Re: [Patch 04/18] include/linux/logfs.h

[David Woodhouse <dwmw2@infradead.org>]

On Tue, 2007-06-05 at 20:49 +0200, Segher Boessenkool wrote:
> >>> It would be better if GCC had a 'nopadding' attribute which gave us
> >>> what we need without the _extra_ implications about alignment.
> >>
> >> That's impossible; removing the padding from a struct
> >> _will_ make accesses to its members unaligned (think
> >> about arrays of that struct).
> >
> > It _might_ make accesses to _some_ of its members unaligned.
> 
> It _will_ make accesses to _at least one_ of the members
> unaligned, in the second array element.

It _might_, but not necessarily.

Won't a simple struct { uint16_t } get padded to a size of 4 bytes on
ARM? Even if I'm misremembering that, I certainly can't guarantee that
such a thing will _never_ happen on any newly-invented ABI. If you had
'nopadding' instead of 'packed', then there's no need to emit code to
handle unaligned loads.

Likewise, with a struct which looks like this: 
   { uint32_t, uint16_t, uint16_t, uint32_t, uint32_t }
I cannot _guarantee_ that there will never be an architecture on which
we'll end up using 32 bits of space for each uint16_t. I might _guess_
that and hope, but that's precisely the kind of moronic empirical
behaviour which caused Linux to have so many problems with newer
compilers in the past. If it isn't _guaranteed_ then I shouldn't be
assuming it.

Thus, I want a way to tell the compiler not to insert _any_ padding. But
without the compiler making extra inferences about the whole thing being
found at arbitrary alignments.

And if I had something like this (which is admittedly contrived, but
hardware people _do_ do stupid things to us):
   { uint32_t, uint8_t, uint16_t, uint8_t, uint32_t, uint32_t }

With the 'packed' attribute the compiler would assume arbitrary
alignment of all the 32-bit integers. But in reality it's only necessary
for the uint16_t in the middle. A 'nopadding' attribute would deal with
that correctly.

-- 
dwmw2

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