On Tue, Feb 04, 2020 at 02:38:51PM +0100, David Hildenbrand wrote: > On 04.02.20 14:13, Segher Boessenkool wrote: > > On Tue, Feb 04, 2020 at 01:41:06PM +0100, David Hildenbrand wrote: > >> 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) . > > You can just write the easy to understand > > > > ... ALIGN_UP(x) - x ... > > you mean > > ALIGN_UP(x, PAGES_PER_SECTION) - x > > but ... > > > 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 ;-) > 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 (and replace 1 by any other minimum size required). This *also* is easy to read, without having to have any details (and quirks :-/ ) of those utility functions memorised. Segher
