feat: refactor SSA passes to run on individual functions (#6072)

# Description

## Problem\*

~Resolves #5839

Refactors passes that only act on individual functions into `impl
Function { fn do_pass() }`

## Summary\*



## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_evaluator/src/ssa/opt/array_set.rs

@@ -2,6 +2,7 @@ use crate::ssa::{
     ir::{
         basic_block::BasicBlockId,
         dfg::DataFlowGraph,
+        function::Function,
         instruction::{Instruction, InstructionId, TerminatorInstruction},
         types::Type::{Array, Slice},
         value::ValueId,
@@ -17,32 +18,38 @@ impl Ssa {
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn array_set_optimization(mut self) -> Self {
         for func in self.functions.values_mut() {
-            let reachable_blocks = func.reachable_blocks();
-
-            if !func.runtime().is_entry_point() {
-                assert_eq!(reachable_blocks.len(), 1, "Expected there to be 1 block remaining in Acir function for array_set optimization");
-            }
-            let mut array_to_last_use = HashMap::default();
-            let mut instructions_to_update = HashSet::default();
-            let mut arrays_from_load = HashSet::default();
-
-            for block in reachable_blocks.iter() {
-                analyze_last_uses(
-                    &func.dfg,
-                    *block,
-                    &mut array_to_last_use,
-                    &mut instructions_to_update,
-                    &mut arrays_from_load,
-                );
-            }
-            for block in reachable_blocks {
-                make_mutable(&mut func.dfg, block, &instructions_to_update);
-            }
+            func.array_set_optimization();
         }
         self
     }
 }
 
+impl Function {
+    pub(crate) fn array_set_optimization(&mut self) {
+        let reachable_blocks = self.reachable_blocks();
+
+        if !self.runtime().is_entry_point() {
+            assert_eq!(reachable_blocks.len(), 1, "Expected there to be 1 block remaining in Acir function for array_set optimization");
+        }
+        let mut array_to_last_use = HashMap::default();
+        let mut instructions_to_update = HashSet::default();
+        let mut arrays_from_load = HashSet::default();
+
+        for block in reachable_blocks.iter() {
+            analyze_last_uses(
+                &self.dfg,
+                *block,
+                &mut array_to_last_use,
+                &mut instructions_to_update,
+                &mut arrays_from_load,
+            );
+        }
+        for block in reachable_blocks {
+            make_mutable(&mut self.dfg, block, &instructions_to_update);
+        }
+    }
+}
+
 /// Builds the set of ArraySet instructions that can be made mutable
 /// because their input value is unused elsewhere afterward.
 fn analyze_last_uses(

compiler/noirc_evaluator/src/ssa/opt/as_slice_length.rs

@@ -20,13 +20,19 @@ impl Ssa {
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn as_slice_optimization(mut self) -> Self {
         for func in self.functions.values_mut() {
-            let known_slice_lengths = known_slice_lengths(func);
-            replace_known_slice_lengths(func, known_slice_lengths);
+            func.as_slice_optimization();
         }
         self
     }
 }
 
+impl Function {
+    pub(crate) fn as_slice_optimization(&mut self) {
+        let known_slice_lengths = known_slice_lengths(self);
+        replace_known_slice_lengths(self, known_slice_lengths);
+    }
+}
+
 fn known_slice_lengths(func: &Function) -> HashMap<InstructionId, usize> {
     let mut known_slice_lengths = HashMap::default();
     for block_id in func.reachable_blocks() {

compiler/noirc_evaluator/src/ssa/opt/assert_constant.rs

@@ -26,22 +26,31 @@ impl Ssa {
         mut self,
     ) -> Result<Ssa, RuntimeError> {
         for function in self.functions.values_mut() {
-            for block in function.reachable_blocks() {
-                // Unfortunately we can't just use instructions.retain(...) here since
-                // check_instruction can also return an error
-                let instructions = function.dfg[block].take_instructions();
-                let mut filtered_instructions = Vec::with_capacity(instructions.len());
+            function.evaluate_static_assert_and_assert_constant()?;
+        }
+        Ok(self)
+    }
+}
 
-                for instruction in instructions {
-                    if check_instruction(function, instruction)? {
-                        filtered_instructions.push(instruction);
-                    }
-                }
+impl Function {
+    pub(crate) fn evaluate_static_assert_and_assert_constant(
+        &mut self,
+    ) -> Result<(), RuntimeError> {
+        for block in self.reachable_blocks() {
+            // Unfortunately we can't just use instructions.retain(...) here since
+            // check_instruction can also return an error
+            let instructions = self.dfg[block].take_instructions();
+            let mut filtered_instructions = Vec::with_capacity(instructions.len());
 
