ffdf8c08c0
> iov_iter: separate direction from flavour
Except it also had the direction flag inside the enum.
That caused its own piles of grief.
IIRC Linus had type:6 - that doesn't leave any headroom
for additional types (even though they shouldn't proliferate).
It may be best to avoid bits 15+ (in a bitfield) due to
issues with large constants and sign extension.
On x86 (I think) 'and immediate' and 'bit test' are the same
size for bit 0 to 7, BIT wins for higher bits.
gcc generates strange code for some initialisers (see yesterday's
thread) and you definitely mustn't leave unused bits in a bitfield.
Might be better is the fields are assigned later!
(I also saw clang carefully preserving %eax on stabck!)
David
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