From d3fcfc1b78d34bf52631d4b4d32c635a18c19737 Mon Sep 17 00:00:00 2001
From: Christopher Bate <cbate@nvidia.com>
Date: Mon, 9 Dec 2024 18:33:07 +0000
Subject: [PATCH] [mlir][IR] Fix bug in AffineExpr simplifier `lhs % rhs` where
 `lhs = lhs floordiv rhs`

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 https://github.com/llvm/llvm-project/issues/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 https://github.com/llvm/llvm-project/issues/114654.
---
 mlir/lib/IR/AffineExpr.cpp           |  2 +-
 mlir/unittests/IR/AffineExprTest.cpp | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/mlir/lib/IR/AffineExpr.cpp b/mlir/lib/IR/AffineExpr.cpp
index 2291d64c50a56..59df0cd6833db 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 9e89a5b79e2e2..8a2d697540d5c 100644
--- a/mlir/unittests/IR/AffineExprTest.cpp
+++ b/mlir/unittests/IR/AffineExprTest.cpp
@@ -129,3 +129,21 @@ 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);
+
+  // Manually replace variables by constants to avoid constant folding.
+  AffineExpr expr = (d0 - (d0 + 2)).floorDiv(8) % 8;
+  AffineExpr result = mlir::simplifyAffineExpr(expr, 1, 0);
+
+  ASSERT_TRUE(isa<AffineConstantExpr>(result));
+  ASSERT_EQ(cast<AffineConstantExpr>(result).getValue(), 7);
+}