# Check for borrows in bitwise operations

2020-05-03: Had to add a complement step in the ULEB section. Seems I couldn’t actually avoid that crummy-looking notation. Spotted by redditor /u/Y_Less.

In the fourth installment of his series on sorting with AVX2, @damageboy has a short aside where he tries to detect partitioning (pivot) patterns where elements less than and greater than or equal to the pivot are already in the correct order: in that case, the partitioning routine does not need to permute the block of values. The practical details are irrelevant for this post; what matters is that we wish to quickly identify whether a byte value matches any of the follow nine cases:

• 0b11111111
• 0b11111110
• 0b11111100
• 0b11111000
• 0b11110000
• 0b11100000
• 0b11000000
• 0b10000000
• 0b00000000

Looking at the bit patterns,1 the OP’s solution with popcount and bitscan is pretty natural. These instructions are somewhat complex (latency closer to 3 cycles than 1, and often port restricted), and it seems like the sort of problem that would have had efficient solutions before SSE4 finally graced x86 with a population count instruction.

In the context of a sorting library’s partition loop, popcnt and bsf is probably more than good enough: the post shows that the real issue is branch mispredictions being slower than permuting unconditionally. This is just a fun challenge to think about (:

## Warm-up: is_power_of_two

Detecting whether a machine integer is a power of two (or zero) is another task that has a straightforward solution in terms of popcount or bitscan. There’s also a simpler classic solution to this problem:

x == 0 || is_power_of_two(x) <==> (x & (x - 1)) == 0

How does that expression work? Say x is a power of two. Its binary representation is 0b0...010...0: any number of leading zeros,2 a single “1” bit, and trailing zeros (maybe none). Let’s see what happens when we subtract 1 from x:

x           = 0b00...0010...0
x - 1  = 0b00...0001...1
x & (x - 1) = 0b00...0000...0


The subtraction triggered a chain of borrows throughout the trailing zeros, until we finally hit that 1 bit. In decimal, subtracting one from 10...0 yields 09...9; in binary we instead find 01...1. If you ever studied the circuit depth (latency) of carry chains (for me, that was for circuit complexity theory), you know that this is difficult to do well. Luckily for us, chip makers work hard to pull it off, and we can just use carries as a data-controlled primitive to efficiently flip ranges of bits.

When x is a power of two, x and x - 1 have no “1” bit in common, so taking the bitwise and yields zero. That’s also true when x is 0, since anding anything with 0 yields zero. Let’s see what happens for non-zero, non-power-of-two values x = 0bxx...xx10..0, i.e., where x consists of an arbitrary non-zero sequence of bits xx..xx followed by the least set bit (there’s at least one, since x is neither zero nor a power of two), and the trailing zeros:

x           = 0bxx...xx10...0
x - 1  = 0bxx...xx01...1
x & (x - 1) = 0bxx...xx000000


The leading not-all-zero 0bxx...xx is unaffected by the subtraction, so it passes through the bitwise and unscathed (anding any bit with itself yields that same bit), and we know there’s at least one non-zero bit in there; our test correctly rejects it!

## Stretching: decoding varints

When decoding variable length integers in ULEB format, e.g., for protocol buffers, it quickly becomes clear that, in order to avoid byte-at-a-time logic, we must rapidly segment (lex or tokenize, in a way) our byte stream to determine where each ULEB ends. Let’s focus on the fast path, when the encoded ULEB fits in a machine register.

We have uleb = 0bnnnnnnnnmmmmmmmm...0zzzzzzz1yyyyyyy1...: a sequence of bytes3 with the topmost bit equal to 1, terminated by a byte with the top bit set to 0, and, finally, arbitrary nuisance bytes (m...m, n...n, etc.) we wish to ignore. Ideally, we’d extract data = 0b0000000000000000...?zzzzzzz?yyyyyyy?... from uleb: we want to clear the nuisance bytes, and are fine with arbitrary values in the ULEB’s control bits.

It’s much easier to find bits set to 1 than to zero, so the first thing to do is to complement the ULEB data and clear out everything but potential ULEB control bits (the high bit of each byte), with c = ~uleb & (128 * (WORD_MAX / 255)), i.e., compute the bitwise and of ~uleb with a bitmask of the high bit in each byte.

   uleb = 0bnnnnnnnnmmmmmmmm...0zzzzzzz1yyyyyyy1...
~uleb = 0b̅n̅n̅n̅n̅n̅n̅n̅n̅m̅m̅m̅m̅m̅m̅m̅m̅...1z̅z̅z̅z̅z̅z̅z0y̅y̅y̅y̅y̅y̅y0...
c = 0b̅n̅0000000̅m̅0000000...10000000000000000...


