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fix topk decomposer crash #41074

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fix topk decomposer crash #41074

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84 changes: 69 additions & 15 deletions xla/service/topk_rewriter.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -462,31 +462,77 @@ class TopkDecomposerVisitor : public DfsHloRewriteVisitor {
return absl::OkStatus();
}
HloComputation* comparator = call->to_apply();
return DecomposeTopK(call, comparator);
bool skip_iota = IndicesAreUnused(inst);
comparator = CreateWrapperComparator(comparator, skip_iota);
return DecomposeTopK(call, comparator, skip_iota);
}

HloComputation* CreateWrapperComparator(HloComputation* old_comp,
bool skip_iota) {
if ((skip_iota ? 2 : 4) == old_comp->num_parameters()) {
return old_comp;
}
HloModule* module = old_comp->parent();
HloComputation::Builder builder("wrapper_comparator");

auto p0 = builder.AddInstruction(HloInstruction::CreateParameter(
0, old_comp->parameter_instruction(0)->shape(), "p0"));
auto p1 = builder.AddInstruction(HloInstruction::CreateParameter(
1, old_comp->parameter_instruction(1)->shape(), "p1"));

if (!skip_iota) {
builder.AddInstruction(HloInstruction::CreateParameter(
2, ShapeUtil::MakeShape(S32, {}), "p2"));
builder.AddInstruction(HloInstruction::CreateParameter(
3, ShapeUtil::MakeShape(S32, {}), "p3"));
}

if (old_comp->num_parameters() == 2) {
auto call = builder.AddInstruction(HloInstruction::CreateCall(
ShapeUtil::MakeShape(PRED, {}), {p0, p1}, old_comp));
return module->AddEmbeddedComputation(builder.Build(call));
} else {
CHECK_EQ(old_comp->num_parameters(), 4);
HloInstruction* d = builder.AddInstruction(
HloInstruction::CreateConstant(LiteralUtil::CreateR0<int32_t>(0)));
auto call = builder.AddInstruction(HloInstruction::CreateCall(
ShapeUtil::MakeShape(PRED, {}), {p0, p1, d, d}, old_comp));
return module->AddEmbeddedComputation(builder.Build(call));
}
}

absl::Status HandleTopK(HloInstruction* topk) override {
if (should_decompose_ && !should_decompose_(topk)) {
return absl::OkStatus();
}
bool skip_iota = IndicesAreUnused(topk);
TF_ASSIGN_OR_RETURN(HloComputation * comparator,
CreateVariadicComparator(topk));
return DecomposeTopK(topk, comparator);
CreateVariadicComparator(topk, skip_iota));
return DecomposeTopK(topk, comparator, skip_iota);
}

private:
bool HasSingleUserReadingOnlyTheValueOutput(HloInstruction* inst) {
return inst->user_count() == 1 && inst->users().front()->tuple_index() == 0;
bool IndicesAreUnused(HloInstruction* inst) {
if (inst == inst->parent()->root_instruction()) {
return false;
}
for (auto user : inst->users()) {
if (user->opcode() != HloOpcode::kGetTupleElement ||
user->tuple_index() != 0) {
return false;
}
}
return true;
}

absl::StatusOr<HloComputation*> CreateVariadicComparator(
HloInstruction* inst) {
absl::StatusOr<HloComputation*> CreateVariadicComparator(HloInstruction* inst,
bool skip_iota) {
HloTopKInstruction* topk = DynCast<HloTopKInstruction>(inst);
XlaBuilder b(absl::StrCat("comparator_", topk->name()));
std::vector<PrimitiveType> ptypes = {
topk->operand(0)->shape().element_type()};

if (!HasSingleUserReadingOnlyTheValueOutput(inst)) {
if (!skip_iota) {
ptypes.emplace_back(PrimitiveType::S32);
}

Expand All @@ -500,7 +546,8 @@ class TopkDecomposerVisitor : public DfsHloRewriteVisitor {
}

absl::Status DecomposeTopK(HloInstruction* call,
HloComputation* variadic_comparator) {
HloComputation* variadic_comparator,
bool skip_iota) {
HloInstruction* input = call->mutable_operand(0);
Shape iota_shape = input->shape();
iota_shape.set_element_type(S32);
Expand All @@ -509,17 +556,24 @@ class TopkDecomposerVisitor : public DfsHloRewriteVisitor {
std::vector<int64_t> ones(iota_shape.dimensions().size(), 1);
CHECK_NE(variadic_comparator, nullptr);
// If only the topk values are necessary, skip the iota.
if (HasSingleUserReadingOnlyTheValueOutput(call) &&
variadic_comparator->num_parameters() == 2) {
if (skip_iota) {
CHECK_EQ(variadic_comparator->num_parameters(), 2);
HloInstruction* sort = call->AddInstruction(HloInstruction::CreateSort(
input->shape(), sort_dimension, {input}, variadic_comparator,
/*is_stable=*/true));
HloInstruction* slice = call->AddInstruction(HloInstruction::CreateSlice(
call->shape().tuple_shapes(0), sort, zeroes,
call->shape().tuple_shapes(0).dimensions(), ones));
// Create a dummy value for indices since they are unused.
HloInstruction* dummy_indices = call->AddInstruction(
HloInstruction::CreateConstant(LiteralUtil::CreateR0<int32_t>(0)));
dummy_indices = call->AddInstruction(HloInstruction::CreateBroadcast(
call->shape().tuple_shapes(1), dummy_indices, {}));
TF_RETURN_IF_ERROR(ReplaceInstruction(
call->users().front(),
call->AddInstruction(HloInstruction::CreateSlice(
call->shape().tuple_shapes(0), sort, zeroes,
call->shape().tuple_shapes(0).dimensions(), ones))));
call, call->AddInstruction(
HloInstruction::CreateTuple({slice, dummy_indices}))));
} else {
CHECK_EQ(variadic_comparator->num_parameters(), 4);
HloInstruction* iota = call->AddInstruction(HloInstruction::CreateIota(
iota_shape, iota_shape.dimensions().size() - 1));
HloInstruction* sort = call->AddInstruction(HloInstruction::CreateSort(
Expand Down
1 change: 1 addition & 0 deletions xla/service/topk_rewriter_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -427,6 +427,7 @@ ENTRY cluster {
TF_ASSERT_OK_AND_ASSIGN(bool decomposer_changed,
TopkDecomposer().Run(module.get()));
EXPECT_TRUE(decomposer_changed);
TF_ASSERT_OK(TupleSimplifier().Run(module.get()).status());
TF_ASSERT_OK(HloDCE().Run(module.get()).status());
auto root = module->entry_computation()->root_instruction();
HloInstruction* sort;
Expand Down
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