Fix threadgroup memory in arg reduce (#723)

Author: jagrit06

mlx/backend/metal/kernels/arg_reduce.metal

@@ -11,8 +11,6 @@ template <typename U>
 struct IndexValPair {
   uint32_t index;
   U val;
-
-  IndexValPair(uint32_t _index, U _val) : index(_index), val(_val) {}
 };
 
 template <typename U>
@@ -65,10 +63,10 @@ struct ArgMax {
 
 template <typename U>
 IndexValPair<U> simd_shuffle_down(IndexValPair<U> data, uint16_t delta) {
-  return IndexValPair<U>(
+  return IndexValPair<U>{
     simd_shuffle_down(data.index, delta),
     simd_shuffle_down(data.val, delta)
-  );
+  };
 }
 
 
@@ -82,7 +80,6 @@ template <typename T, typename Op, int N_READS>
     const device size_t& ndim [[buffer(5)]],
     const device size_t& axis_stride [[buffer(6)]],
     const device size_t& axis_size [[buffer(7)]],
-    threadgroup IndexValPair<T> *local_data [[threadgroup(0)]],
     uint gid [[thread_position_in_grid]],
     uint lid [[thread_position_in_threadgroup]],
     uint lsize [[threads_per_threadgroup]],
@@ -111,7 +108,9 @@ template <typename T, typename Op, int N_READS>
   auto in_idx = elem_to_loc(gid / lsize, shape, in_strides, ndim);
   auto out_idx = elem_to_loc(gid / lsize, shape, out_strides, ndim);
 
-  IndexValPair<T> best(0, Op::init);
+  IndexValPair<T> best{0, Op::init};
+
+  threadgroup IndexValPair<T> local_data[32];
 
   // Loop over the reduction axis in lsize*N_READS buckets
   for (uint r=0; r < ceildiv(axis_size, N_READS*lsize); r++) {
@@ -172,7 +171,6 @@ template <typename T, typename Op, int N_READS>
       const device size_t& ndim [[buffer(5)]], \
       const device size_t& axis_stride [[buffer(6)]], \
       const device size_t& axis_size [[buffer(7)]], \
-      threadgroup IndexValPair<itype> *local_data [[threadgroup(0)]], \
       uint gid [[thread_position_in_grid]], \
       uint lid [[thread_position_in_threadgroup]], \
       uint lsize [[threads_per_threadgroup]], \

mlx/backend/metal/primitives.cpp

@@ -430,8 +430,6 @@ void ArgReduce::eval_gpu(const std::vector<array>& inputs, array& out) {
     compute_encoder->setBytes(&ndim, sizeof(size_t), 5);
     compute_encoder->setBytes(&axis_stride, sizeof(size_t), 6);
     compute_encoder->setBytes(&axis_size, sizeof(size_t), 7);
-    compute_encoder->setThreadgroupMemoryLength(
-        simd_size * (sizeof(uint32_t) + in.itemsize()), 0);
     compute_encoder->dispatchThreads(grid_dims, group_dims);
   }
 }