Fix CGemV with empty A

pytensor/tensor/blas_c.py

@@ -417,20 +417,6 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, must_initialize_y=False, params=N
         }
     }
 
-    if (%(must_initialize_y)d && dbeta == 0)
-    {
-        // Most BLAS implementations of GEMV ignore y=nan when beta=0
-        // PyTensor considers that the correct behavior,
-        // and even exploits it to avoid copying or initializing outputs.
-        // By deciding to exploit this, however, it becomes our responsibility
-        // to ensure the behavior even in the rare cases BLAS deviates,
-        // or users will get errors, even for graphs that had no nan to begin with.
-        PyObject *zero = PyFloat_FromDouble(0.);
-        if (zero == NULL) %(fail)s;
-        if (PyArray_FillWithScalar(%(z)s, zero) != 0) %(fail)s;
-        Py_DECREF(zero);
-    }
-
     {
         int NA0 = PyArray_DIMS(%(A)s)[0];
         int NA1 = PyArray_DIMS(%(A)s)[1];
@@ -439,6 +425,17 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, must_initialize_y=False, params=N
         {
             // Non-empty A matrix
 
+            if (%(must_initialize_y)d && dbeta == 0)
+            {
+                // Most BLAS implementations of GEMV ignore y=nan when beta=0
+                // PyTensor considers that the correct behavior,
+                // and even exploits it to avoid copying or initializing outputs.
+                // By deciding to exploit this, however, it becomes our responsibility
+                // to ensure the behavior even in the rare cases BLAS deviates,
+                // or users will get errors, even for graphs that had no nan to begin with.
+                PyArray_FILLWBYTE(%(z)s, 0);
+            }
+
             /* In the case where A is actually a row or column matrix,
              * the strides corresponding to the dummy dimension don't matter,
              * but BLAS requires these to be no smaller than the number of elements in the array.
@@ -567,6 +564,18 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, must_initialize_y=False, params=N
                                 "A is neither C nor F-contiguous, it should have been copied into a memory-contiguous array;");
                 %(fail)s
             }
+        } else
+        {
+            // Empty A matrix, just scale y by beta
+            if (dbeta != 1.0)
+            {
+                npy_intp Sz = PyArray_STRIDES(%(z)s)[0] / elemsize;
+                dtype_%(z)s* z_data = (dtype_%(z)s*) PyArray_DATA(%(z)s);
+                for (npy_intp i = 0; i < NA0; ++i)
+                {
+                    z_data[i * Sz] = (dbeta == 0.0) ? 0 : z_data[i * Sz] * dbeta;
+                }
+            }
         }
     }
     """
@@ -598,7 +607,7 @@ class CGemv(BaseBLAS, Gemv):
         return code
 
     def c_code_cache_version(self):
-        return (17, blas_header_version(), must_initialize_y_gemv())
+        return (18, blas_header_version(), must_initialize_y_gemv())
 
 
 cgemv_inplace = CGemv(inplace=True)

tests/tensor/test_blas_c.py

@@ -8,7 +8,15 @@ import pytensor.tensor as pt
 from pytensor.tensor.basic import AllocEmpty
 from pytensor.tensor.blas import Ger
 from pytensor.tensor.blas_c import CGemv, CGer, must_initialize_y_gemv
-from pytensor.tensor.type import dmatrix, dvector, matrix, scalar, tensor, vector
+from pytensor.tensor.type import (
+    dmatrix,
+    dscalar,
+    dvector,
+    matrix,
+    scalar,
+    tensor,
+    vector,
+)
 from tests import unittest_tools
 from tests.tensor.test_blas import BaseGemv, TestBlasStrides
 from tests.unittest_tools import OptimizationTestMixin
@@ -143,19 +151,21 @@ class TestCGemv(OptimizationTestMixin):
     def test_nan_beta_0(self, inplace):
         mode = self.mode.including()
         mode.check_isfinite = False
+        beta = self.a.type("beta")
         f = pytensor.function(
-            [self.A, self.x, pytensor.In(self.y, mutable=inplace), self.a],
-            self.a * self.y + pt.dot(self.A, self.x),
+            [self.A, self.x, pytensor.In(self.y, mutable=inplace), beta],
+            beta * self.y + pt.dot(self.A, self.x),
             mode=mode,
         )
         [node] = f.maker.fgraph.apply_nodes
         assert isinstance(node.op, CGemv) and node.op.inplace == inplace
-        for rows in (3, 1):
-            Aval = np.ones((rows, 1), dtype=self.dtype)
-            xval = np.ones((1,), dtype=self.dtype)
-            yval = np.full((rows,), np.nan, dtype=self.dtype)
-            zval = f(Aval, xval, yval, 0)
-            assert not np.isnan(zval).any()
+        for rows in (3, 1, 0):
+            for cols in (1, 0):
+                Aval = np.ones((rows, cols), dtype=self.dtype)
+                xval = np.ones((cols,), dtype=self.dtype)
+                yval = np.full((rows,), np.nan, dtype=self.dtype)
+                zval = f(Aval, xval, yval, beta=0)
+                assert not np.isnan(zval).any(), f"{rows=}, {cols=}"
 
     def test_optimizations_vm(self):
         skip_if_blas_ldflags_empty()
@@ -294,6 +304,26 @@ class TestCGemv(OptimizationTestMixin):
             == 2
         )
 
+    def test_empty_A(self):
+        A = dmatrix("A")
+        x = dvector("x")
+        y = dvector("y")
+        alpha = 1.0
+        beta = dscalar("beta")
+        gemv = CGemv(inplace=True)(y, alpha, A, x, beta)
+        fn = pytensor.function(
+            [A, x, y, beta],
+            gemv,
+            accept_inplace=True,
+        )
+        test_A = np.empty((10, 0))
+        test_x = np.empty((0,))
+        test_y = np.random.random((10,))
+        for test_beta in [0.0, 1.0, 2.0]:
+            out = fn(test_A, test_x, test_y.copy(), test_beta)
+            expected = test_beta * test_y
+            np.testing.assert_allclose(out, expected)
+
 
 class TestCGemvFloat32(BaseGemv, OptimizationTestMixin):
     mode = mode_blas_opt