diff --git a/compiler/noirc_evaluator/src/ssa/opt/die.rs b/compiler/noirc_evaluator/src/ssa/opt/die.rs index 8d3fa9cc615..e331efda894 100644 --- a/compiler/noirc_evaluator/src/ssa/opt/die.rs +++ b/compiler/noirc_evaluator/src/ssa/opt/die.rs @@ -1,6 +1,6 @@ //! Dead Instruction Elimination (DIE) pass: Removes any instruction without side-effects for //! which the results are unused. -use fxhash::{FxHashMap as HashMap, FxHashSet as HashSet}; +use fxhash::FxHashSet as HashSet; use im::Vector; use noirc_errors::Location; use rayon::iter::{IntoParallelRefMutIterator, ParallelIterator}; @@ -18,8 +18,6 @@ use crate::ssa::{ ssa_gen::{Ssa, SSA_WORD_SIZE}, }; -use super::rc::{pop_rc_for, RcInstruction}; - impl Ssa { /// Performs Dead Instruction Elimination (DIE) to remove any instructions with /// unused results. @@ -108,8 +106,6 @@ impl Context { let instructions_len = block.instructions().len(); - let mut rc_tracker = RcTracker::default(); - // Indexes of instructions that might be out of bounds. // We'll remove those, but before that we'll insert bounds checks for them. let mut possible_index_out_of_bounds_indexes = Vec::new(); @@ -127,8 +123,9 @@ impl Context { .push(instructions_len - instruction_index - 1); } } else { - use Instruction::*; - if matches!(instruction, IncrementRc { .. } | DecrementRc { .. }) { + // We can't remove rc instructions if they're loaded from a reference + // since we'd have no way of knowing whether the reference is still used. + if Self::is_inc_dec_instruction_on_known_array(instruction, &function.dfg) { self.rc_instructions.push((*instruction_id, block_id)); } else { instruction.for_each_value(|value| { @@ -136,13 +133,8 @@ impl Context { }); } } - - rc_tracker.track_inc_rcs_to_remove(*instruction_id, function); } - self.instructions_to_remove.extend(rc_tracker.get_non_mutated_arrays()); - self.instructions_to_remove.extend(rc_tracker.rc_pairs_to_remove); - // If there are some instructions that might trigger an out of bounds error, // first add constrain checks. Then run the DIE pass again, which will remove those // but leave the constrains (any any value needed by those constrains) @@ -337,6 +329,28 @@ impl Context { inserted_check } + + /// True if this is a `Instruction::IncrementRc` or `Instruction::DecrementRc` + /// operating on an array directly from a `Instruction::MakeArray` or an + /// intrinsic known to return a fresh array. + fn is_inc_dec_instruction_on_known_array( + instruction: &Instruction, + dfg: &DataFlowGraph, + ) -> bool { + use Instruction::*; + if let IncrementRc { value } | DecrementRc { value } = instruction { + if let Value::Instruction { instruction, .. } = &dfg[*value] { + return match &dfg[*instruction] { + MakeArray { .. } => true, + Call { func, .. } => { + matches!(&dfg[*func], Value::Intrinsic(_) | Value::ForeignFunction(_)) + } + _ => false, + }; + } + } + false + } } fn instruction_might_result_in_out_of_bounds( @@ -499,103 +513,6 @@ fn apply_side_effects( (lhs, rhs) } -#[derive(Default)] -struct RcTracker { - // We can track IncrementRc instructions per block to determine whether they are useless. - // IncrementRc and DecrementRc instructions are normally side effectual instructions, but we remove - // them if their value is not used anywhere in the function. However, even when their value is used, their existence - // is pointless logic if there is no array set between the increment and the decrement of the reference counter. - // We track per block whether an IncrementRc instruction has a paired DecrementRc instruction - // with the same value but no array set in between. - // If we see an inc/dec RC pair within a block we can safely remove both instructions. - rcs_with_possible_pairs: HashMap>, - rc_pairs_to_remove: HashSet, - // We also separately track all IncrementRc instructions and all arrays which have been mutably borrowed. - // If an array has not been mutably borrowed we can then safely remove all IncrementRc instructions