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[XLA:CPU] Fuse reductions in all cases #23933

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Mar 20, 2025
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[XLA:CPU] Fuse reductions in all cases #23933

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17 changes: 0 additions & 17 deletions xla/service/cpu/cpu_instruction_fusion.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -252,23 +252,6 @@ FusionDecision CpuInstructionFusion::ShouldFuse(HloInstruction* consumer,
}
}

// Don't fuse reductions over the major dimensions. These have an efficient
// lowering that's only implemented for the unfused case.
if (consumer->opcode() == HloOpcode::kReduce &&
!absl::c_linear_search(
consumer->dimensions(),
LayoutUtil::Minor(consumer->operand(0)->shape().layout(), 0))) {
return FusionDecision::Forbid(
"Not fusing reductions over major dimensions");
}
if (producer->opcode() == HloOpcode::kReduce &&
!absl::c_linear_search(
producer->dimensions(),
LayoutUtil::Minor(producer->operand(0)->shape().layout(), 0))) {
return FusionDecision::Forbid(
"Not fusing reductions over major dimensions");
}

if (consumer->IsLoopFusion()) {
VLOG(2) << "Fusing: consumer is a fusion node.";
return FusionDecision::Allow();
Expand Down
7 changes: 3 additions & 4 deletions xla/service/cpu/cpu_instruction_fusion_test.cc
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Expand Up @@ -881,7 +881,7 @@ INSTANTIATE_TEST_SUITE_P(GatherLoopFusionTestInstantiation,
::testing::ValuesIn(GetGatherLoopFusionTestSpecs()),
GatherLoopFusionTestSpec::Name);

TEST_F(InstructionFusionTest, NoFuseReduceMajor) {
TEST_F(InstructionFusionTest, FuseReduceMajor) {
absl::string_view module_string = R"(
HloModule module

Expand All @@ -904,9 +904,8 @@ ENTRY main {
ParseAndReturnVerifiedModule(module_string));
TF_ASSERT_OK_AND_ASSIGN(bool fused_something,
CpuInstructionFusion().Run(module.get()));
EXPECT_FALSE(fused_something);
EXPECT_THAT(module->entry_computation()->root_instruction(),
Not(op::Fusion()));
EXPECT_TRUE(fused_something);
EXPECT_THAT(module->entry_computation()->root_instruction(), op::Fusion());
}

TEST_F(InstructionFusionTest, FuseReduceMinor) {
Expand Down
20 changes: 12 additions & 8 deletions xla/service/cpu/tests/cpu_fusion_test.cc
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Expand Up @@ -166,15 +166,19 @@ TEST_F(CpuFusionTest, ElementwiseOpChainWithNonfusibleInstruction) {
HloInstruction::CreateParameter(1, r0f32, "y"))));
auto add_f32 = module->AddEmbeddedComputation(embedded_builder.Build());

Window window;
WindowDimension* window_dimension = window.add_dimensions();
window_dimension->set_size(1);
window_dimension->set_stride(1);
window_dimension->set_base_dilation(1);
window_dimension->set_window_dilation(1);
// This is a nop reduction.
auto reduce = builder.AddInstruction(HloInstruction::CreateReduce(
cshape,
builder.AddInstruction(HloInstruction::CreateReshape(
ShapeUtil::MakeShape(F32, {1, 6}), concatenate)),
auto reduce = builder.AddInstruction(HloInstruction::CreateReduceWindow(
cshape, concatenate,
/*init_value=*/
builder.AddInstruction(
HloInstruction::CreateConstant(LiteralUtil::CreateR0<float>(0))),
/*dimensions_to_reduce=*/{0}, add_f32));
window, add_f32));

auto exp = builder.AddInstruction(
HloInstruction::CreateUnary(cshape, HloOpcode::kExp, reduce));
Expand Down Expand Up @@ -208,11 +212,11 @@ TEST_F(CpuFusionTest, ElementwiseOpChainWithNonfusibleInstruction) {

auto fusion_instruction2 = reduce->operand(0);
EXPECT_EQ(HloOpcode::kFusion, fusion_instruction1->opcode());
EXPECT_EQ(HloOpcode::kReshape,
EXPECT_EQ(HloOpcode::kConcatenate,
fusion_instruction2->fused_expression_root()->opcode());
// There should be 5 fused instructions in the second fusion instruction: 1
// parameter, negate, ceil, concat, and reshape.
EXPECT_EQ(5, fusion_instruction2->fused_instruction_count())
// parameter, negate, ceil and concat.
EXPECT_EQ(4, fusion_instruction2->fused_instruction_count())
<< fusion_instruction2->fused_instructions_computation()->ToString();

// Compile and execute the computation.
Expand Down
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