From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Mon, 1 Jan 2024 01:46:15 +0000 (-0700)
Subject: Optimize setting a matrix to zero.
X-Git-Tag: relicensing~210^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=97ec2ef545f2c173f4a77cfbd70326584aab28e8;p=dealii.git

Optimize setting a matrix to zero.
---

diff --git a/source/lac/trilinos_sparse_matrix.cc b/source/lac/trilinos_sparse_matrix.cc
index b9c74f49b4..231e9e3f83 100644
--- a/source/lac/trilinos_sparse_matrix.cc
+++ b/source/lac/trilinos_sparse_matrix.cc
@@ -1761,14 +1761,23 @@ namespace TrilinosWrappers
   SparseMatrix &
   SparseMatrix::operator=(const double d)
   {
+    (void)d;
     Assert(d == 0, ExcScalarAssignmentOnlyForZeroValue());
     compress(VectorOperation::unknown); // TODO: why do we do this? Should we
                                         // not check for is_compressed?
 
-    const int ierr = matrix->PutScalar(d);
+    // As checked above, we are only allowed to use d==0.0, so pass
+    // a constant zero (instead of a run-time value 'd' that *happens* to
+    // have a zero value) to the underlying class in hopes that the compiler
+    // can optimize this somehow.
+    const int ierr = matrix->PutScalar(/*d=*/0.0);
     AssertThrow(ierr == 0, ExcTrilinosError(ierr));
+
     if (nonlocal_matrix.get() != nullptr)
-      nonlocal_matrix->PutScalar(d);
+      {
+        const int ierr = nonlocal_matrix->PutScalar(/*d=*/0.0);
+        AssertThrow(ierr == 0, ExcTrilinosError(ierr));
+      }
 
     return *this;
   }