On Wed, 13 Mar 2019 at 23:29:40 +0100, Rasmus Villemoes wrote: > On 21/02/2019 09.21, George Spelvin wrote: >> +/** >> + * parent - given the offset of the child, find the offset of the parent. >> + * @i: the offset of the heap element whose parent is sought. Non-zero. >> + * @lsbit: a precomputed 1-bit mask, equal to "size & -size" >> + * @size: size of each element >> + * >> + * In terms of array indexes, the parent of element j = i/size is simply >> + * (j-1)/2. But when working in byte offsets, we can't use implicit >> + * truncation of integer divides. >> + * >> + * Fortunately, we only need one bit of the quotient, not the full divide. >> + * size has a least significant bit. That bit will be clear if i is >> + * an even multiple of size, and set if it's an odd multiple. >> + * >> + * Logically, we're doing "if (i & lsbit) i -= size;", but since the >> + * branch is unpredictable, it's done with a bit of clever branch-free >> + * code instead. >> + */ >> +__attribute_const__ __always_inline >> +static size_t parent(size_t i, unsigned int lsbit, size_t size) >> +{ >> + i -= size; >> + i -= size & -(i & lsbit); >> + return i / 2; >> +} >> + > > Really nice :) I had to work through this by hand, but it's solid.
Thank you! Yes, the way the mask doesn't include the low-order bits that don't matter anyway is a bit subtle. When the code is subtle, use lots of comments. The entire reason for making this a separate helper function is to leave room for the large comment. >> + unsigned const lsbit = size & -size; /* Used to find parent */ > > Nit: qualifier before type, "const unsigned". And this sets ZF, so a > paranoid check for zero size (cf. the other mail) by doing "if (!lsbit) > return;" is practically free. Though it's probably a bit obscure doing > it that way... Actually, this is a personal style thing which I can ignore for the sake of the kernel, but I believe that it's better to put the qualifier *after* the type. This is due to C's pointer declaration syntax. The standard example of the issue is: typedef char *pointer; const char *a; char const *b; char * const c; const pointer d; pointer const e; Now, which variables are the same types? The answer is that a & b are the same (mutable pointer to const char), and c, d & e are the same (const pointer to mutable char). I you make a habit of putting the qualifier *after* the type, then a simple "textual substitution" mental model for the typedef works, and it's clear that c and e are the same. It's also clear that b cannot be represented by the typedef because the const is between "char" and "*", and you obviously can't do that with the typedef. But if you put the qualifier first, it's annoying to rememeber why a and d are not the same type. So I've deliberately cultivated the style of putting the qualifier after the type. But if the kernel prefers it before... >> + if (!n) >> + return; > > I'd make that n <= 1. Shouldn't be much more costly. (Actually, it's "num <= 1"; n is the pre-multiplied form so n <= 1 can only happen when sorting one one-byte value.) I actually thought about this and decided not to bother. I did it this way during development to stress the general-case code. But should I change it? === NEVER MIND === I had written a long reply justifying leaving it alone to save one instruction when the light dawned: I can do *both* tests in one step with size_t n = num * size, a = (num/2) * size; unsigned const lsbit = size & -size; /* Used to find parent */ if (!a) /* num < 2 || size == 0 */ return; So now everyone's happy. > Nice! Thank you. May I translate that into Acked-by?