svd_truncated: restore scipy fallback

docs/changelog.md

@@ -10,6 +10,11 @@ Release notes for `quimb`.
 - [qu.randn](quimb.randn): support `dist="rademacher"`.
 - support `dist` and other `randn` options in various TN builders.
 
+**Bug fixes:**
+
+- restore fallback (to `scipy.linalg.svd` with driver='gesvd') behavior for
+  truncated SVD with numpy backend.
+
 
 (whats-new-1-7-2)=
 ## v1.7.2 (2024-01-30)

quimb/tensor/decomp.py

@@ -3,10 +3,12 @@
 
 import functools
 import operator
+import warnings
 
 import numpy as np
-import scipy.sparse.linalg as spla
+import scipy.linalg as scla
 import scipy.linalg.interpolative as sli
+import scipy.sparse.linalg as spla
 from autoray import (
     astype,
     backend_like,
@@ -313,7 +315,6 @@ def _trim_and_renorm_svd_result_numba(
     return U, None, VH
 
 
-@svd_truncated.register("numpy")
 @njit  # pragma: no cover
 def svd_truncated_numba(
     x, cutoff=-1.0, cutoff_mode=4, max_bond=-1, absorb=0, renorm=0
@@ -325,6 +326,25 @@ def svd_truncated_numba(
     )
 
 
+@svd_truncated.register("numpy")
+def svd_truncated_numpy(
+    x, cutoff=-1.0, cutoff_mode=4, max_bond=-1, absorb=0, renorm=0
+):
+    """Numpy version of ``svd_truncated``, trying the accelerated version
+    first, then falling back to the more stable scipy version.
+    """
+    try:
+        return svd_truncated_numba(
+            x, cutoff, cutoff_mode, max_bond, absorb, renorm
+        )
+    except np.linalg.LinAlgError as e:  # pragma: no cover
+        warnings.warn(f"Got: {e}, falling back to scipy gesvd driver.")
+        U, s, VH = scla.svd(x, full_matrices=False, lapack_driver="gesvd")
+        return _trim_and_renorm_svd_result_numba(
+            U, s, VH, cutoff, cutoff_mode, max_bond, absorb, renorm
+        )
+
+
 @svd_truncated.register("autoray.lazy")
 @lazy.core.lazy_cache("svd_truncated")
 def svd_truncated_lazy(