Fix sign of largest known divisor of div. (#100081)

There's a missing abs, so it returns a negative value if the divisor is
negative. Later this is then cast to uint.

Author: jreiffers

mlir/lib/IR/AffineExpr.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <cmath>
 #include <cstdint>
 #include <limits>
 #include <utility>
@@ -257,7 +258,7 @@ int64_t AffineExpr::getLargestKnownDivisor() const {
     if (rhs && rhs.getValue() != 0) {
       int64_t lhsDiv = binExpr.getLHS().getLargestKnownDivisor();
       if (lhsDiv % rhs.getValue() == 0)
-        return lhsDiv / rhs.getValue();
+        return std::abs(lhsDiv / rhs.getValue());
     }
     return 1;
   }

mlir/unittests/IR/AffineExprTest.cpp

@@ -106,3 +106,9 @@ TEST(AffineExprTest, modSimplificationRegression) {
   auto sum = d0 + d0.floorDiv(3).floorDiv(-3);
   ASSERT_EQ(sum.getKind(), AffineExprKind::Add);
 }
+
+TEST(AffineExprTest, divisorOfNegativeFloorDiv) {
+  MLIRContext ctx;
+  OpBuilder b(&ctx);
+  ASSERT_EQ(b.getAffineDimExpr(0).floorDiv(-1).getLargestKnownDivisor(), 1);
+}