PR #8151: [mhlo] AllGather variadic operands support

Imported from GitHub PR openxla/xla#8151

Currently AllGather in HLO supports multiple operands/results, while MHLO only supports a single operand/result. This change addresses the parity gap by adding MHLO AllGather variadic operands support.

This change was inspired by previous commit [2457fc1](openxla/xla@2457fc1) - [mhlo] AllReduce tuple support. Jun 7, 2023 by @GleasonK

Related commit:
- [PR-5740](openxla/xla#5740) [hlo] Add tuple input support to all-gather and reduce-scatter (Oct 16, 2023 by @jeffhataws)

@GleasonK @cheshire @burmako @jurahul @thomasjoerg Could you review this PR?
Copybara import of the project:

--
fb53ead74cbb40177a3680c8f807149d39c396b7 by Alexander Pivovarov <[email protected]>:

[mhlo] AllGather variadic operands support

Merging this change closes #8151

PiperOrigin-RevId: 599175008

mhlo/IR/hlo_ops.cc

@@ -2017,18 +2017,30 @@ LogicalResult AllGatherOp::verify() {
   if (auto channelHandleAttr = getChannelHandleAttr())
     channelId = channelHandleAttr.getHandle();
 
-  return hlo::verifyAllGatherOp(getLoc(), getOperand(), getAllGatherDim(),
-                                getReplicaGroups(), channelId,
-                                getUseGlobalDeviceIds(), getResult());
+  if (getOperands().empty())
+    return emitOptionalError(getLoc(),
+                             "AllGather must have have at least one operand");
+  if (getNumOperands() != getNumResults())
+    return emitOptionalError(
+        getLoc(), "AllGather requires the same number of operands and results");
+
+  for (unsigned i = 0; i < getNumOperands(); ++i) {
+    if (failed(hlo::verifyAllGatherOp(
+            getLoc(), getOperand(i), getAllGatherDim(), getReplicaGroups(),
+            channelId, getUseGlobalDeviceIds(), getResult(i)))) {
+      return failure();
+    }
+  }
+  return success();
 }
 
 void AllGatherOp::build(OpBuilder& odsBuilder, OperationState& odsState,
                         Type resultType, Value operand,
                         IntegerAttr allGatherDim,
                         DenseIntElementsAttr replicaGroups,
                         ChannelHandleAttr channelHandle) {
-  AllGatherOp::build(odsBuilder, odsState, resultType, operand, allGatherDim,
-                     replicaGroups, channelHandle,
+  AllGatherOp::build(odsBuilder, odsState, resultType, ValueRange(operand),
+                     allGatherDim, replicaGroups, channelHandle,
                      /*use_global_device_ids=*/nullptr);
 }
 

mhlo/IR/hlo_ops.td

@@ -1449,8 +1449,9 @@ def MHLO_AllGatherOp : MHLO_Op<"all_gather", [SameOperandsAndResultElementType]>
   string summary = "AllGather operation";
   string description = [{
     Within each process group in the process grid, concatenates the values of the
-    `operand` tensor from each process along `all_gather_dim` and produces a
-    `result` tensor.
+    operand tensor from each process along `all_gather_dim` and produces a
+    result tensor. The `computation` is applied separately for each operand in
+    `operands`, producing one result per operand.
 
     See:
     https://github.com/openxla/stablehlo/blob/main/docs/spec.md#all_gather
@@ -1468,13 +1469,13 @@ def MHLO_AllGatherOp : MHLO_Op<"all_gather", [SameOperandsAndResultElementType]>
   }];
 
   let arguments = (ins
-    MHLO_Tensor:$operand,
+    Variadic<MHLO_Tensor>:$operands,
     I64Attr:$all_gather_dim,
     I64ElementsAttr:$replica_groups,
     OptionalAttr<MHLO_ChannelHandle>:$channel_handle,
     UnitAttr:$use_global_device_ids
   );
-  let results = (outs MHLO_Tensor);
+  let results = (outs Variadic<MHLO_Tensor>);
   // use_global_device_ids is rarely used, so we add simplified builder methods
   // for convenience.
   let builders = [

tests/Dialect/mhlo/ops.mlir

@@ -753,6 +753,17 @@ func.func @all_to_all_i5(%data: tensor<4x16xf32>) -> tensor<16x4xf32> {
 
 // -----
 
+func.func @all_gather_variadic(%arg0: tensor<8x2xf32>, %arg1: tensor<8x4xf32>) -> (tensor<8x8xf32>, tensor<8x16xf32>) {
+  %0:2 = "mhlo.all_gather"(%arg0, %arg1) {
+    all_gather_dim = 1 : i64,
+    channel_handle = #mhlo.channel_handle<handle = 1, type = 0>,
+    replica_groups = dense<[[0, 2, 4, 6], [1, 3, 5, 7]]> : tensor<2x4xi64>
+  } : (tensor<8x2xf32>, tensor<8x4xf32>) -> (tensor<8x8xf32>, tensor<8x16xf32>)
+  func.return %0#0, %0#1 : tensor<8x8xf32>, tensor<8x16xf32>
+}
+
+// -----
+
 func.func @allgather_gather_along_zero_dimension(%arg0: tensor<128x0xf32>) -> tensor<128x100xf32> {
   // expected-error@+1 {{dimension size of operand at 'all_gather_dim' cannot be zero}}
   %0 = "mhlo.all_gather"(%arg0) {