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Propagate loop metadata in double_buffer_loop_unrolling. #25275

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Propagate loop metadata in double_buffer_loop_unrolling. #25275

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6bd4459
Keep loop metadata after loop unrolling.
jreiffers Apr 4, 2025
369ad9b
Fix init update and add tests.
jreiffers Apr 15, 2025
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15 changes: 15 additions & 0 deletions xla/service/gpu/transforms/double_buffer_loop_unrolling.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -519,6 +519,21 @@ absl::StatusOr<bool> DoubleBufferingUnroll(HloInstruction* while_instr,

WhileLoopBackendConfig new_config;
new_config.mutable_known_trip_count()->set_n(exact_trip_count / 2);

// Keep known induction variable metadata if it was present before.
if (config.has_known_induction_variable()) {
*new_config.mutable_known_induction_variable() =
config.known_induction_variable();
}

// Update the init/step metadata if it was present before.
if (config.has_known_init_step()) {
int64_t step = config.known_init_step().step();
new_config.mutable_known_init_step()->set_step(step * 2);
new_config.mutable_known_init_step()->set_init(
config.known_init_step().init() + (peel_one_iteration ? step : 0));
}

TF_RETURN_IF_ERROR(while_instr->set_backend_config(new_config));
return true; // changed
}
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93 changes: 92 additions & 1 deletion xla/service/gpu/transforms/double_buffer_loop_unrolling_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -92,7 +92,9 @@ ENTRY main {
param_0 = f32[] parameter(0)
param_2 = s32[] constant(0)
tuple = (f32[], s32[]) tuple(param_0, param_2)
ROOT while = (f32[], s32[]) while(tuple), condition=condition, body=body, backend_config={"known_trip_count":{"n":"10"}}
ROOT while = (f32[], s32[]) while(tuple), condition=condition, body=body,
backend_config={"known_trip_count":{"n":"10"},
"known_induction_variable":{"tuple_index":"1"}}
})";

TF_ASSERT_OK_AND_ASSIGN(std::unique_ptr<xla::HloModule> module,
Expand All @@ -110,6 +112,7 @@ ENTRY main {
WhileLoopBackendConfig config,
while_instruction->backend_config<WhileLoopBackendConfig>());
EXPECT_EQ(config.known_trip_count().n(), 5);
EXPECT_EQ(config.known_induction_variable().tuple_index(), 1);
EXPECT_EQ(CountInstructions((*while_instruction->while_body()),
HloOpcode::kAllReduceStart),
2);
Expand Down Expand Up @@ -1397,6 +1400,94 @@ ENTRY main {
EXPECT_THAT(double_buffer.Run(module.get()), IsOkAndHolds(false));
}

TEST_F(GpuLoopDoubleBufferTransformerTest, UpdateInitStepOddTripCount) {
absl::string_view kModuleString = R"(
HloModule m
condition {
input_tuple = (s32[]) parameter(0)
iter = s32[] get-tuple-element(input_tuple), index=0
c12 = s32[] constant(12)
ROOT continue = pred[] compare(iter, c12), direction=LT
}

body {
input_tuple = (s32[]) parameter(0)
iter = s32[] get-tuple-element(input_tuple), index=0
c2 = s32[] constant(2)
next_iter = s32[] add(iter, c2)
ROOT output_tuple = (s32[]) tuple(next_iter)
}

ENTRY main {
c3 = s32[] constant(3)
tuple = (s32[]) tuple(c3)
// Values: 3, 5, 7, 9, 11
ROOT while = (s32[]) while(tuple), condition=condition, body=body,
backend_config={"known_trip_count":{"n":"5"},
"known_init_step":{"init":"3","step":"2"}}
})";

TF_ASSERT_OK_AND_ASSIGN(std::unique_ptr<xla::HloModule> module,
ParseAndReturnVerifiedModule(kModuleString));
DoubleBufferLoopUnrolling unroller(
DoubleBufferLoopUnrolling::UnrollStrategy::kDoubleBuffer);
TF_ASSERT_OK_AND_ASSIGN(bool changed, unroller.Run(module.get()));
EXPECT_TRUE(changed);

HloInstruction* while_instruction = hlo_query::GetFirstInstructionWithOpcode(
*module->entry_computation(), HloOpcode::kWhile);
TF_ASSERT_OK_AND_ASSIGN(
WhileLoopBackendConfig config,
while_instruction->backend_config<WhileLoopBackendConfig>());
EXPECT_EQ(config.known_trip_count().n(), 2);
EXPECT_EQ(config.known_init_step().init(), 5);
EXPECT_EQ(config.known_init_step().step(), 4);
}

TEST_F(GpuLoopDoubleBufferTransformerTest, UpdateInitStepEvenTripCount) {
absl::string_view kModuleString = R"(
HloModule m
condition {
input_tuple = (s32[]) parameter(0)
iter = s32[] get-tuple-element(input_tuple), index=0
c14 = s32[] constant(14)
ROOT continue = pred[] compare(iter, c14), direction=LT
}

body {
input_tuple = (s32[]) parameter(0)
iter = s32[] get-tuple-element(input_tuple), index=0
c2 = s32[] constant(2)
next_iter = s32[] add(iter, c2)
ROOT output_tuple = (s32[]) tuple(next_iter)
}

ENTRY main {
c3 = s32[] constant(3)
tuple = (s32[]) tuple(c3)
// Values: 3, 5, 7, 9, 11, 13
ROOT while = (s32[]) while(tuple), condition=condition, body=body,
backend_config={"known_trip_count":{"n":"6"},
"known_init_step":{"init":"3","step":"2"}}
})";

TF_ASSERT_OK_AND_ASSIGN(std::unique_ptr<xla::HloModule> module,
ParseAndReturnVerifiedModule(kModuleString));
DoubleBufferLoopUnrolling unroller(
DoubleBufferLoopUnrolling::UnrollStrategy::kDoubleBuffer);
TF_ASSERT_OK_AND_ASSIGN(bool changed, unroller.Run(module.get()));
EXPECT_TRUE(changed);

HloInstruction* while_instruction = hlo_query::GetFirstInstructionWithOpcode(
*module->entry_computation(), HloOpcode::kWhile);
TF_ASSERT_OK_AND_ASSIGN(
WhileLoopBackendConfig config,
while_instruction->backend_config<WhileLoopBackendConfig>());
EXPECT_EQ(config.known_trip_count().n(), 3);
EXPECT_EQ(config.known_init_step().init(), 3);
EXPECT_EQ(config.known_init_step().step(), 4);
}

} // namespace
} // namespace gpu
} // namespace xla