on that array. - inc_rcs: HashMap>, - mut_borrowed_arrays: HashSet, - // The SSA often creates patterns where after simplifications we end up with repeat - // IncrementRc instructions on the same value. We track whether the previous instruction was an IncrementRc, - // and if the current instruction is also an IncrementRc on the same value we remove the current instruction. - // `None` if the previous instruction was anything other than an IncrementRc - previous_inc_rc: Option, -} - -impl RcTracker { - fn track_inc_rcs_to_remove(&mut self, instruction_id: InstructionId, function: &Function) { - let instruction = &function.dfg[instruction_id]; - - if let Instruction::IncrementRc { value } = instruction { - if let Some(previous_value) = self.previous_inc_rc { - if previous_value == *value { - self.rc_pairs_to_remove.insert(instruction_id); - } - } - self.previous_inc_rc = Some(*value); - } else { - self.previous_inc_rc = None; - } - - // DIE loops over a block in reverse order, so we insert an RC instruction for possible removal - // when we see a DecrementRc and check whether it was possibly mutated when we see an IncrementRc. - match instruction { - Instruction::IncrementRc { value } => { - if let Some(inc_rc) = - pop_rc_for(*value, function, &mut self.rcs_with_possible_pairs) - { - if !inc_rc.possibly_mutated { - self.rc_pairs_to_remove.insert(inc_rc.id); - self.rc_pairs_to_remove.insert(instruction_id); - } - } - - self.inc_rcs.entry(*value).or_default().insert(instruction_id); - } - Instruction::DecrementRc { value } => { - let typ = function.dfg.type_of_value(*value); - - // We assume arrays aren't mutated until we find an array_set - let dec_rc = - RcInstruction { id: instruction_id, array: *value, possibly_mutated: false }; - self.rcs_with_possible_pairs.entry(typ).or_default().push(dec_rc); - } - Instruction::ArraySet { array, .. } => { - let typ = function.dfg.type_of_value(*array); - if let Some(dec_rcs) = self.rcs_with_possible_pairs.get_mut(&typ) { - for dec_rc in dec_rcs { - dec_rc.possibly_mutated = true; - } - } - - self.mut_borrowed_arrays.insert(*array); - } - Instruction::Store { value, .. } => { - // We are very conservative and say that any store of an array value means it has the potential - // to be mutated. This is done due to the tracking of mutable borrows still being per block. - let typ = function.dfg.type_of_value(*value); - if matches!(&typ, Type::Array(..) | Type::Slice(..)) { - self.mut_borrowed_arrays.insert(*value); - } - } - _ => {} - } - } - - fn get_non_mutated_arrays(&self) -> HashSet { - self.inc_rcs - .keys() - .filter_map(|value| { - if !self.mut_borrowed_arrays.contains(value) { - Some(&self.inc_rcs[value]) - } else { - None - } - }) - .flatten() - .copied() - .collect() - } -} #[cfg(test)] mod test { use std::sync::Arc; @@ -604,7 +521,7 @@ mod test { use crate::ssa::{ function_builder::FunctionBuilder, - ir::{instruction::Instruction, map::Id, types::Type}, + ir::{map::Id, types::Type}, opt::assert_normalized_ssa_equals, Ssa, }; @@ -676,30 +593,6 @@ mod test { assert_normalized_ssa_equals(ssa, expected); } - #[test] - fn remove_useless_paired_rcs_even_when_used() { - let src = " - acir(inline) fn main f0 { - b0(v0: [Field; 2]): - inc_rc v0 - v2 = array_get v0, index u32 0 -> Field - dec_rc v0 - return v2 - } - "; - let ssa = Ssa::from_str(src).unwrap(); - - let expected = " - acir(inline) fn main f0 { - b0(v0: [Field; 2]): - v2 = array_get v0, index u32 0 -> Field - return v2 - } - "; - let ssa = ssa.dead_instruction_elimination(); - assert_normalized_ssa_equals(ssa, expected); - } - #[test] fn keep_paired_rcs_with_array_set() { let src = " @@ -770,92 +663,23 @@ mod test { } #[test] - fn keep_inc_rc_on_borrowed_array_set() { - // acir(inline) fn main f0 { - // b0(v0: [u32; 2]): - // inc_rc v0 - // v3 = array_set v0, index u32 0, value u32 1 - // inc_rc v0 - // inc_rc v0 - // inc_rc v0 - // v4 = array_get v3, index u32 1 - // return v4 - // } - let