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[codegen 1.5] Fix register cleanup pass if all writes are no-ops #3691

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Merged
copybara-service merged 1 commit into from
Jan 16, 2026
+13 −7
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[codegen 1.5] Fix register cleanup pass if all writes are no-ops #3691

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20 changes: 13 additions & 7 deletions xls/codegen_v_1_5/register_cleanup_pass.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -75,7 +75,9 @@ absl::StatusOr<bool> RegisterCleanupPass::RemoveTrivialLoadEnables(

absl::StatusOr<bool> RegisterCleanupPass::RemoveImpossibleWrites(
Block* block, QueryEngine& query_engine) const {
for (Register* reg : block->GetRegisters()) {
std::vector<Register*> registers(block->GetRegisters().begin(),
block->GetRegisters().end());
for (Register* reg : registers) {
XLS_ASSIGN_OR_RETURN(std::vector<RegisterWrite*> writes,
GetRegisterWrites(block, reg));
for (RegisterWrite* write : writes) {
Expand All @@ -89,19 +91,23 @@ absl::StatusOr<bool> RegisterCleanupPass::RemoveImpossibleWrites(
}

XLS_ASSIGN_OR_RETURN(writes, GetRegisterWrites(block, reg));
if (writes.empty() || (reg->reset_value().has_value() &&
absl::c_all_of(writes, [&](RegisterWrite* write) {
return query_engine.KnownValue(write->data()) ==
*reg->reset_value();
}))) {
if (absl::c_all_of(writes, [&](RegisterWrite* write) {
return reg->reset_value().has_value() &&
query_engine.KnownValue(write->data()) == *reg->reset_value();
})) {
// Nothing can ever write a new value to this register, so it's equivalent
// to its reset value.
// to its reset value. We can remove the register & all ops.
XLS_ASSIGN_OR_RETURN(RegisterRead * read, block->GetRegisterRead(reg));
XLS_RETURN_IF_ERROR(
read->ReplaceUsesWithNew<Literal>(
reg->reset_value().value_or(ZeroOfType(reg->type())))
.status());
XLS_RETURN_IF_ERROR(block->RemoveNode(read));
for (RegisterWrite* write : writes) {
XLS_RETURN_IF_ERROR(
write->ReplaceUsesWithNew<Literal>(Value::Tuple({})).status());
XLS_RETURN_IF_ERROR(block->RemoveNode(write));
}
XLS_RETURN_IF_ERROR(block->RemoveRegister(reg));
}
}
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