Avoid copy of flipped A matrices in GEMV

pytensor/tensor/blas_c.py

@@ -423,6 +423,7 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, force_init_beta=False, params=Non
         int Sz = PyArray_STRIDES(%(z)s)[0] / elemsize;
         int Sx = PyArray_STRIDES(%(x)s)[0] / elemsize;
 
+        dtype_%(A)s* A_data = (dtype_%(A)s*) PyArray_DATA(%(A)s);
         dtype_%(x)s* x_data = (dtype_%(x)s*) PyArray_DATA(%(x)s);
         dtype_%(z)s* z_data = (dtype_%(z)s*) PyArray_DATA(%(z)s);
         // gemv expects pointers to the beginning of memory arrays,
@@ -435,17 +436,25 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, force_init_beta=False, params=Non
 
         if (NA0 * NA1)
         {
-            // If A is neither C- nor F-contiguous, we make a copy.
-            // TODO:
-            // - if one stride is equal to "- elemsize", we can still call
-            //   gemv on reversed matrix and vectors
-            // - if the copy is too long, maybe call vector/vector dot on
-            //   each row instead
-            if ((PyArray_STRIDES(%(A)s)[0] < 0)
-                || (PyArray_STRIDES(%(A)s)[1] < 0)
-                || ((PyArray_STRIDES(%(A)s)[0] != elemsize)
-                    && (PyArray_STRIDES(%(A)s)[1] != elemsize)))
+            if ( ((SA0 < 0) || (SA1 < 0)) && (abs(SA0) == 1 || (abs(SA1) == 1)) )
             {
+                // We can treat the array A as C-or F-contiguous by changing the order of iteration
+                if (SA0 < 0){
+                    A_data += (NA0 -1) * SA0;  // Jump to first row
+                    SA0 = -SA0;  // Iterate over rows in reverse
+                    Sz = -Sz;  // Iterate over y in reverse
+                }
+                if (SA1 < 0){
+                    A_data += (NA1 -1) * SA1;  // Jump to first column
+                    SA1 = -SA1;  // Iterate over columns in reverse
+                    Sx = -Sx;  // Iterate over x in reverse
+                }
+            } else if ((SA0 < 0) || (SA1 < 0) || ((SA0 != 1) && (SA1 != 1)))
+            {
+                // Array isn't contiguous, we have to make a copy
+                // - if the copy is too long, maybe call vector/vector dot on
+                //   each row instead
+                // printf("GEMV: Making a copy SA0=%%d, SA1=%%d\\n", SA0, SA1);
                 npy_intp dims[2];
                 dims[0] = NA0;
                 dims[1] = NA1;
@@ -458,16 +467,17 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, force_init_beta=False, params=Non
                 %(A)s = A_copy;
                 SA0 = (NA0 > 1) ? (PyArray_STRIDES(%(A)s)[0] / elemsize) : (NA1 + 1);
                 SA1 = (NA1 > 1) ? (PyArray_STRIDES(%(A)s)[1] / elemsize) : (NA0 + 1);
+                A_data = (dtype_%(A)s*) PyArray_DATA(%(A)s);
             }
 
