Cover more cases of log1mexp stabilization (#1483)

* Created some tests that fail due to #1476

* Fixes 1476 and other ways to create a log1mexp

* Reimplemented logmexpm1_to_log1mexp by tracking expm1 and then looking through the clients

* Absorbed the rewrite log1pexp_to_softplus into the new rewrite for log1mexp

* Fixed bug where I forgot to check whether result of is_neg was None or not before proceeding

---------

Co-authored-by: Luca Citi <[email protected]>

Author: lciti

pytensor/tensor/rewriting/math.py

@@ -64,6 +64,7 @@
     log,
     log1mexp,
     log1p,
+    log1pexp,
     makeKeepDims,
     maximum,
     mul,
@@ -2999,12 +3000,6 @@ def _is_1(expr):
     tracks=[sigmoid],
     get_nodes=get_clients_at_depth2,
 )
-log1pexp_to_softplus = PatternNodeRewriter(
-    (log1p, (exp, "x")),
-    (softplus, "x"),
-    values_eq_approx=values_eq_approx_remove_inf,
-    allow_multiple_clients=True,
-)
 log1p_neg_sigmoid = PatternNodeRewriter(
     (log1p, (neg, (sigmoid, "x"))),
     (neg, (softplus, "x")),
@@ -3016,7 +3011,6 @@ def _is_1(expr):
 
 register_stabilize(logsigm_to_softplus, name="logsigm_to_softplus")
 register_stabilize(log1msigm_to_softplus, name="log1msigm_to_softplus")
-register_stabilize(log1pexp_to_softplus, name="log1pexp_to_softplus")
 register_stabilize(log1p_neg_sigmoid, name="log1p_neg_sigmoid")
 register_specialize(log1p_neg_sigmoid, name="log1p_neg_sigmoid")
 
@@ -3582,12 +3576,40 @@ def local_reciprocal_1_plus_exp(fgraph, node):
 register_specialize(local_1msigmoid)
 
 
-log1pmexp_to_log1mexp = PatternNodeRewriter(
-    (log1p, (neg, (exp, "x"))),
-    (log1mexp, "x"),
-    allow_multiple_clients=True,
-)
-register_stabilize(log1pmexp_to_log1mexp, name="log1pmexp_to_log1mexp")
+@register_stabilize
+@node_rewriter([log1p])
+def local_log1p_plusminus_exp(fgraph, node):
+    """Transforms log1p of ±exp(x) into log1pexp (aka softplus) / log1mexp
+    ``log1p(exp(x))  -> log1pexp(x)``
+    ``log1p(-exp(x)) -> log1mexp(x)``
+    where "-" can be "neg" or any other expression detected by "is_neg"
+    """
+    (log1p_arg,) = node.inputs
+    exp_info = is_exp(log1p_arg)
+    if exp_info is not None:
+        exp_neg, exp_arg = exp_info
+        if exp_neg:
+            return [log1mexp(exp_arg)]
+        else:
+            return [log1pexp(exp_arg)]  # aka softplus
+
+
+@register_stabilize
+@node_rewriter([expm1])
+def logmexpm1_to_log1mexp(fgraph, node):
+    """``log(-expm1(x)) -> log1mexp(x)``
+    where "-" can be "neg" or any other expression detected by "is_neg"
+    """
+    rewrites = {}
+    for node in get_clients_at_depth(fgraph, node, depth=2):
+        if node.op == log:
+            (log_arg,) = node.inputs
+            neg_arg = is_neg(log_arg)
+            if neg_arg is not None and neg_arg.owner and neg_arg.owner.op == expm1:
+                (expm1_arg,) = neg_arg.owner.inputs
+                rewrites[node.outputs[0]] = log1mexp(expm1_arg)
+    return rewrites
+
 
 # log(exp(a) - exp(b)) -> a + log1mexp(b - a)
 logdiffexp_to_log1mexpdiff = PatternNodeRewriter(

tests/tensor/rewriting/test_math.py

@@ -4438,11 +4438,22 @@ def test_local_add_neg_to_sub(first_negative):
     assert np.allclose(f(x_test, y_test), exp)
 
 
-def test_log1mexp_stabilization():
+@pytest.mark.parametrize(
+    "op_name",
+    ["log_1_minus_exp", "log1p_minus_exp", "log_minus_expm1", "log_minus_exp_minus_1"],
+)
+def test_log1mexp_stabilization(op_name):
     mode = Mode("py").including("stabilize")
 
     x = vector()
-    f = function([x], log(1 - exp(x)), mode=mode)
+    if op_name == "log_1_minus_exp":
+        f = function([x], log(1 - exp(x)), mode=mode)
+    elif op_name == "log1p_minus_exp":
+        f = function([x], log1p(-exp(x)), mode=mode)
+    elif op_name == "log_minus_expm1":
+        f = function([x], log(-expm1(x)), mode=mode)
+    elif op_name == "log_minus_exp_minus_1":
+        f = function([x], log(-(exp(x) - 1)), mode=mode)
 
     nodes = [node.op for node in f.maker.fgraph.toposort()]
     assert nodes == [pt.log1mexp]