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

[Daria Sukhonina]

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

>From 43f8303a443a74a656b5b24ce21735dea495a26a 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