faster cpu indexing (#1450)

mlx/backend/common/indexing.cpp

@@ -1,5 +1,4 @@
 // Copyright © 2023 Apple Inc.
-
 #include <algorithm>
 #include <cassert>
 #include <cmath>
@@ -81,11 +80,18 @@ void gather(
   T* dst_ptr = out.data<T>();
   size_t out_idx = 0;
 
+  std::vector<ContiguousIterator<size_t>> its(inds.begin(), inds.end());
+  ContiguousIterator<size_t> src_it;
+  if (!can_copy && src.ndim() > 0) {
+    src_it = std::move(
+        ContiguousIterator<size_t>(slice_sizes, src.strides(), src.ndim()));
+  }
   for (int idx = 0; idx < ind_size; idx++) {
     size_t src_idx = 0;
     for (int ii = 0; ii < inds.size(); ++ii) {
       auto ax = axes[ii];
-      auto idx_loc = elem_to_loc(idx, inds[ii]);
+      auto idx_loc = its[ii].loc;
+      its[ii].step();
       auto idx_val =
           offset_neg_idx(inds[ii].data<IdxT>()[idx_loc], src.shape(ax));
       src_idx += (idx_val * src.strides()[ax]);
@@ -99,9 +105,10 @@ void gather(
       out_idx += slice_size;
     } else {
       for (int jj = 0; jj < slice_size; jj++) {
-        auto src_offset = elem_to_loc(jj, slice_sizes, src.strides());
+        dst_ptr[out_idx++] = src_ptr[src_idx + src_it.loc];
-        dst_ptr[out_idx++] = src_ptr[src_idx + src_offset];
+        src_it.step();
       }
+      src_it.reset();
     }
   }
 }
@@ -223,21 +230,29 @@ void scatter(
     update_size *= us;
   }
 
+  std::vector<ContiguousIterator<size_t>> its(inds.begin(), inds.end());
+  ContiguousIterator<size_t> update_it(updates);
+  ContiguousIterator<size_t> out_it(update_shape, out.strides(), out.ndim());
+
   for (int i = 0; i < n_updates; ++i) {
     size_t out_offset = 0;
     for (int j = 0; j < nind; ++j) {
       auto ax = axes[j];
-      auto idx_loc = elem_to_loc(i, inds[j]);
+      auto idx_loc = its[j].loc;
+      its[j].step();
       auto idx_val =
           offset_neg_idx(inds[j].data<IdxT>()[idx_loc], out.shape(ax));
       out_offset += (idx_val * out.strides()[ax]);
     }
+    update_it.seek(i * update_size);
     for (int j = 0; j < update_size; ++j) {
-      auto update_loc = elem_to_loc(i * update_size + j, updates);
+      op(updates.data<InT>()[update_it.loc],
-      auto out_loc = elem_to_loc(j, update_shape, out.strides());
+         out.data<InT>() + out_offset + out_it.loc);
-      op(updates.data<InT>()[update_loc],
+      update_it.step();
-         out.data<InT>() + out_offset + out_loc);
+      out_it.step();
     }
+    out_it.reset();
+    update_it.reset();
   }
 }
 

mlx/backend/common/utils.h

@@ -88,7 +88,11 @@ std::pair<std::vector<int>, std::vector<size_t>> collapse_contiguous_dims(
 template <typename StrideT>
 struct ContiguousIterator {
   inline void step() {
-    int i = dims_;
+    int dims = shape_.size();
+    if (dims == 0) {
+      return;
+    }
+    int i = dims - 1;
     while (pos_[i] == (shape_[i] - 1) && i > 0) {
       pos_[i] = 0;
       loc -= (shape_[i] - 1) * strides_[i];
@@ -98,15 +102,41 @@ struct ContiguousIterator {
     loc += strides_[i];
   }
 
+  void seek(StrideT n) {
+    loc = 0;
+    for (int i = shape_.size() - 1; i >= 0; --i) {
+      auto q_and_r = ldiv(n, shape_[i]);
+      loc += q_and_r.rem * strides_[i];
+      pos_[i] = q_and_r.rem;
+      n = q_and_r.quot;
+    }
+  }
+
+  void reset() {
+    loc = 0;
+    std::fill(pos_.begin(), pos_.end(), 0);
+  }
+
+  ContiguousIterator() {};
+
+  explicit ContiguousIterator(const array& a)
+      : shape_(a.shape()), strides_(a.strides()) {
+    if (!shape_.empty()) {
+      std::tie(shape_, strides_) = collapse_contiguous_dims(shape_, strides_);
+      pos_ = std::vector<int>(shape_.size(), 0);
+    }
+  }
+
   explicit ContiguousIterator(
       const std::vector<int>& shape,
       const std::vector<StrideT>& strides,
       int dims)
       : shape_(shape.begin(), shape.begin() + dims),
         strides_(strides.begin(), strides.begin() + dims) {
-    std::tie(shape_, strides_) = collapse_contiguous_dims(shape_, strides_);
+    if (!shape_.empty()) {
-    dims_ = shape_.size() - 1;
+      std::tie(shape_, strides_) = collapse_contiguous_dims(shape_, strides_);
-    pos_ = std::vector<int>(dims_ + 1, 0);
+      pos_ = std::vector<int>(shape_.size(), 0);
+    }
   }
 
   StrideT loc{0};
@@ -115,7 +145,6 @@ struct ContiguousIterator {
   std::vector<int> shape_;
   std::vector<StrideT> strides_;
   std::vector<int> pos_;
-  int dims_;
 };
 
 template <typename StrideT>