We could now bitscan to find the index of the first 1 (marking the last ULEB byte), and then generate a mask. However, it seems wasteful to generate an index with a scan, only to convert it back into bitmap space with a shift. We’ll probably still want that index to know how far to advance the decoder’s cursor, but we can hopefully update the cursor in parallel with decoding the current ULEB value.

When we were trying to detect powers of two, we subtracted 1 from x, a value kind of like c, in order to generate a new value that differed from x in all the bits up to and including the first set (equal to 1) bit of x, and identical in the remaining bits. We then used the fact that anding a bit with itself yields that same bit to detect whether there was any non-zero bit in the remainder.

Here, we wish to do something else with the remaining untouched bits, we wish to set them all to zero. Another bitwise operator does what we want: xoring a bit with itself always yields zero, while xoring bits that differ yields 1. That’s the plan for ULEB. We’ll subtract 1 from c and xor that back with c.

       uleb = 0bnnnnnnnnmmmmmmmm...0zzzzzzz1yyyyyyy1...
~uleb = 0b̅n̅n̅n̅n̅n̅n̅n̅n̅m̅m̅m̅m̅m̅m̅m̅m̅...1z̅z̅z̅z̅z̅z̅z0y̅y̅y̅y̅y̅y̅y0...
c           = 0b̅n̅0000000̅m̅0000000...10000000000000000...
c - 1  = 0b̅n̅0000000̅m̅0000000...01111111111111111...
c ^ (c - 1) = 0b0000000000000000...11111111111111111...


We now just have to bitwise and uleb with c ^ (c - 1) to obtain the bits of the first ULEB value in uleb, while overwriting everything else with 0. Once we have that, we can either extract data bits with PEXT on recent Intel chips, or otherwise dust off interesting stunts for SWAR shifts by variable amounts.

## Now for damageboy’s problem

Let’s first repeat the question that motivated this post. We want to detect when a byte p is one of the following nine values:

• 0b11111111
• 0b11111110
• 0b11111100
• 0b11111000
• 0b11110000
• 0b11100000
• 0b11000000
• 0b10000000
• 0b00000000

These bit patterns feel similar to those for power of two bytes: if we complement the bits, these values are all 1 less than a power of two (or -1, one less than zero). We already know how to detect when a value x is zero or a power of two (x & (x - 1) == 0), so it’s easy to instead determine whether ~p is one less than zero or a power of two: (~p + 1) & ~p == 0.

This is already pretty good: bitwise not the byte p, and check if it’s one less than zero or a power of two (three simple instructions on the critical path). We can do better.

There’s another name for ~p + 1, i.e., for bitwise complementing a value and adding one: that’s simply -p, the additive inverse of p in two’s complement! We can use -p & ~p == 0. That’s one fewer instruction on the critical path of our dependency graph (down to two, since we can test whether anding yields zero), and still only uses simple instructions that are unlikely to be port constrained.

Let’s check our logic by enumerating all byte-sized values.

CL-USER> (dotimes (p 256)
(when (zerop (logand (- p) (lognot p) 255))
(format t "0b~2,8,'0r~%" p)))
0b00000000
0b10000000
0b11000000
0b11100000
0b11110000
0b11111000
0b11111100
0b11111110
0b11111111


These are the bytes we’re looking for (in ascending rather than descending order)!

## Remember the power of borrows

I hope the examples above communicated a pattern I often observe when mangling bits: operations that are annoying (not hard, just a bit more complex than we’d like) in the bitmap domain can be simpler in two’s complement arithmetic. Arithmetic operations are powerful mutators for bitmaps, but they’re often hard to control. Subtracting or adding 1 are the main exceptions: it’s easy to describe their impact in terms of the low bits of the bitmap. In fact, we can extend that trick to subtracting or adding powers of two: it’s the same carry/borrow chain effect as for 1, except that bits smaller than the power of two pass straight through… which might be useful when we expect a known tag followed by a ULEB value that must be decoded.

If you find yourself wishing for a way to flip ranges of bits in a data-dependent fashion, it’s always worth considering the two’s complement representation of the problem for a couple minutes. Adding or subtracting powers of two doesn’t always work, but the payoff is pretty good when it does.

P.S., Wojciech Muła offers a different 3-operation sequence with -p to solve damageboy’s problem. That’s another nice primitive to generate bitmasks dynamically.

Thank you Ruchir for helping me clarify the notation around the ULEB section.

1. I use the 0b... syntax throughout this post to denote bit literals, similarly to the usual 0x... hexadecimal literals.

2. While I prefer to work with little-endian bytes, I find everything makes more sense with big-endian bits.

3. Remember, while ULEB is little-endian, we use big bit-endianness.

May 2nd, 2020
In categories: Puzzle