-                *function.dfg[block].instructions_mut() = filtered_instructions;
+            for instruction in instructions {
+                if check_instruction(self, instruction)? {
+                    filtered_instructions.push(instruction);
+                }
             }
+
+            *self.dfg[block].instructions_mut() = filtered_instructions;
         }
-        Ok(self)
+        Ok(())
     }
 }
 

compiler/noirc_evaluator/src/ssa/opt/constant_folding.rs

@@ -44,7 +44,7 @@ impl Ssa {
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn fold_constants(mut self) -> Ssa {
         for function in self.functions.values_mut() {
-            constant_fold(function, false);
+            function.constant_fold(false);
         }
         self
     }
@@ -57,25 +57,27 @@ impl Ssa {
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn fold_constants_using_constraints(mut self) -> Ssa {
         for function in self.functions.values_mut() {
-            constant_fold(function, true);
+            function.constant_fold(true);
         }
         self
     }
 }
 
-/// The structure of this pass is simple:
-/// Go through each block and re-insert all instructions.
-fn constant_fold(function: &mut Function, use_constraint_info: bool) {
-    let mut context = Context { use_constraint_info, ..Default::default() };
-    context.block_queue.push(function.entry_block());
+impl Function {
+    /// The structure of this pass is simple:
+    /// Go through each block and re-insert all instructions.
+    pub(crate) fn constant_fold(&mut self, use_constraint_info: bool) {
+        let mut context = Context { use_constraint_info, ..Default::default() };
+        context.block_queue.push(self.entry_block());
 
-    while let Some(block) = context.block_queue.pop() {
-        if context.visited_blocks.contains(&block) {
-            continue;
-        }
+        while let Some(block) = context.block_queue.pop() {
+            if context.visited_blocks.contains(&block) {
+                continue;
+            }
 
-        context.visited_blocks.insert(block);
-        context.fold_constants_in_block(function, block);
+            context.visited_blocks.insert(block);
+            context.fold_constants_in_block(self, block);
+        }
     }
 }
 

compiler/noirc_evaluator/src/ssa/opt/die.rs

@@ -25,44 +25,46 @@ impl Ssa {
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn dead_instruction_elimination(mut self) -> Ssa {
         for function in self.functions.values_mut() {
-            dead_instruction_elimination(function, true);
+            function.dead_instruction_elimination(true);
         }
         self
     }
 }
 
-/// Removes any unused instructions in the reachable blocks of the given function.
-///
-/// The blocks of the function are iterated in post order, such that any blocks containing
-/// instructions that reference results from an instruction in another block are evaluated first.
-/// If we did not iterate blocks in this order we could not safely say whether or not the results
-/// of its instructions are needed elsewhere.
-fn dead_instruction_elimination(function: &mut Function, insert_out_of_bounds_checks: bool) {
-    let mut context = Context::default();
-    for call_data in &function.dfg.data_bus.call_data {
-        context.mark_used_instruction_results(&function.dfg, call_data.array_id);
-    }
+impl Function {
+    /// Removes any unused instructions in the reachable blocks of the given function.
+    ///
+    /// The blocks of the function are iterated in post order, such that any blocks containing
+    /// instructions that reference results from an instruction in another block are evaluated first.
+    /// If we did not iterate blocks in this order we could not safely say whether or not the results
+    /// of its instructions are needed elsewhere.
+    pub(crate) fn dead_instruction_elimination(&mut self, insert_out_of_bounds_checks: bool) {
+        let mut context = Context::default();
+        for call_data in &self.dfg.data_bus.call_data {
+            context.mark_used_instruction_results(&self.dfg, call_data.array_id);
+        }
 
-    let mut inserted_out_of_bounds_checks = false;
+        let mut inserted_out_of_bounds_checks = false;
 
-    let blocks = PostOrder::with_function(function);
-    for block in blocks.as_slice() {
-        inserted_out_of_bounds_checks |= context.remove_unused_instructions_in_block(
-            function,
-            *block,
-            insert_out_of_bounds_checks,
-        );
-    }
+        let blocks = PostOrder::with_function(self);
+        for block in blocks.as_slice() {
+            inserted_out_of_bounds_checks |= context.remove_unused_instructions_in_block(
+                self,
+                *block,
+                insert_out_of_bounds_checks,
+            );
+        }
 
-    // If we inserted out of bounds check, let's run the pass again with those new
-    // instructions (we don't want to remove those checks, or instructions that are
-    // dependencies of those checks)
-    if inserted_out_of_bounds_checks {
-        dead_instruction_elimination(function, false);
-        return;
-    }
+        // If we inserted out of bounds check, let's run the pass again with those new
+        // instructions (we don't want to remove those checks, or instructions that are
+        // dependencies of those checks)
+        if inserted_out_of_bounds_checks {
+            self.dead_instruction_elimination(false);
+            return;
+        }
 
