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Fix an issue in PartitionGatherTrivialSlicedOperandDimensions when handling out-of-bound indices. #20401

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Dec 13, 2024
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Fix an issue in PartitionGatherTrivialSlicedOperandDimensions when handling out-of-bound indices. #20401

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50 changes: 45 additions & 5 deletions xla/service/spmd/gather_scatter_handler.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -193,6 +193,44 @@ std::vector<int64_t> GatherOutputDimsByPriority(
return priority_dims_for_output;
}

PartitionedHlo ClampGatherIndices(const PartitionedHlo& indices,
const Shape& operand_base_shape,
absl::Span<const int64_t> start_index_map,
int64_t index_vector_dim, SpmdBuilder* b) {
const PrimitiveType indices_type = indices.hlo()->shape().element_type();

HloInstruction* max_indices;
if (index_vector_dim < indices.rank()) {
std::vector<int32_t> max_indices_values;
max_indices_values.reserve(start_index_map.size());
for (int64_t operand_dim : start_index_map) {
max_indices_values.push_back(operand_base_shape.dimensions(operand_dim) -
1);
}
max_indices = b->AddInstruction(HloInstruction::CreateConstant(
LiteralUtil::CreateR1<int32_t>(max_indices_values)));
max_indices = b->AddInstruction(HloInstruction::CreateBroadcast(
indices.hlo()->shape(), max_indices, {index_vector_dim}));
} else {
CHECK_EQ(start_index_map.size(), 1);
max_indices = CreateR0WithType<int32_t>(
indices_type, operand_base_shape.dimensions(start_index_map[0]) - 1, b);
max_indices = b->AddInstruction(HloInstruction::CreateBroadcast(
indices.hlo()->shape(), max_indices, {}));
}

HloInstruction* constant_zero = CreateR0WithType<int32_t>(indices_type, 0, b);
HloInstruction* min_indices =
b->AddInstruction(HloInstruction::CreateBroadcast(indices.hlo()->shape(),
constant_zero, {}));

HloInstruction* clamped_indices = b->AddInstruction(
HloInstruction::CreateTernary(indices.hlo()->shape(), HloOpcode::kClamp,
min_indices, indices.hlo(), max_indices));
clamped_indices->set_sharding(indices.sharding());
return PartitionedHlo(clamped_indices, indices.base_shape(), indices.state());
}

// Returns the min and max for the indices in a scatter/gather which has the
// operand partitioned on trivial slice dimensions (slice size 1).
std::pair<HloInstruction*, HloInstruction*>
Expand Down Expand Up @@ -451,11 +489,9 @@ absl::StatusOr<HloInstruction*> PartitionGatherTrivialSlicedOperandDimensions(

