[Mlir-commits] [mlir] Fix mlir-reduce splitting smallest range instead of the largest one (PR #214738)

[Daria Sukhonina]

https://github.com/zetanumbers created https://github.com/llvm/llvm-project/pull/214738

The function `max_element` expects comparison lambda function to return true if first argument is **less** than the second one. As such behavior is different from the one described in the comment above. The difference can become obvious when trying to remove the last line in the `except_one` MLIR function which was newly added to tests.

>From 37a3fb8e1da8e537c872a2bd30fc883c73349bc5 Mon Sep 17 00:00:00 2001
From: Daria Sukhonina <dariasukhonina at gmail.com>
Date: Fri, 7 Aug 2026 16:49:06 +0300
Subject: [PATCH] Fix mlir-reduce splitting smallest range instead of the
 largest one

---
 mlir/lib/Reducer/ReductionNode.cpp              |  2 +-
 .../mlir-reduce/reduction-tree/doc-example.mlir |  9 +++++----
 .../mlir-reduce/reduction-tree/except-last.mlir | 17 +++++++++++++++++
 mlir/test/mlir-reduce/script/except-last.sh     |  7 +++++++
 4 files changed, 30 insertions(+), 5 deletions(-)
 create mode 100644 mlir/test/mlir-reduce/reduction-tree/except-last.mlir
 create mode 100755 mlir/test/mlir-reduce/script/except-last.sh

diff --git a/mlir/lib/Reducer/ReductionNode.cpp b/mlir/lib/Reducer/ReductionNode.cpp
index 11aeaf77b4642..f298c12665b83 100644
--- a/mlir/lib/Reducer/ReductionNode.cpp
+++ b/mlir/lib/Reducer/ReductionNode.cpp
@@ -75,7 +75,7 @@ ArrayRef<ReductionNode *> ReductionNode::generateNewVariants() {
   // final ranges vector will be {{1, 3}, {4, 6}, {6, 9}}.
   auto maxElement =
       llvm::max_element(ranges, [](const Range &lhs, const Range &rhs) {
-        return (lhs.second - lhs.first) > (rhs.second - rhs.first);
+        return (lhs.second - lhs.first) < (rhs.second - rhs.first);
       });
 
   // The length of range is less than 1, we can't split it to create new
diff --git a/mlir/test/mlir-reduce/reduction-tree/doc-example.mlir b/mlir/test/mlir-reduce/reduction-tree/doc-example.mlir
index 7f194025a7ef0..0593db2168afe 100644
--- a/mlir/test/mlir-reduce/reduction-tree/doc-example.mlir
+++ b/mlir/test/mlir-reduce/reduction-tree/doc-example.mlir
@@ -26,7 +26,8 @@ func.func @func2(%arg0: i1) -> f32 {
 // CHECK-LABEL: func @func2
 // CHECK-SAME: (%arg0: i1) -> f32
 // CHECK-DAG: %[[C22:.*]] = arith.constant 2.200000e+00 : f32
-// CHECK-DAG: %[[C75:.*]] = arith.constant 7.500000e+00 : f32
-// CHECK: %[[SEL:.*]] = arith.select %arg0, %[[C75]], %[[C22]] : f32
-// CHECK: %[[ADD:.*]] = arith.addf %[[SEL]], %[[C22]] : f32
-// CHECK: return %[[ADD]] : f32
+// CHECK-DAG: %[[C53:.*]] = arith.constant 5.300000e+00 : f32
+// CHECK: %[[ADD1:.*]] = arith.addf %[[C22]], %[[C53]] : f32
+// CHECK: %[[SEL:.*]] = arith.select %arg0, %[[ADD1]], %[[C22]] : f32
+// CHECK: %[[ADD2:.*]] = arith.addf %[[C22]], %[[SEL]] : f32
+// CHECK: return %[[ADD2]] : f32
diff --git a/mlir/test/mlir-reduce/reduction-tree/except-last.mlir b/mlir/test/mlir-reduce/reduction-tree/except-last.mlir
new file mode 100644
index 0000000000000..fa5c59e33177a
--- /dev/null
+++ b/mlir/test/mlir-reduce/reduction-tree/except-last.mlir
@@ -0,0 +1,17 @@
+// UNSUPPORTED: system-windows
+// RUN: mlir-reduce %s -reduction-tree='traversal-mode=0 test=%S/../script/except-last.sh' | FileCheck %s
+// We are testing the ability of keeping trivially-dead yet interesting code
+
+func.func @except_last() {
+  %0 = arith.constant 1 : i32
+  %1 = arith.constant 2 : i32
+  %2 = arith.constant 3 : i32
+  %3 = arith.constant 4 : i32
+  return
+}
+
+// CHECK-LABEL: func @except_last
+//  CHECK-NEXT:   arith.constant 1 : i32
+//  CHECK-NEXT:   arith.constant 2 : i32
+//  CHECK-NEXT:   arith.constant 3 : i32
+//  CHECK-NEXT: return
diff --git a/mlir/test/mlir-reduce/script/except-last.sh b/mlir/test/mlir-reduce/script/except-last.sh
new file mode 100755
index 0000000000000..b6dce52a9403f
--- /dev/null
+++ b/mlir/test/mlir-reduce/script/except-last.sh
@@ -0,0 +1,7 @@
+#!/bin/sh
+
+file=$1
+
+if grep -q "arith.constant 1 : i32" $file && grep -q "arith.constant 2 : i32" $file && grep -q "arith.constant 3 : i32" $file; then
+  exit 1
+fi