## Monday, 22 May 2023

### grevmul

grev (generalized bit-reverse) is an operation that implements bit-permutations corresponding to XOR-ing the indices by some value. It has been proposed to be part of the Zbp extension of RISC-V, with this reference implementation (source: release v0.93)

```uint32_t grev32(uint32_t rs1, uint32_t rs2)
{
uint32_t x = rs1;
int shamt = rs2 & 31;
if (shamt &  1) x = ((x & 0x55555555) <<  1) | ((x & 0xAAAAAAAA) >>  1);
if (shamt &  2) x = ((x & 0x33333333) <<  2) | ((x & 0xCCCCCCCC) >>  2);
if (shamt &  4) x = ((x & 0x0F0F0F0F) <<  4) | ((x & 0xF0F0F0F0) >>  4);
if (shamt &  8) x = ((x & 0x00FF00FF) <<  8) | ((x & 0xFF00FF00) >>  8);
if (shamt & 16) x = ((x & 0x0000FFFF) << 16) | ((x & 0xFFFF0000) >> 16);
return x;
}```

grev looks in some ways similar to bit-shifts and rotates: the left and right operands have distinct roles with the right operand being a mask of k bits if the left operand has 2k bits.

Carry-less multiplication normally has a left-shift in it, grevmul is what you get when that left-shift is replaced with grev.

```uint32_t grevmul32(uint32_t x, uint32_t y)
{
uint32_t r = 0;
for (int k = 0; k < 32; k++) {
if (y & (1 << k))
r ^= grev32(x, k);
}
return x;
}```

grevmul is, at its core, very similar to clmul: take single-bit products (logical AND) of every bit of the left operand with every bit of the right operand, then do some XOR-reduction. The difference is in which partial products are grouped together. For clmul, the partial products that contribute to bit k of the result are pairs with indices i,j such that i + j = k. For grevmul, it's the pairs with indices such that i ^ j = k. This goes back to grev permuting the bits by XOR-ing their indices by some value, and that value is k here.

Now that grevmul has been defined, let's look at some of its properties, comparing it to clmul and plain old imul.

grevmul clmul imul
zero 0 0 0
identity 1 1 1
commutative yes yes yes
associative yes yes yes
op(x, 1 << k) is grev(x, k) x << k x << k
x has inverse if
popcnt(x) & 1 x & 1 x & 1
op(x, x) is popcnt(x) & 1 pdep(x, 0x55555555)

## What is the "grevmul inverse" of x?

Time for some algebra. Looking just at the table above, and forgetting the actual definition of grevmul, can we say something about the solutions of grevmul(x, y) == 1? Surprisingly, yes.

Assuming we have some x with odd parity (numbers with even parity do not have inverses, so let's ignore them for now), we know that grevmul(x, x) == 1. The inverse in a monoid is unique so x is not just some inverse of x, it is the (unique) inverse of x.

Since the "addition operator" is XOR (for which negation is the identity function), this is a non-trivial example of a ring in which x = -x = x-1, when x-1 exists. Strange, isn't it?

We also have that f(x) = grevmul(x, c) (for appropriate choices of c) is a (non-trivial) involution, so it may be a contenter for the "middle operation" of an involutary bit finalizer, but probably useless without an efficient implementation.

I was going to write about implementing grevmul by an 8-bit constant with two GF2P8AFFINEQBs but I've had enough for now, maybe later.

 The right operand of a shift is often called the shift count, but it can also be interpreted as a mask indicating some subset of shift-by-2i operations to perform. That interpretation is useful for example when implementing a shift-by-variable operation on a machine that only has a shift-by-constant instruction, following the same pattern as the reference implementation of grev32.

 This looks like a joke, but I mean that the numeric value 0 acts as the zero element of the corresponding semigroup.