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Do not mark literals as direct-in, as this can make loop feedbacks un-decomposable. #3750

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Merged
copybara-service merged 1 commit into from
Jan 28, 2026
+26 −17
Merged

Do not mark literals as direct-in, as this can make loop feedbacks un-decomposable. #3750

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14 changes: 11 additions & 3 deletions xls/contrib/xlscc/continuations.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -280,14 +280,18 @@ OptimizationContext::GetSourcesSetTreeNodeInfoForFunction(

absl::StatusOr<bool> OptimizationContext::CheckNodeSourcesInSet(
xls::FunctionBase* in_function, xls::Node* node,
absl::flat_hash_set<const xls::Param*> sources_set) {
absl::flat_hash_set<const xls::Param*> sources_set,
bool allow_empty_sources_result) {
// Save lazy node analysis for each function for efficiency
XLS_ASSIGN_OR_RETURN(SourcesSetNodeInfo * info,
GetSourcesSetNodeInfoForFunction(in_function));

ParamSet param_sources = info->GetSingleInfoForNode(node);

// No param sources will return true
if (param_sources.empty()) {
return allow_empty_sources_result;
}

bool all_in_set = true;
for (const xls::Param* param : param_sources) {
CHECK_EQ(param->function_base(), in_function);
Expand Down Expand Up @@ -2090,10 +2094,14 @@ absl::Status Translator::MarkDirectIns(GeneratedFunction& func,
continue;
}

// Don't count things that don't actually use any direct-ins as direct-in,
// for example literals. Allow literal substitution pass to prevent these
// from being stored in state elements.
XLS_ASSIGN_OR_RETURN(
continuation_out.direct_in,
context.CheckNodeSourcesInSet(
slice.function, continuation_out.output_node, direct_in_sources));
slice.function, continuation_out.output_node, direct_in_sources,
/*allow_empty_sources_result=*/false));
}

first_slice = false;
Expand Down
3 changes: 3 additions & 0 deletions xls/contrib/xlscc/generate_fsm.cc
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Expand Up @@ -539,6 +539,9 @@ absl::Status NewFSMGenerator::LayoutValuesToSaveForNewFSMStates(
if (key.value->direct_in) {
continue;
}
if (key.value->literal.has_value()) {
continue;
}
state.values_to_save.insert(key.value);
}
}
Expand Down
3 changes: 2 additions & 1 deletion xls/contrib/xlscc/translator_types.h
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Expand Up @@ -1304,7 +1304,8 @@ class OptimizationContext {

absl::StatusOr<bool> CheckNodeSourcesInSet(
xls::FunctionBase* in_function, xls::Node* node,
absl::flat_hash_set<const xls::Param*> sources_set);
absl::flat_hash_set<const xls::Param*> sources_set,
bool allow_empty_sources_result = true);

private:
absl::flat_hash_map<xls::FunctionBase*, std::unique_ptr<SourcesSetNodeInfo>>
Expand Down
23 changes: 10 additions & 13 deletions xls/contrib/xlscc/unit_tests/continuations_test.cc
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Expand Up @@ -1060,9 +1060,9 @@ TEST_F(ContinuationsTest, PipelinedLoopBackwardsPropagation) {

EXPECT_EQ(SliceInputsDeclCount(third_slice, "i"), 2);
EXPECT_EQ(SliceInputsDeclCount(third_slice, "a"), 2);
EXPECT_TRUE(SliceInputsDecl(third_slice, "a", /*direct_in=*/true,
EXPECT_TRUE(SliceInputsDecl(third_slice, "a", /*direct_in=*/false,
/*is_feedback=*/false, /*func=*/func));
EXPECT_TRUE(SliceInputsDecl(third_slice, "i", /*direct_in=*/true,
EXPECT_TRUE(SliceInputsDecl(third_slice, "i", /*direct_in=*/false,
/*is_feedback=*/false, /*func=*/func));
EXPECT_TRUE(SliceInputsDecl(third_slice, "a", /*direct_in=*/false,
/*is_feedback=*/true, /*func=*/func));
Expand Down Expand Up @@ -1322,9 +1322,9 @@ TEST_F(ContinuationsTest, PipelinedLoopNothingOutside) {
++slice_it;
const xlscc::GeneratedFunctionSlice& third_slice = *slice_it;

EXPECT_TRUE(SliceOutputsDecl(first_slice, "a", /*direct_in*/ true));
EXPECT_TRUE(SliceOutputsDecl(first_slice, "i", /*direct_in*/ true));
EXPECT_TRUE(SliceOutputsDecl(first_slice, "c", /*direct_in*/ true));
EXPECT_TRUE(SliceOutputsDecl(first_slice, "a", /*direct_in*/ false));
EXPECT_TRUE(SliceOutputsDecl(first_slice, "i", /*direct_in*/ false));
EXPECT_TRUE(SliceOutputsDecl(first_slice, "c", /*direct_in*/ false));

EXPECT_FALSE(SliceInputsDecl(second_slice, "i"));
EXPECT_FALSE(SliceInputsDecl(second_slice, "a"));
Expand Down Expand Up @@ -2131,8 +2131,8 @@ TEST_F(ContinuationsTest, PipelinedLoopBackwardsPropagationInSubroutine) {
const xlscc::GeneratedFunctionSlice& fourth_slice = *slice_it;

EXPECT_TRUE(SliceOutputsDecl(second_slice, "ctrl"));
EXPECT_TRUE(SliceOutputsDecl(second_slice, "a", /*direct_in=*/true));
EXPECT_TRUE(SliceOutputsDecl(second_slice, "i", /*direct_in=*/true));
EXPECT_TRUE(SliceOutputsDecl(second_slice, "a", /*direct_in=*/false));
EXPECT_TRUE(SliceOutputsDecl(second_slice, "i", /*direct_in=*/false));

EXPECT_EQ(SliceInputsDeclCount(third_slice, "i"), 2);
EXPECT_EQ(SliceInputsDeclCount(third_slice, "a"), 2);
Expand Down Expand Up @@ -2420,15 +2420,12 @@ TEST_F(ContinuationsTest, ContinuationLiteralDecomposed) {
++slice_it;
const xlscc::GeneratedFunctionSlice& second_slice = *slice_it;

// TODO(seanhaskell): Calling literals direct-in is stopped them from being
// decomposed. This makes loops less efficient

EXPECT_TRUE(SliceInputsDecl(second_slice, "x", /*direct_in=*/true,
EXPECT_TRUE(SliceInputsDecl(second_slice, "x", /*direct_in=*/false,
/*is_feedback=*/false, /*func=*/func,
/*decl_index=*/-1));
/*decl_index=*/0));
EXPECT_TRUE(SliceInputsDecl(second_slice, "x", /*direct_in=*/false,
/*is_feedback=*/true, /*func=*/func,
/*decl_index=*/-1));
/*decl_index=*/0));
}

TEST_F(ContinuationsTest, StaticThisDecomposed) {
Expand Down
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