[mlir][IR] Fix bug in AffineExpr simplifier lhs % rhs where lhs = lhs floordiv rhs

[christopherbate]

Fixes an issue where the SimpleAffineExprFlattener would simplify
lhs % rhs to just -(lhs floordiv rhs) instead of lhs - (lhs floordiv rhs)
if lhs happened to be equal to lhs floordiv rhs.

The reported failure case was (d0, d1) -> (((d1 - (d1 + 2)) floordiv 8) % 8)
from #114654.

Note that many paths that simplify AffineMaps (e.g. the AffineApplyOp
folder and canonicalization) would not observe this bug because of
of slightly different paths taken by the code. Slightly different grouping
of the terms could also result in avoiding the bug. The way to reproduce was
by constructing the map directly, replacing d1 with 1 and calling
mlir::simplifyAffineExpr.

Resolves #114654.

[llvmbot]

@llvm/pr-subscribers-mlir

@llvm/pr-subscribers-mlir-core

Author: Christopher Bate (christopherbate)

Changes

Fixes an issue where the SimpleAffineExprFlattener would simplify
lhs % rhs to just -(lhs floordiv rhs) instead of lhs - (lhs floordiv rhs)
if lhs happened to be equal to lhs floordiv rhs.

The reported failure case was (d0, d1) -> (((d1 - (d1 + 2)) floordiv 8) % 8)
from #114654.

Note that many paths that simplify AffineMaps (e.g. the AffineApplyOp
folder and canonicalization) would not observe this bug because of
of slightly different paths taken by the code. Slightly different grouping
of the terms could also result in avoiding the bug. The way to reproduce was
by constructing the map directly, replacing d1 with 1 and calling
mlir::simplifyAffineExpr.

Resolves #114654.

---

Full diff: https://github.com/llvm/llvm-project/pull/119245.diff

2 Files Affected:

• (modified) mlir/lib/IR/AffineExpr.cpp (+1-1)
• (modified) mlir/unittests/IR/AffineExprTest.cpp (+21)

diff --git a/mlir/lib/IR/AffineExpr.cpp b/mlir/lib/IR/AffineExpr.cpp
index 2291d64c50a560..59df0cd6833db3 100644
--- a/mlir/lib/IR/AffineExpr.cpp
+++ b/mlir/lib/IR/AffineExpr.cpp
@@ -1385,7 +1385,7 @@ LogicalResult SimpleAffineExprFlattener::visitModExpr(AffineBinaryOpExpr expr) {
     lhs[getLocalVarStartIndex() + numLocals - 1] = -rhsConst;
   } else {
     // Reuse the existing local id.
-    lhs[getLocalVarStartIndex() + loc] = -rhsConst;
+    lhs[getLocalVarStartIndex() + loc] -= rhsConst;
   }
   return success();
 }
diff --git a/mlir/unittests/IR/AffineExprTest.cpp b/mlir/unittests/IR/AffineExprTest.cpp
index 9e89a5b79e2e2e..7005b2004267c8 100644
--- a/mlir/unittests/IR/AffineExprTest.cpp
+++ b/mlir/unittests/IR/AffineExprTest.cpp
@@ -129,3 +129,24 @@ TEST(AffineExprTest, d0PlusD0FloorDivNeg2) {
   auto sum = d0 + d0.floorDiv(-2) * 2;
   ASSERT_EQ(toString(sum), "d0 + (d0 floordiv -2) * 2");
 }
+
+TEST(AffineExprTEst, simpleAffineExprFlattenerRegression) {
+
+  // Regression test for a bug where mod simplification was not handled
+  // properly when `lhs % rhs` was happened to have the property that `lhs
+  // floordiv rhs = lhs`.
+  MLIRContext ctx;
+  OpBuilder b(&ctx);
+
+  auto d0 = b.getAffineDimExpr(0);
+  auto d1 = b.getAffineDimExpr(1);
+
+  // Manually replace variables by constants to avoid constant folding.
+  AffineExpr expr = (d0 - (d1 + 2)).floorDiv(8) % 8;
+  expr = expr.replaceDims(
+      {b.getAffineConstantExpr(1), b.getAffineConstantExpr(1)});
+  AffineExpr result = mlir::simplifyAffineExpr(expr, 2, 0);
+
+  ASSERT_TRUE(isa<AffineConstantExpr>(result));
+  ASSERT_EQ(cast<AffineConstantExpr>(result).getValue(), 7);
+}
\ No newline at end of file

[Groverkss]

LGTM, please fix the test before landing.