[Mlir-commits] [mlir] [mlir][presburger] Preserve relative ordering of symbols and non-symbol vars when setting up SymbolicLexSimplex (PR #119036)

[Arjun P]

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@@ -64,13 +64,36 @@ SimplexBase::SimplexBase(unsigned nVar, bool mustUseBigM,
                          const llvm::SmallBitVector &isSymbol)
     : SimplexBase(nVar, mustUseBigM) {
   assert(isSymbol.size() == nVar && "invalid bitmask!");
-  // Invariant: nSymbol is the number of symbols that have been marked
-  // already and these occupy the columns
-  // [getNumFixedCols(), getNumFixedCols() + nSymbol).
-  for (unsigned symbolIdx : isSymbol.set_bits()) {
-    var[symbolIdx].isSymbol = true;
-    swapColumns(var[symbolIdx].pos, getNumFixedCols() + nSymbol);
-    ++nSymbol;
+  // Iterate through all the variables. Move symbols to the left and non-symbols
+  // to the right while preserving relative ordering.
+  for (unsigned i = 0; i < nVar; ++i) {
+    if (isSymbol[i]) {
+
+      // Move the column from its current position to the end of
+      // the symbols segment and update the position metadata for each column
+      // unknown (which should all be vars at construction time). The
+      // segment of non-symbol up until the current variable's column are
+      // shifted to the right and the current column is then moved before the
+      // right-shifted segment.
+      tableau.moveColumns(var[i].pos, 1, getNumFixedCols() + nSymbol);
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Superty wrote:

Probably a good idea to leave a comment to that effect.

https://github.com/llvm/llvm-project/pull/119036