From cba7502e4a74bd92cd7ca96ba87643191b55b1d4 Mon Sep 17 00:00:00 2001
From: obchain <riteshnikhoriya94@gmail.com>
Date: Wed, 17 Jun 2026 12:44:53 +0530
Subject: [PATCH] Implement jvp for cumulative logsumexp

Scan::jvp only handled the Sum reduction and threw for everything else,
so forward-mode differentiation through mx.logcumsumexp raised. The jvp
is the running softmax-weighted sum of the tangents,

    d/dt logcumsumexp(x)_k = sum_{i<=k} softmax(x)_i * t_i,

computed in log space by splitting the tangent into its positive and
negative parts, mirroring the existing vjp. Exclusive scans leave the
first element with no inputs (output -inf, locally constant), so its
tangent is set to zero, which also avoids an inf - inf there.
---
 mlx/primitives.cpp            | 36 +++++++++++++++++++++++++++++++++++
 python/tests/test_autograd.py | 35 ++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+)

diff --git a/mlx/primitives.cpp b/mlx/primitives.cpp
index e2f31f4954..c42171a0e4 100644
--- a/mlx/primitives.cpp
+++ b/mlx/primitives.cpp
@@ -4296,6 +4296,42 @@ std::vector<array> Scan::jvp(
 
   if (reduce_type_ == Scan::Sum) {
     return {cumsum(tangents[0], axis_, reverse_, inclusive_, stream())};
+  } else if (reduce_type_ == Scan::LogAddExp) {
+    // d/dt logcumsumexp(x)_k = sum_{i<=k} softmax(x)_i * t_i. Compute it in log
+    // space for stability by splitting the tangent into its positive and
+    // negative parts, mirroring the vjp.
+    auto x = primals[0];
+    auto t = tangents[0];
+    auto y = logcumsumexp(x, axis_, reverse_, inclusive_, stream());
+
+    auto zero = zeros({1}, t.dtype(), stream());
+    auto log_min = array(finfo(t.dtype()).min, t.dtype());
+    auto log_abs_t = log(abs(t, stream()), stream());
+    auto log_t_positive =
+        where(greater(t, zero, stream()), log_abs_t, log_min, stream());
+    auto log_t_negative =
+        where(less(t, zero, stream()), log_abs_t, log_min, stream());
+
+    auto masked_scan = [&](const array& log_t) {
+      return exp(
+          subtract(
+              logcumsumexp(
+                  add(log_t, x, stream()),
+                  axis_,
+                  reverse_,
+                  inclusive_,
+                  stream()),
+              y,
+              stream()),
+          stream());
+    };
+    auto out = subtract(
+        masked_scan(log_t_positive), masked_scan(log_t_negative), stream());
+    // An exclusive scan leaves the first element with no inputs, so the output
+    // is -inf and locally constant there: its jvp is zero (this also avoids an
+    // inf - inf in the expression above).
+    return {
+        where(isneginf(y, stream()), zeros_like(out, stream()), out, stream())};
   } else {
     throw std::runtime_error(
         "JVP is not implemented for cumulative prod/min/max");
diff --git a/python/tests/test_autograd.py b/python/tests/test_autograd.py
index 221f9229b1..4a204222b7 100644
--- a/python/tests/test_autograd.py
+++ b/python/tests/test_autograd.py
@@ -590,6 +590,41 @@ def fun(y):
         expected = mx.array([0.0, 0.0, 0.0, 9.0, 1.0])
         self.assertTrue(mx.allclose(out, expected))
 
+    def test_logcumsumexp_grad(self):
+        # The jvp of logcumsumexp is the running softmax-weighted sum of the
+        # tangents; check it against an explicit reference and against the vjp.
+        x = mx.array([1.0, 3.0, 2.0, 4.0])
+        v = mx.array([1.0, -1.0, 2.0, 0.5])
+        jv = mx.jvp(lambda z: mx.logcumsumexp(z), (x,), (v,))[1][0]
+        # d/dt logcumsumexp(x)_k = sum_{i<=k} softmax(x[:k+1])_i * v_i
+        ref = []
+        for k in range(x.size):
+            w = mx.softmax(x[: k + 1])
+            ref.append(mx.sum(w * v[: k + 1]).item())
+        self.assertTrue(mx.allclose(jv, mx.array(ref)))
+
+        # vjp must be the transpose of the jvp (adjoint test) for every
+        # combination of the reverse / inclusive flags and across axes.
+        mx.random.seed(0)
+        for reverse in (False, True):
+            for inclusive in (True, False):
+                for axis in (0, 1, -1):
+                    a = mx.random.normal((4, 6))
+                    v = mx.random.normal(a.shape)
+                    w = mx.random.normal(a.shape)
+
+                    def fun(z):
+                        return mx.logcumsumexp(
+                            z, axis=axis, reverse=reverse, inclusive=inclusive
+                        )
+
+                    jv = mx.jvp(fun, (a,), (v,))[1][0]
+                    jtw = mx.vjp(fun, (a,), (w,))[1][0]
+                    self.assertTrue(mx.all(mx.isfinite(jv)))
+                    self.assertAlmostEqual(
+                        mx.sum(w * jv).item(), mx.sum(v * jtw).item(), places=4
+                    )
+
     def test_topk_grad(self):
         a = mx.array([[1, 2, 6, 4, 5], [9, 5, 6, 7, 8]], mx.float32)