-            if (PyArray_STRIDES(%(A)s)[0] == elemsize)
+            if (SA0 == 1)
             {
                 if (PyArray_DESCR(%(A)s)->type_num == NPY_FLOAT)
                 {
                     float alpha = ((dtype_%(alpha)s*)PyArray_DATA(%(alpha)s))[0];
                     sgemv_(&NOTRANS, &NA0, &NA1,
                         &alpha,
-                        (float*)(PyArray_DATA(%(A)s)), &SA1,
+                        (float*)(A_data), &SA1,
                         (float*)x_data, &Sx,
                         &fbeta,
                         (float*)z_data, &Sz);
@@ -477,7 +487,7 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, force_init_beta=False, params=Non
                     double alpha = ((dtype_%(alpha)s*)PyArray_DATA(%(alpha)s))[0];
                     dgemv_(&NOTRANS, &NA0, &NA1,
                         &alpha,
-                        (double*)(PyArray_DATA(%(A)s)), &SA1,
+                        (double*)(A_data), &SA1,
                         (double*)x_data, &Sx,
                         &dbeta,
                         (double*)z_data, &Sz);
@@ -489,7 +499,7 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, force_init_beta=False, params=Non
                     %(fail)s
                 }
             }
-            else if (PyArray_STRIDES(%(A)s)[1] == elemsize)
+            else if (SA1 == 1)
             {
                 if (PyArray_DESCR(%(A)s)->type_num == NPY_FLOAT)
                 {
@@ -506,14 +516,14 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, force_init_beta=False, params=Non
                           z_data[0] = 0.f;
                         }
                         z_data[0] += alpha*sdot_(&NA1,
-                              (float*)(PyArray_DATA(%(A)s)), &SA1,
+                              (float*)(A_data), &SA1,
                               (float*)x_data, &Sx);
                     }
                     else
                     {
                         sgemv_(&TRANS, &NA1, &NA0,
                             &alpha,
-                            (float*)(PyArray_DATA(%(A)s)), &SA0,
+                            (float*)(A_data), &SA0,
                             (float*)x_data, &Sx,
                             &fbeta,
                             (float*)z_data, &Sz);
@@ -534,14 +544,14 @@ def gemv_c_code(y, A, x, z, alpha, beta, fail, force_init_beta=False, params=Non
                           z_data[0] = 0.;
                         }
                         z_data[0] += alpha*ddot_(&NA1,
-                              (double*)(PyArray_DATA(%(A)s)), &SA1,
+                              (double*)(A_data), &SA1,
                               (double*)x_data, &Sx);
                     }
                     else
                     {
                         dgemv_(&TRANS, &NA1, &NA0,
                             &alpha,
-                            (double*)(PyArray_DATA(%(A)s)), &SA0,
+                            (double*)(A_data), &SA0,
                             (double*)x_data, &Sx,
                             &dbeta,
                             (double*)z_data, &Sz);
@@ -603,7 +613,7 @@ class CGemv(BaseBLAS, Gemv):
         return code
 
     def c_code_cache_version(self):
-        return (14, blas_header_version(), check_force_gemv_init())
+        return (15, blas_header_version(), check_force_gemv_init())
 
 
 cgemv_inplace = CGemv(inplace=True)

tests/tensor/test_blas_c.py

@@ -411,3 +411,45 @@ class TestSdotNoFlags(TestCGemvNoFlags):
 
 class TestBlasStridesC(TestBlasStrides):
     mode = mode_blas_opt
+
+
+@pytest.mark.parametrize(
+    "neg_stride1", (True, False), ids=["neg_stride1", "pos_stride1"]
+)
+@pytest.mark.parametrize(
+    "neg_stride0", (True, False), ids=["neg_stride0", "pos_stride0"]
+)
+@pytest.mark.parametrize("F_layout", (True, False), ids=["F_layout", "C_layout"])
+def test_gemv_negative_strides_perf(neg_stride0, neg_stride1, F_layout, benchmark):
+    A = pt.matrix("A", shape=(512, 512))
+    x = pt.vector("x", shape=(A.type.shape[-1],))
+    y = pt.vector("y", shape=(A.type.shape[0],))
+
+    out = CGemv(inplace=False)(
+        y,
+        1.0,
+        A,
+        x,
+        1.0,
+    )
+    fn = pytensor.function([A, x, y], out, trust_input=True)
+
+    rng = np.random.default_rng(430)
+    test_A = rng.normal(size=A.type.shape)
+    test_x = rng.normal(size=x.type.shape)
+    test_y = rng.normal(size=y.type.shape)
+
+    if F_layout:
+        test_A = test_A.T
+    if neg_stride0:
+        test_A = test_A[::-1]
+    if neg_stride1:
+        test_A = test_A[:, ::-1]
+    assert (test_A.strides[0] < 0) == neg_stride0
+    assert (test_A.strides[1] < 0) == neg_stride1
+
+    # Check result is correct by using a copy of A with positive strides
+    res = fn(test_A, test_x, test_y)
+    np.testing.assert_allclose(res, fn(test_A.copy(), test_x, test_y))
+
+    benchmark(fn, test_A, test_x, test_y)