main_id = Id::test_new(0); - - // Compiling main - let mut builder = FunctionBuilder::new("main".into(), main_id); - let array_type = Type::Array(Arc::new(vec![Type::unsigned(32)]), 2); - let v0 = builder.add_parameter(array_type.clone()); - builder.increment_array_reference_count(v0); - let zero = builder.numeric_constant(0u128, Type::unsigned(32)); - let one = builder.numeric_constant(1u128, Type::unsigned(32)); - let v3 = builder.insert_array_set(v0, zero, one); - builder.increment_array_reference_count(v0); - builder.increment_array_reference_count(v0); - builder.increment_array_reference_count(v0); - - let v4 = builder.insert_array_get(v3, one, Type::unsigned(32)); - - builder.terminate_with_return(vec![v4]); - - let ssa = builder.finish(); - let main = ssa.main(); - - // The instruction count never includes the terminator instruction - assert_eq!(main.dfg[main.entry_block()].instructions().len(), 6); - - // We expect the output to be unchanged - // Expected output: - // - // acir(inline) fn main f0 { - // b0(v0: [u32; 2]): - // inc_rc v0 - // v3 = array_set v0, index u32 0, value u32 1 - // inc_rc v0 - // v4 = array_get v3, index u32 1 - // return v4 - // } - let ssa = ssa.dead_instruction_elimination(); - let main = ssa.main(); - - let instructions = main.dfg[main.entry_block()].instructions(); - // We expect only the repeated inc_rc instructions to be collapsed into a single inc_rc. - assert_eq!(instructions.len(), 4); - - assert!(matches!(&main.dfg[instructions[0]], Instruction::IncrementRc { .. })); - assert!(matches!(&main.dfg[instructions[1]], Instruction::ArraySet { .. })); - assert!(matches!(&main.dfg[instructions[2]], Instruction::IncrementRc { .. })); - assert!(matches!(&main.dfg[instructions[3]], Instruction::ArrayGet { .. })); - } - - #[test] - fn remove_inc_rcs_that_are_never_mutably_borrowed() { + fn does_not_remove_inc_or_dec_rc_of_if_they_are_loaded_from_a_reference() { let src = " - acir(inline) fn main f0 { - b0(v0: [Field; 2]): - inc_rc v0 - inc_rc v0 - inc_rc v0 - v2 = array_get v0, index u32 0 -> Field - inc_rc v0 - return v2 + brillig(inline) fn borrow_mut f0 { + b0(v0: &mut [Field; 3]): + v1 = load v0 -> [Field; 3] + inc_rc v1 // this one shouldn't be removed + v2 = load v0 -> [Field; 3] + inc_rc v2 // this one shouldn't be removed + v3 = load v0 -> [Field; 3] + v6 = array_set v3, index u32 0, value Field 5 + store v6 at v0 + dec_rc v6 + return } "; let ssa = Ssa::from_str(src).unwrap(); - let main = ssa.main(); - - // The instruction count never includes the terminator instruction - assert_eq!(main.dfg[main.entry_block()].instructions().len(), 5); - - let expected = " - acir(inline) fn main f0 { - b0(v0: [Field; 2]): - v2 = array_get v0, index u32 0 -> Field - return v2 - } - "; let ssa = ssa.dead_instruction_elimination(); - assert_normalized_ssa_equals(ssa, expected); + assert_normalized_ssa_equals(ssa, src); } } diff --git a/test_programs/execution_success/reference_counts/src/main.nr b/test_programs/execution_success/reference_counts/src/main.nr index 7ab7de893fa..86b5c812472 100644 --- a/test_programs/execution_success/reference_counts/src/main.nr +++ b/test_programs/execution_success/reference_counts/src/main.nr @@ -1,6 +1,6 @@ fn main() { let mut array = [0, 1, 2]; - assert_refcount(array, 1); + assert_refcount(array, 2); borrow(array, std::mem::array_refcount(array)); borrow_mut(&mut array, std::mem::array_refcount(array)); @@ -13,13 +13,13 @@ fn borrow(array: [Field; 3], rc_before_call: u32) { } fn borrow_mut(array: &mut [Field; 3], rc_before_call: u32) { - assert_refcount(*array, rc_before_call + 0); // Issue! This should be rc_before_call + 1 + assert_refcount(*array, rc_before_call + 1); array[0] = 5; println(array[0]); } fn copy_mut(mut array: [Field; 3], rc_before_call: u32) { - assert_refcount(array, rc_before_call + 0); // Issue! This should be rc_before_call + 1 + assert_refcount(array, rc_before_call + 1); array[0] = 6; println(array[0]); }