SpmdBuilder* b = visitor->builder();
const GatherDimensionNumbers& dnums = gather->gather_dimension_numbers();
std::vector<int64_t> start_index_map(dnums.start_index_map().begin(),
dnums.start_index_map().end());
if (std::optional<std::vector<int64_t>> trivial_slice_dims =
GatherScatterOperandPartitionedOnTrivialSliceDims(
operand, start_index_map, slice_sizes)) {
operand, dnums.start_index_map(), slice_sizes)) {
const HloSharding original_operand_sharding = operand.sharding();
const int64_t num_groups = operand.sharding().NumTiles(*trivial_slice_dims);
const int64_t num_tiles = operand.sharding().TotalNumTiles();
Expand Down Expand Up @@ -504,6 +540,9 @@ absl::StatusOr<HloInstruction*> PartitionGatherTrivialSlicedOperandDimensions(
// Reshard indices to its intended sharding before clamping and adjusting.
indices =
indices.Reshard(hlo_sharding_util::UngroupSharding(indices_grouped));
indices = ClampGatherIndices(indices, operand.base_shape(),
dnums.start_index_map(),
dnums.index_vector_dim(), b);
// Now the operand is partitioned in trivial slice dimensions, and the
// indices are replicated. We execute a gather on partitioned operand,
// with full number of indices, where out-of-bounds indices are clamped,
Expand All @@ -514,8 +553,9 @@ absl::StatusOr<HloInstruction*> PartitionGatherTrivialSlicedOperandDimensions(
HloInstruction* indices_max;
std::tie(indices_min, indices_max) =
IndexBoundsForGatherScatterOperandPartitionedOnTrivialSliceDims(
operand, indices, operand.state().partition_id, start_index_map,
*trivial_slice_dims, dnums.index_vector_dim(), b);
operand, indices, operand.state().partition_id,
dnums.start_index_map(), *trivial_slice_dims,
dnums.index_vector_dim(), b);
// Clamp the indices.
auto adjusted_indices = b->AddInstruction(
HloInstruction::CreateTernary(indices.hlo()->shape(), HloOpcode::kClamp,
Expand Down
26 changes: 16 additions & 10 deletions xla/service/spmd/spmd_partitioner_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -7926,10 +7926,13 @@ ENTRY entry {
auto min = AllOf(op::Broadcast(offset), op::Shape("s32[2,3]"));
auto max = AllOf(op::Broadcast(op::Add(offset, op::Constant())),
op::Shape("s32[2,3]"));
auto clamp = op::Clamp(min, op::Parameter(1), max);
auto clamped_indices =
op::Clamp(op::Broadcast(op::Constant()), op::Parameter(1),
op::Broadcast(op::Constant()));
auto clamp = op::Clamp(min, clamped_indices, max);
auto gather = op::Gather(op::Parameter(0), op::Subtract(clamp, min));
auto mask =
op::Or(op::Lt(op::Parameter(1), min), op::Gt(op::Parameter(1), max));
op::Or(op::Lt(clamped_indices, min), op::Gt(clamped_indices, max));
auto masked =
op::Select(op::Broadcast(mask), op::Broadcast(op::Constant()), gather);
HloInstruction* root = module->entry_computation()->root_instruction();
Expand All @@ -7952,15 +7955,18 @@ ENTRY entry {
TF_ASSERT_OK_AND_ASSIGN(auto module,
PartitionComputation(hlo_string, /*num_devices=*/4));
VLOG(1) << module->ToString();
auto clamped_indices =
op::Clamp(op::Broadcast(op::Constant()), op::Parameter(1),
op::Broadcast(op::Constant()));
auto offset =
op::Reshape(op::DynamicSlice(op::Constant(), op::PartitionId()));
auto min = AllOf(op::Broadcast(offset), op::Shape("s32[2,3]"));
auto max = AllOf(op::Broadcast(op::Add(offset, op::Constant())),
op::Shape("s32[2,3]"));
auto clamp = op::Clamp(min, op::Parameter(1), max);
auto clamp = op::Clamp(min, clamped_indices, max);
auto gather = op::Gather(op::Parameter(0), op::Subtract(clamp, min));
auto mask =
op::Or(op::Lt(op::Parameter(1), min), op::Gt(op::Parameter(1), max));
op::Or(op::Lt(clamped_indices, min), op::Gt(clamped_indices, max));
auto masked =
op::Select(op::Broadcast(mask), op::Broadcast(op::Constant()), gather);
HloInstruction* root = module->entry_computation()->root_instruction();
Expand Down Expand Up @@ -11919,11 +11925,10 @@ ENTRY entry {
VLOG(1) << module->ToString();
HloInstruction* root = module->entry_computation()->root_instruction();
EXPECT_THAT(root, op::AllReduce(op::Select(_, _, op::Gather(_, _))));
EXPECT_THAT(root->operand(0)->operand(2)->operand(1),
op::Subtract(op::Clamp(_, op::Parameter(1), _), _));
EXPECT_THAT(
root->operand(0)->operand(2)->operand(1),
op::Subtract(op::Clamp(_, op::Clamp(_, op::Parameter(1), _), _), _));

auto clamp = FindInstruction(module.get(), HloOpcode::kClamp);
EXPECT_THAT(clamp->operand(1), op::Parameter(1));
auto dynamic_slice = FindInstruction(module.get(), HloOpcode::kDynamicSlice);
EXPECT_THAT(dynamic_slice->operand(1), op::PartitionId());
auto collective_permute =
Expand Down Expand Up @@ -11955,8 +11960,9 @@ ENTRY entry {
_, op::AllReduce(op::Select(_, _, op::Gather(op::AllReduce(_), _))),
_, _, _)));
auto gather = FindInstruction(module.get(), HloOpcode::kGather);
EXPECT_THAT(gather->operand(1),
op::Subtract(op::Clamp(_, op::Parameter(1), _), _));
EXPECT_THAT(
gather->operand(1),
op::Subtract(op::Clamp(_, op::Clamp(_, op::Parameter(1), _), _), _));
auto collective_permute =
FindInstruction(module.get(), HloOpcode::kCollectivePermute);
EXPECT_NE(collective_permute, nullptr);
Expand Down
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