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feat(perf): Remove last store in return block if last load is before that store #5910

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Merged
TomAFrench merged 3 commits into from
Sep 4, 2024
Merged

feat(perf): Remove last store in return block if last load is before that store #5910

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b750da1
remove last store even if last stores exist in last block
vezenovm Sep 3, 2024
08f129f
Update compiler/noirc_evaluator/src/ssa/opt/mem2reg.rs
TomAFrench Sep 4, 2024
71ba2f8
chore: fix semicolons
TomAFrench Sep 4, 2024
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32 changes: 27 additions & 5 deletions compiler/noirc_evaluator/src/ssa/opt/mem2reg.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -116,7 +116,7 @@ struct PerFunctionContext<'f> {

/// Track a value's last load across all blocks.
/// If a value is not used in anymore loads we can remove the last store to that value.
last_loads: HashMap<ValueId, InstructionId>,
last_loads: HashMap<ValueId, (InstructionId, BasicBlockId)>,
}

impl<'f> PerFunctionContext<'f> {
Expand Down Expand Up @@ -152,9 +152,31 @@ impl<'f> PerFunctionContext<'f> {
// This rule does not apply to reference parameters, which we must also check for before removing these stores.
for (block_id, block) in self.blocks.iter() {
let block_params = self.inserter.function.dfg.block_parameters(*block_id);
for (value, store_instruction) in block.last_stores.iter() {
let is_reference_param = block_params.contains(value);
if self.last_loads.get(value).is_none() && !is_reference_param {
for (store_address, store_instruction) in block.last_stores.iter() {
let is_reference_param = block_params.contains(store_address);
let terminator = self.inserter.function.dfg[*block_id].unwrap_terminator();

let is_return = matches!(terminator, TerminatorInstruction::Return { .. });
let remove_load = if is_return {
// Determine whether the last store is used in the return value
let mut is_return_value = false;
terminator.for_each_value(|return_value| {
is_return_value = return_value == *store_address || is_return_value;
});

// If the last load of a store is not part of the block with a return terminator,
// we can safely remove this store.
let last_load_not_in_return = self
.last_loads
.get(store_address)
.map(|(_, last_load_block)| *last_load_block != *block_id)
.unwrap_or(true);
!is_return_value && last_load_not_in_return
} else {
self.last_loads.get(store_address).is_none()
};

if remove_load && !is_reference_param {
self.instructions_to_remove.insert(*store_instruction);
}
}
Expand Down Expand Up @@ -259,7 +281,7 @@ impl<'f> PerFunctionContext<'f> {
} else {
references.mark_value_used(address, self.inserter.function);

self.last_loads.insert(address, instruction);
self.last_loads.insert(address, (instruction, block_id));
}
}
Instruction::Store { address, value } => {
Expand Down