From 7d7d1d69822091d9d88809a69948f575721058df Mon Sep 17 00:00:00 2001
From: Manuel Candales <mcandales@meta.com>
Date: Wed, 5 Nov 2025 14:36:11 -0800
Subject: [PATCH] Enable fast path for negative indices

Differential Revision: D86351194
---
 kernels/portable/cpu/op_index.cpp | 28 +++++-----------
 kernels/test/op_index_test.cpp    | 53 +++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+), 20 deletions(-)

diff --git a/kernels/portable/cpu/op_index.cpp b/kernels/portable/cpu/op_index.cpp
index 6ce9fb375de..d8eb992b85a 100644
--- a/kernels/portable/cpu/op_index.cpp
+++ b/kernels/portable/cpu/op_index.cpp
@@ -49,23 +49,6 @@ bool check_fast_path_conditions(
       if (index.dim() != 1) {
         return false;
       }
-
-      // Fast path only supports non-negative indices.
-      if (ix_type == ScalarType::Int) {
-        const int32_t* const data = index.const_data_ptr<int32_t>();
-        if (std::any_of(data, data + index.numel(), [](const auto x) {
-              return x < 0;
-            })) {
-          return false;
-        }
-      } else { // ScalarType::Long
-        const int64_t* const data = index.const_data_ptr<int64_t>();
-        if (std::any_of(data, data + index.numel(), [](const auto x) {
-              return x < 0;
-            })) {
-          return false;
-        }
-      }
     }
   }
 
@@ -96,8 +79,10 @@ bool check_fast_path_args(
       Long, Int, index.scalar_type(), ctx, "index.Tensor", CTYPE, [&]() {
         const CTYPE* const index_arr = index.const_data_ptr<CTYPE>();
         for (const auto i : c10::irange(index.numel())) {
-          if (index_arr[i] < 0 ||
-              index_arr[i] >= static_cast<CTYPE>(in.size(dim))) {
+          CTYPE index_val = index_arr[i];
+          CTYPE dim_size = static_cast<CTYPE>(in.size(dim));
+          index_val = index_val < 0 ? index_val + dim_size : index_val;
+          if (index_val < 0 || index_val >= dim_size) {
             ET_LOG(
                 Error,
                 "Index %" PRId64
@@ -189,11 +174,14 @@ Tensor& fast_path(
 
   ET_SWITCH_TWO_TYPES(Long, Int, index_type, ctx, op_name, CTYPE, [&]() {
     const CTYPE* const index_arr = index.const_data_ptr<CTYPE>();
+    CTYPE dim_size = static_cast<CTYPE>(in.size(dim));
     for (const auto i : c10::irange(leading_dims)) {
       const char* src = in_data + i * in_dim_length * length_per_step;
       char* dest = out_data + i * out_dim_length * length_per_step;
       for (const auto j : c10::irange(out_dim_length)) {
-        const char* copy_src = src + index_arr[j] * length_per_step;
+        auto index_val =
+            index_arr[j] < 0 ? index_arr[j] + dim_size : index_arr[j];
+        const char* copy_src = src + index_val * length_per_step;
         char* copy_dest = dest + j * length_per_step;
         memcpy(copy_dest, copy_src, length_per_step);
       }
diff --git a/kernels/test/op_index_test.cpp b/kernels/test/op_index_test.cpp
index 787eb4612d8..8816d0a8d3f 100644
--- a/kernels/test/op_index_test.cpp
+++ b/kernels/test/op_index_test.cpp
@@ -947,3 +947,56 @@ TEST_F(OpIndexTensorOutTest, FastPathEmptyInput) {
 
   EXPECT_TENSOR_EQ(out, expected);
 }
+
+TEST_F(OpIndexTensorOutTest, FastPathNegativeIndex) {
+  TensorFactory<ScalarType::Float> tf;
+  TensorFactory<ScalarType::Long> tfl;
+
+  // clang-format off
+  Tensor x = tf.make(
+    {2, 3, 4},
+    {
+        // [0, :, :]
+        1.,   2.,   3.,   4., // [0, 0, :]
+        5.,   6.,   7.,   8., // [0, 1, :]
+        9.,  10.,  11.,  12., // [0, 2, :]
+
+        // [1, :, :]
+       -1.,  -2.,  -3.,  -4., // [1, 0, :]
+       -5.,  -6.,  -7.,  -8., // [1, 1, :]
+       -9., -10., -11., -12., // [1, 2, :]
+    });
+  // clang-format on
+
+  // Use negative indices in the first dimension: -1, 0, -2
+  std::array<optional<Tensor>, 3> indices = {
+      optional<Tensor>(tfl.make({3}, {-1, 0, -2})),
+      optional<Tensor>(),
+      optional<Tensor>()};
+
+  Tensor out = tf.zeros({3, 3, 4});
+  // clang-format off
+  Tensor expected = tf.make(
+    {3, 3, 4},
+    {
+        // [1, :, :]
+       -1.,  -2.,  -3.,  -4., // [1, 0, :]
+       -5.,  -6.,  -7.,  -8., // [1, 1, :]
+       -9., -10., -11., -12., // [1, 2, :]
+
+        // [0, :, :]
+        1.,   2.,   3.,   4., // [0, 0, :]
+        5.,   6.,   7.,   8., // [0, 1, :]
+        9.,  10.,  11.,  12., // [0, 2, :]
+
+        // [0, :, :] again (since -2 wraps to 0)
+        1.,   2.,   3.,   4., // [0, 0, :]
+        5.,   6.,   7.,   8., // [0, 1, :]
+        9.,  10.,  11.,  12., // [0, 2, :]
+    });
+  // clang-format on
+
+  op_index_tensor_out(x, indices, out);
+
+  EXPECT_TENSOR_EQ(out, expected);
+}