Updates based on new semantics

rfcs/20231017-collective-broadcast.md

@@ -2,7 +2,7 @@
 
 Status: Review<br/>
 Initial version: 10/17/20223<br/>
-Last updated: 10/17/2023<br/>
+Last updated: 11/1/2023<br/>
 Discussion thread: [GitHub](https://github.com/openxla/stablehlo/pull/1809)
 
 ## Motivation
@@ -30,49 +30,44 @@ Unfortunately, the op name `broadcast` is already taken by [an op in XLA proper]
 
 Within each process group in the StableHLO process grid, send the value of the
 `operand` tensor from the source process to the target processes and produce a
-`result` tensor. The first process in each of the `replica_groups` is treated as the 
-source process. For any process `n` not in any replica group, that process will just return 
-zeros.
+`result` tensor. For any process `n` not in any replica group, that process will just
+return zeros.
 
 The operation splits the StableHLO process grid into `process_groups` which is
 defined as follows:
 
-* `cross_replica(replica_groups)`
-  if `channel_id <= 0 and use_global_device_ids = false`.
-* `cross_replica_and_partition(replica_groups)`
-  if `channel_id > 0 and use_global_device_ids = false`.
-* `flattened_ids(replica_groups)`
-  if `channel_id > 0 and use_global_device_ids = true`.
+* `cross_replica(replica_groups)` if `channel_id <= 0`.
+* `cross_partition(replica_groups)` if `channel_id > 0`.
 
-Afterwards, within each `process_group`:
+Afterwards, `result@process` is given by:
+
+* `operand@process_groups[i, 0]` if there exists an `i` such that
+  the process is in `process_groups[i]`.
+* `broadcast_in_dim(constant(0, element_type(result)), [], type(result))`
+  if process is never found in `process_groups`.
 
-* `result@process[result_index] = operand@process[0]`
 
 #### Inputs
 
 | Label | Name                    | Type                                                             | Constraints |
 |-------|-------------------------|------------------------------------------------------------------|-------------|
-| (I1)  | `operand`               | tensor                                                           | (C5)        |
-| (I2)  | `replica_groups`        | variadic number of 1-dimensional tensor constants of type `si64` | (C1-C3)     |
-| (I3)  | `channel_id`            | constant of type `si64`                                          | (C4)        |
-| (I4)  | `use_global_device_ids` | constant of type `i1`                                            | (C4)        |
+| (I1)  | `operand`               | tensor                                                           | (C3)        |
+| (I2)  | `replica_groups`        | variadic number of 1-dimensional tensor constants of type `si64` | (C1-C2)     |
+| (I3)  | `channel_id`            | constant of type `si64`                                          |             |
 
 #### Outputs
 
 | Name     | Type   | Constraints |
 |----------|--------|-------------|
-| `result` | tensor | (C5)        |
+| `result` | tensor | (C3)        |
 
 #### Constraints
 
-* (C1) `is_unique(replica_groups)`.
-* (C2) `size(replica_groups)` is defined as:
+* (C1) is_unique(replica_groups).
+* (C2) `0 <= replica_groups < N` where `N` is defined as:
   * `num_replicas` if `cross_replica` is used.
-  * `num_replicas` if `cross_replica_and_partition` is used.
-  * `num_processes` if `flattened_ids` is used.
-* (C3) `0 <= replica_groups < size(replica_groups)`.
-* (C4) If `use_global_device_ids = true`, then `channel_id > 0`.
-* (C5) `type(result) = type(operand)`.
+  * `num_partitions` if `cross_partition` is used.
+* (C3) `type(result) = type(operand)`.
 
 #### Examples
 
@@ -85,9 +80,8 @@ Afterwards, within each `process_group`:
 // %operand@(0, 3): [[7, 8]]
 %result = "stablehlo.collective_broadcast"(%operand) {
   replica_groups = dense<[[2, 0], [1, 3]]> : tensor<2x2xi64>,
-  // channel_id = 0
-  channel_handle = #stablehlo.channel_handle<handle = 0, type = 0>
-  // use_global_device_ids = false
+  // channel_id = 1
+  channel_handle = #stablehlo.channel_handle<handle = 1, type = 0>
 } : (tensor1x2xi64>) -> tensor<1x2xi64>
 // %result@(0, 0): [[5, 6]]
 // %result@(0, 1): [[3, 4]]
@@ -105,9 +99,8 @@ Example with missing processes
 // %operand@(0, 3): [[7, 8]]
 %result = "stablehlo.collective_broadcast"(%operand) {
   replica_groups = dense<[[2, 1]]> : tensor<1x2xi64>,
-  // channel_id = 0
-  channel_handle = #stablehlo.channel_handle<handle = 0, type = 0>
-  // use_global_device_ids = false
+  // channel_id = 1
+  channel_handle = #stablehlo.channel_handle<handle = 1, type = 0>
 } : (tensor1x2xi64>) -> tensor<1x2xi64>
 // %result@(0, 0): [[0, 0]]
 // %result@(0, 1): [[5, 6]]