-    context.remove_rc_instructions(&mut function.dfg);
+        context.remove_rc_instructions(&mut self.dfg);
+    }
 }
 
 /// Per function context for tracking unused values and which instructions to remove.

compiler/noirc_evaluator/src/ssa/opt/mem2reg.rs

@@ -123,16 +123,22 @@ impl Ssa {
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn mem2reg(mut self) -> Ssa {
         for function in self.functions.values_mut() {
-            let mut context = PerFunctionContext::new(function);
-            context.mem2reg();
-            context.remove_instructions();
-            context.update_data_bus();
+            function.mem2reg();
         }
 
         self
     }
 }
 
+impl Function {
+    pub(crate) fn mem2reg(&mut self) {
+        let mut context = PerFunctionContext::new(self);
+        context.mem2reg();
+        context.remove_instructions();
+        context.update_data_bus();
+    }
+}
+
 struct PerFunctionContext<'f> {
     cfg: ControlFlowGraph,
     post_order: PostOrder,

compiler/noirc_evaluator/src/ssa/opt/rc.rs

@@ -22,7 +22,7 @@ impl Ssa {
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn remove_paired_rc(mut self) -> Ssa {
         for function in self.functions.values_mut() {
-            remove_paired_rc(function);
+            function.remove_paired_rc();
         }
         self
     }
@@ -44,26 +44,28 @@ pub(crate) struct RcInstruction {
     pub(crate) possibly_mutated: bool,
 }
 
-/// This function is very simplistic for now. It takes advantage of the fact that dec_rc
-/// instructions are currently issued only at the end of a function for parameters and will
-/// only check the first and last block for inc & dec rc instructions to be removed. The rest
-/// of the function is still checked for array_set instructions.
-///
-/// This restriction lets this function largely ignore merging intermediate results from other
-/// blocks and handling loops.
-fn remove_paired_rc(function: &mut Function) {
-    // `dec_rc` is only issued for parameters currently so we can speed things
-    // up a bit by skipping any functions without them.
-    if !contains_array_parameter(function) {
-        return;
-    }
+impl Function {
+    /// This function is very simplistic for now. It takes advantage of the fact that dec_rc
+    /// instructions are currently issued only at the end of a function for parameters and will
+    /// only check the first and last block for inc & dec rc instructions to be removed. The rest
+    /// of the function is still checked for array_set instructions.
+    ///
+    /// This restriction lets this function largely ignore merging intermediate results from other
+    /// blocks and handling loops.
+    pub(crate) fn remove_paired_rc(&mut self) {
+        // `dec_rc` is only issued for parameters currently so we can speed things
+        // up a bit by skipping any functions without them.
+        if !contains_array_parameter(self) {
+            return;
+        }
 
-    let mut context = Context::default();
+        let mut context = Context::default();
 
-    context.find_rcs_in_entry_block(function);
-    context.scan_for_array_sets(function);
-    let to_remove = context.find_rcs_to_remove(function);
-    remove_instructions(to_remove, function);
+        context.find_rcs_in_entry_block(self);
+        context.scan_for_array_sets(self);
+        let to_remove = context.find_rcs_to_remove(self);
+        remove_instructions(to_remove, self);
+    }
 }
 
 fn contains_array_parameter(function: &mut Function) -> bool {

compiler/noirc_evaluator/src/ssa/opt/remove_bit_shifts.rs

@@ -21,24 +21,30 @@ impl Ssa {
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn remove_bit_shifts(mut self) -> Ssa {
         for function in self.functions.values_mut() {
-            remove_bit_shifts(function);
+            function.remove_bit_shifts();
         }
         self
     }
 }
 
-/// The structure of this pass is simple:
-/// Go through each block and re-insert all instructions.
-fn remove_bit_shifts(function: &mut Function) {
-    if let RuntimeType::Brillig = function.runtime() {
-        return;
-    }
+impl Function {
+    /// The structure of this pass is simple:
+    /// Go through each block and re-insert all instructions.
+    pub(crate) fn remove_bit_shifts(&mut self) {
+        if let RuntimeType::Brillig = self.runtime() {
+            return;
+        }
 
-    let block = function.entry_block();
-    let mut context =
-        Context { function, new_instructions: Vec::new(), block, call_stack: CallStack::default() };
+        let block = self.entry_block();
+        let mut context = Context {
+            function: self,
+            new_instructions: Vec::new(),
+            block,
+            call_stack: CallStack::default(),
+        };
 
-    context.remove_bit_shifts();
+        context.remove_bit_shifts();
+    }
 }
 
 struct Context<'f> {