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

[Christopher Bate]

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@@ -64,13 +64,14 @@ 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]) {
+      var[i].isSymbol = true;
+      swapColumns(var[i].pos, getNumFixedCols() + nSymbol);
+      nSymbol++;
+    }
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christopherbate wrote:

(The old code in `computeReprWithOnlyDivLocals` was not preserving the relative order of symbols while this `SimplexBase` constructor was not preserving the relative order of non-symbols. The updated code ensures the partitioning is now stable.)  

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