Small Bugfix: Remove possibility of bitshifting negative integers (#1111

)

* remove possible bitshifts of negative numbers

* changelog

CHANGELOG.md

@@ -28,6 +28,7 @@
 - [[PR 1004]](https://github.com/parthenon-hpc-lab/parthenon/pull/1004) Allow parameter modification from an input file for restarts
 
 ### Fixed (not changing behavior/API/variables/...)
+- [[PR 1111]](https://github.com/parthenon-hpc-lab/parthenon/pull/1111) Fix undefined behavior due to bitshift of negative number in LogicalLocation
 - [[PR 1092]](https://github.com/parthenon-hpc-lab/parthenon/pull/1092) Updates to DataCollection and MeshData to remove requirement of predefining MeshBlockData
 - [[PR 1113]](https://github.com/parthenon-hpc-lab/parthenon/pull/1113) Prevent division by zero
 - [[PR 1112]](https://github.com/parthenon-hpc-lab/parthenon/pull/1112) Remove shared_ptr cycle in forest::Tree

src/mesh/forest/logical_location.cpp

@@ -118,10 +118,10 @@ bool LogicalLocation::IsNeighbor(const LogicalLocation &in) const {
 
   bool neighbors = true;
   for (int dir = 0; dir < 3; ++dir) {
-    auto low = (l(dir) << level_shift_this) - 1;
+    auto low = l(dir) * block_size_this - 1;
     auto hi = low + block_size_this + 1;
 
-    auto low_in = (in.l(dir) << level_shift_in);
+    auto low_in = in.l(dir) * block_size_in;
     auto hi_in = low_in + block_size_in - 1;
     neighbors = neighbors && !(hi < low_in || low > hi_in);
   }
@@ -140,7 +140,7 @@ bool LogicalLocation::IsNeighborOfTE(const LogicalLocation &in,
 
   bool neighbors = true;
   for (int dir = 0; dir < 3; ++dir) {
-    auto low = (l(dir) << level_shift_this);
+    auto low = l(dir) * block_size_this;
     auto hi = low + block_size_this - 1;
     if (te_offset[dir] == -1) {
       low -= 1;
@@ -150,7 +150,7 @@ bool LogicalLocation::IsNeighborOfTE(const LogicalLocation &in,
       low = hi - 1;
     }
 
-    auto low_in = (in.l(dir) << level_shift_in);
+    auto low_in = in.l(dir) * block_size_in;
     auto hi_in = low_in + block_size_in - 1;
     neighbors = neighbors && !(hi < low_in || low > hi_in);
   }