From e7248b26a1ed53fa030c5c459f7ea095dfd276ac Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Antonio=20S=C3=A1nchez?= <cantonios@google.com>
Date: Thu, 19 May 2022 22:29:48 +0000
Subject: [PATCH] Prevent BDCSVD crash caused by index out of bounds.

(cherry picked from commit 028ab12586ee1244755455107fcba66493b336d6)
---
 Eigen/src/SVD/BDCSVD.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/Eigen/src/SVD/BDCSVD.h b/Eigen/src/SVD/BDCSVD.h
index a76a8dd04..6dcb986c1 100644
--- a/Eigen/src/SVD/BDCSVD.h
+++ b/Eigen/src/SVD/BDCSVD.h
@@ -978,8 +978,8 @@ void BDCSVD<MatrixType>::computeSingVals(const ArrayRef& col0, const ArrayRef& d
     // perturb singular value slightly if it equals diagonal entry to avoid division by zero later
     // (deflation is supposed to avoid this from happening)
     // - this does no seem to be necessary anymore -
-//     if (singVals[k] == left) singVals[k] *= 1 + NumTraits<RealScalar>::epsilon();
-//     if (singVals[k] == right) singVals[k] *= 1 - NumTraits<RealScalar>::epsilon();
+    // if (singVals[k] == left) singVals[k] *= 1 + NumTraits<RealScalar>::epsilon();
+    // if (singVals[k] == right) singVals[k] *= 1 - NumTraits<RealScalar>::epsilon();
   }
 }
 
@@ -1033,7 +1033,14 @@ void BDCSVD<MatrixType>::perturbCol0
             std::cout << "  " << "j=" << j << "\n";
           }
 #endif
-          Index j = i<k ? i : perm(l-1);
+          // Avoid index out of bounds.
+          // Will end up setting zhat(k) = 0.
+          if (l == 0) {
+            m_info = NumericalIssue;
+            prod = 0;
+            break;
+          }
+          Index j = i<k ? i : l > 0 ? perm(l-1) : i;
 #ifdef EIGEN_BDCSVD_SANITY_CHECKS
           if(!(dk!=Literal(0) || diag(i)!=Literal(0)))
           {