[Mlir-commits] [mlir] [mlir][IR] Fix bug in AffineExpr simplifier `lhs % rhs` where `lhs = lhs floordiv rhs` (PR #119245)

[llvmlistbot at llvm.org]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-mlir-core

Author: Christopher Bate (christopherbate)

<details>
<summary>Changes</summary>

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.


---
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
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

``````````

</details>


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