On 05.02.20 13:51, Segher Boessenkool wrote:

On Tue, Feb 04, 2020 at 02:38:51PM +0100, David Hildenbrand wrote:

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On 04.02.20 14:13, Segher Boessenkool wrote:

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On Tue, Feb 04, 2020 at 01:41:06PM +0100, David Hildenbrand wrote:

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It's a pattern commonly used in compilers and emulators to calculate the
number of bytes to the next block/alignment. (we're missing a macro
(like we have ALIGN_UP/IS_ALIGNED) for that - but it's hard to come up
with a good name (e.g., SIZE_TO_NEXT_ALIGN) .

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You can just write the easy to understand

  ...  ALIGN_UP(x) - x  ...

you mean

ALIGN_UP(x, PAGES_PER_SECTION) - x

but ...

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which is better *without* having a separate name.  Does that not
generate good machine code for you?

1. There is no ALIGN_UP. "SECTION_ALIGN_UP(x) - x" would be possible

Erm, you started it ;-)

Yeah, I was thinking in the wrong code base :)

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2. It would be wrong if x is already aligned.

e.g., let's use 4096 for simplicity as we all know that value by heart
(for both x and the block size).

a) -(4096 | -4096) -> 4096

b) #define ALIGN_UP(x, a) ((x + a - 1) & -(a))

ALIGN_UP(4096, 4096) - 4096 -> 0

Not as easy as it seems ...

If you always want to return a number >= 1, it it simply
  ALIGN_UP(x + 1) - x

I'm sorry to have to correct you again for some corner cases:

ALIGN_UP(1, 4096) - 4096 = 0

Again, not as easy as it seems ...

-- 
Thanks,

David / dhildenb