One step downhill
You will learn
How one SGD step moves a weight against its gradient, and why a test with two equal inputs cannot tell them apart.
Recording pending: it waits on tri test and tri mutate plant from gHashTag/t27#7400, the two commands the recording runs, and until then the widget below is a placeholder that shows no run. gft_sgd_step.t27 moves a weight against its gradient: w' = w - eta*g, where eta is the learning rate. Its 3 tests are step, with w = 1.0, g = 0.5 and eta = 0.5; nograd, where g = 0 leaves w alone; and ascend, where g = -1.0 and eta = 1.0 push w up to 2.0. The browser skips all 3 tests because its runner does not know assert_eq yet; the native t27c runs all 3, all pass, none vacuous. The recording multiplies g by itself instead of by eta. In step g equals eta, and in nograd both products are 0. So exactly one test fails, ascend, where the weight falls to 0 instead of rising to 2.0. Every byte in the recording was printed by the command; only the typing is staged.
Try it
In the recording, find the changed line and the test that fails; then in the spec frame find step and decide which of its inputs must change for it to catch the bug.
Recording pending: waits on tri test and tri mutate plant from gHashTag/t27#7400. Until then this page is a placeholder and shows no run.
specs/ternary/gft_sgd_step.t27
module GftSgdStep;
// #1764 + GF-T: a GF-T SGD weight update -- w' = w - eta * g, the final brick of an
// on-device training step (forward softmax -> loss -> gradient g -> THIS update).
// eta is the (positive) learning rate; g the gradient (signed); w the weight (signed).
// Composes the verified primitives: signed multiply (smul over the RNE magnitude
// mul) + subtract (sadd + neg). Bit-exact to the integer oracle; accuracy is to
// GF-T16 precision (<=1 ULP; ~0.03 abs at the largest magnitudes).
//
// Inputs: w, g, eta signed GF-T16 (u32). Output: updated weight w' GF-T16 (u32).
fn magadd(a: i32, b: i32) -> i32 {
var ao : i32 = a >> 9; var am : i32 = a & 511;
var bo : i32 = b >> 9; var bm : i32 = b & 511;
var ho : i32 = bo; var hm : i32 = bm; var lo : i32 = ao; var lm : i32 = am;
if (ao >= bo) { ho = ao; hm = am; lo = bo; lm = bm; }
var hs : i32 = 512 + hm; var ls : i32 = 512 + lm;
var d : i32 = ho - lo; if (d > 11) { d = 11; }
var losh : i32 = ls >> d; var rem : i32 = ls - (losh << d);
var s : i32 = hs + losh; var off : i32 = ho; var mant : i32 = s - 512;
if (s >= 1024) {
var g : i32 = s & 1; var pre : i32 = s >> 1; mant = pre - 512;
if (g == 1) { if (rem > 0) { mant = mant + 1; } else { if ((pre & 1) == 1) { mant = mant + 1; } } }
off = ho + 1; if (off >= 80) { off = 80; }
} else {
var t : i32 = rem << 1; var hf : i32 = 1 << d;
if (t > hf) { mant = mant + 1; } else { if (t == hf) { if ((s & 1) == 1) { mant = mant + 1; } } }
}
if (mant >= 512) { mant = 0; off = off + 1; if (off >= 80) { off = 80; } }
return (off << 9) | mant;
}
fn magsub(hi: i32, lo: i32) -> i32 {
if (hi == lo) { return 0; }
var ho : i32 = hi >> 9; var hm : i32 = hi & 511;
var lo_o : i32 = lo >> 9; var lm : i32 = lo & 511;
var d : i32 = ho - lo_o; var hs : i32 = (512 + hm) << 14;
var la : i32 = 0; var sticky : i32 = 0;
if (d >= 26) { la = 0; sticky = 1; }
else { var ls : i32 = (512 + lm) << 14; la = ls >> d; if ((ls - (la << d)) > 0) { sticky = 1; } }
var diff : i32 = hs - la; var off : i32 = ho;
var cap : i32 = 12; if (off - 1 < cap) { cap = off - 1; } if (cap < 0) { cap = 0; }
var sh : i32 = 0;
if (diff != 0) {
var t : i32 = diff;
if (t < 65536) { if (sh + 8 <= cap) { t = t << 8; sh = sh + 8; } }
if (t < 1048576) { if (sh + 4 <= cap) { t = t << 4; sh = sh + 4; } }
if (t < 4194304) { if (sh + 2 <= cap) { t = t << 2; sh = sh + 2; } }
if (t < 8388608) { if (sh + 1 <= cap) { t = t << 1; sh = sh + 1; } }
}
diff = diff << sh; off = off - sh;
var q : i32 = diff >> 14; var rem : i32 = diff - (q << 14); var half : i32 = 8192; var mant : i32 = q - 512;
if (rem > half) { mant = mant + 1; }
