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Drop codegen support of gather (but not takeAlongAxis) #5907

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Merged
naoyam merged 4 commits into from
Feb 4, 2026
+18 −136
Merged

Drop codegen support of gather (but not takeAlongAxis) #5907

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c4b50f5
cleanup
naoyam Jan 31, 2026
a0839c7
Use takeAlongAxis instead of gather
naoyam Feb 3, 2026
617f257
Merge remote-tracking branch 'origin/main' into gather
naoyam Feb 3, 2026
475f7b3
Remove unsupported tests
naoyam Feb 4, 2026
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2 changes: 2 additions & 0 deletions csrc/id_model/indexing.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,8 @@
namespace nvfuser {

TensorIndexer::TensorIndexer(IdModel& id_model) : id_model_(id_model) {
NVF_ERROR(isSupported(id_model.fusion()));

buildLoopIndexMap();

if (isDebugDumpEnabled(DebugDumpOption::IndexingVerbose)) {
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1 change: 1 addition & 0 deletions csrc/scheduler/expr_eval_sched.cpp
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Expand Up @@ -64,6 +64,7 @@ bool ExprEvalScheduler::canScheduleCompileTime(Fusion* fusion) {
// TODO: remove IndexPutAccumulateOp
if (exprs.front()
->isOneOf<
GatherOp,
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@greptile-apps greptile-apps bot Feb 4, 2026

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Adding GatherOp here routes ALL gather operations (including exact-sized takeAlongAxis) to ExprEval/ATen evaluation. The PR title says "but not takeAlongAxis", suggesting exact gather should still be compiled. Consider filtering to only accept non-exact gather:

Suggested change
GatherOp,
!exprs.front()->isa<GatherOp>() || !exprs.front()->as<GatherOp>()->exactSizes() ? GatherOp : void,

Or clarify if the performance regression for takeAlongAxis is intentional.

ScatterOp,
SdpaFwdOp,
SdpaBwdOp,
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10 changes: 10 additions & 0 deletions csrc/scheduler/registry.cpp
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Expand Up @@ -64,6 +64,16 @@ bool checkCanSchedule(Fusion* fusion, SchedulerType scheduler_type) {
return false;
}

// Support of non-exact gather was dropped when the legacy indexer was
// deprecated
if (std::ranges::any_of(
ir_utils::getOpsOfType<GatherOp>(fusion),
[](GatherOp* gather) { return !gather->exactSizes(); })) {
scheduler_debug_utils::canScheduleRejectReason(
scheduler_type, "Non-exact gather ops");
return false;
}

// Fusions with `MatmulOp, LinearOp, MmaOp` can only be accepted by Matmul
// scheduler.
if (scheduler_type != SchedulerType::Matmul &&
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8 changes: 5 additions & 3 deletions tests/cpp/test_gather.cpp
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Expand Up @@ -582,7 +582,7 @@ TEST_F(GatherTest, TakeAlongAxisIntermediateTensorReduction1) {

validateSegmentation(
executor_cache.getMostRecentKernelRuntime(),
{SchedulerType::Reduction, SchedulerType::PointWise});
{SchedulerType::Reduction, SchedulerType::ExprEval});

testValidate(&fusion, outputs, {t0, t1}, __LINE__, __FILE__);
}
Expand Down Expand Up @@ -1127,7 +1127,8 @@ TEST_F(GatherTest, TakeAlongAxisCrossEntropyLoss) {
}

// Test grouped reduction on IterType::GatherScatter
TEST_F(GatherTest, GatherIterGoupedReduction) {
// Codegen support of non-exact gather dropped
TEST_F(GatherTest, DISABLED_GatherIterGoupedReduction) {
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@jjsjann123 jjsjann123 Feb 3, 2026

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Do we still plan to support this later? wondering if we should remove the tests instead.

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@naoyam naoyam Feb 3, 2026

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Let me remove it

const int max_dim_size = 128;
auto options = at::TensorOptions().dtype(at::kFloat).device(at::kCUDA, 0);
auto options_i = at::TensorOptions().dtype(at::kLong).device(at::kCUDA, 0);
Expand Down Expand Up @@ -1211,7 +1212,8 @@ TEST_F(GatherTest, GatherIterGoupedReduction) {
lparams);
}

TEST_F(GatherTest, SameTvUsedAsLookupAndIndex) {
// Codegen support of non-exact gather dropped
TEST_F(GatherTest, DISABLED_SameTvUsedAsLookupAndIndex) {
auto fusion_ptr = std::make_unique<Fusion>();
Fusion& fusion = *fusion_ptr.get();
FusionGuard fg(&fusion);
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4 changes: 3 additions & 1 deletion tests/cpp/test_persistent_buffer.cpp
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Expand Up @@ -1941,7 +1941,9 @@ TEST_F(PersistentBufferTest, BufferGatherLookupTv) {
auto tv2 = sum(tv1, {1});
auto tv3 = broadcast(tv2, {false, true});
auto tv4 = broadcast(index_tv, {false, true});
auto tv5 = gather(tv0, 1, tv4);
// Use takeAlongAxis rather than gather as codegen does not support
// the latter
auto tv5 = takeAlongAxis(tv0, tv4, 1);
auto tv6 = maybeCastOp(DataType::BFloat16, tv5);
auto tv7 = add(tv3, tv6);
auto tv8 = add(tv1, tv7);
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2 changes: 1 addition & 1 deletion tests/cpp/test_reduction.cpp
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Expand Up @@ -2563,7 +2563,7 @@ TEST_F(ReductionTest, CrossEntropyGatherPattern) {
fusion.addInput(labels);

auto tv2 = broadcast(labels, {false, true});
auto tv3 = gather(log_probs, 1, tv2);
auto tv3 = takeAlongAxis(log_probs, tv2, 1);
auto tv4 = squeeze(tv3, std::vector<bool>({false, true}));

fusion.addOutput(tv4);
Expand Down