else { if (rem == half) { if (sticky == 1) { mant = mant + 1; } else { if ((q & 1) == 1) { mant = mant + 1; } } } }
if (mant >= 512) { mant = 0; off = off + 1; if (off >= 80) { off = 80; } }
return (off << 9) | mant;
}
fn sadd(a: u32, b: u32) -> u32 {
if (a == 0) { return b; }
if (b == 0) { return a; }
var sa : i32 = (a >> 16) as i32; var ma : i32 = (a & 65535) as i32;
var sb : i32 = (b >> 16) as i32; var mb : i32 = (b & 65535) as i32;
if (sa == sb) { return ((sa << 16) | magadd(ma, mb)) as u32; }
var bsign : i32 = sa;
var r : i32 = magsub(ma, mb);
if (ma < mb) { r = magsub(mb, ma); bsign = sb; }
if (r == 0) { return 0; }
return ((bsign << 16) | r) as u32;
}
fn neg(v: u32) -> u32 {
if (v == 0) { return 0; }
return v ^ 65536;
}
fn magmul(a16: i32, b16: i32) -> i32 {
var ao : i32 = a16 >> 9; var am : i32 = a16 & 511;
var bo : i32 = b16 >> 9; var bm : i32 = b16 & 511;
var prod : i32 = (512 + am) * (512 + bm);
var carry : i32 = 0; if (prod >= 524288) { carry = 1; }
var q : i32 = prod >> 9; var r : i32 = prod & 511; var half : i32 = 256;
if (carry == 1) { q = prod >> 10; r = prod & 1023; half = 512; }
var mant : i32 = q - 512;
if (r > half) { mant = mant + 1; }
if (r == half) { if ((q & 1) == 1) { mant = mant + 1; } }
var sm : i32 = ao + bo + carry;
var out_off : i32 = 0;
if (sm >= 40) { var res : i32 = sm - 40; if (res >= 80) { out_off = 80; } else { out_off = res; } }
if (mant >= 512) { mant = 0; out_off = out_off + 1; if (out_off >= 80) { out_off = 80; } }
return (out_off << 9) | mant;
}
// softmax: p_sel = 2^(l_sel - M) / sum_i 2^(l_i - M), M = max logit.
// signed GF-T multiply: sign = xor of signs, magnitude = RNE magnitude mul.
fn smul(a: u32, b: u32) -> u32 {
if (a == 0) { return 0; }
if (b == 0) { return 0; }
var sgn : i32 = ((a >> 16) & 1) as i32;
var sb : i32 = ((b >> 16) & 1) as i32;
if (sgn != sb) { sgn = 1; } else { sgn = 0; }
var mag : i32 = magmul((a & 65535) as i32, (b & 65535) as i32);
if (mag == 0) { return 0; }
return ((sgn << 16) | mag) as u32;
}
// w' = w - eta*g.
fn on_comb(w: u32, g: u32, eta: u32) -> u32 {
var delta : u32 = smul(eta, g);
return sadd(w, neg(delta));
}
// w=+1.0, g=+0.5, eta=0.5 -> 1 - 0.25 = 0.75 (offset39,mant256 = 0x4f00 = 20224).
test step { assert_eq(on_comb(20480, 19968, 19968), 20224); }
// g=0 -> weight unchanged.
test nograd { assert_eq(on_comb(20480, 0, 19968), 20480); }
// w=+1.0, g=-1.0, eta=1.0 -> 1 - (1*-1) = 2.0 (offset41 = 0x5200 = 20992).
test ascend { assert_eq(on_comb(20480, 86016, 20480), 20992); }
endmodule
All lessons
Module 1 · Lab: our own research
A number format of our own, an honest scoreboard, and a model's tables multiplied on the board.
Module 2 · AI numbers: the MX block
How AI chips keep weights in a few bits: one shared scale per block, the scale byte itself, and what one outlier does to its neighbours.
Module 3 · Ternary weights
Weights that are only minus, zero or plus a scale, the five rules a ternary alphabet must pass, and a test pass that checked nothing.
Module 4 · The Ternary Network Float
A rule the compiler enforces before any test runs, and a 17-bit float whose exponent is four balanced trits.
Module 5 · Arithmetic on signed numbers
Multiply two signed numbers, add them when their signs differ, and do both at once in a multiply-accumulate.
Module 6 · Parts of a neuron
A ReLU that bends at zero, a power of two for softmax, and an argmax that names the answer.
Module 7 · Learning from a mistake
A loss that prices a wrong guess in bits, one step that moves a weight against its gradient, and the hidden layer that XOR needs.
Module 8 · BitNet: ternary networks
A threshold that squeezes a sum back to three values, one neuron that becomes a different function when its weights change, and a neuron that reads its inputs 27 trits at a time.
Module 9 · The ternary MAC as a chip
The 27-trit dot product as wires with no register, the same sum added into a register on every clock, and a small whole network to close the course.