feat(ssa): Loop invariant code motion (#6563)

compiler/noirc_evaluator/src/ssa.rs

@@ -103,6 +103,7 @@ pub(crate) fn optimize_into_acir(
         Ssa::evaluate_static_assert_and_assert_constant,
         "After `static_assert` and `assert_constant`:",
     )?
+    .run_pass(Ssa::loop_invariant_code_motion, "After Loop Invariant Code Motion:")
     .try_run_pass(Ssa::unroll_loops_iteratively, "After Unrolling:")?
     .run_pass(Ssa::simplify_cfg, "After Simplifying (2nd):")
     .run_pass(Ssa::flatten_cfg, "After Flattening:")

compiler/noirc_evaluator/src/ssa/ir/function_inserter.rs

@@ -25,7 +25,7 @@ pub(crate) struct FunctionInserter<'f> {
     ///
     /// This is optional since caching arrays relies on the inserter inserting strictly
     /// in control-flow order. Otherwise, if arrays later in the program are cached first,
-    /// they may be refered to by instructions earlier in the program.
+    /// they may be referred to by instructions earlier in the program.
     array_cache: Option<ArrayCache>,
 
     /// If this pass is loop unrolling, store the block before the loop to optionally

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

@@ -0,0 +1,378 @@
+//! The loop invariant code motion pass moves code from inside a loop to before the loop
+//! if that code will always have the same result on every iteration of the loop.
+//!
+//! To identify a loop invariant, check whether all of an instruction's values are:
+//! - Outside of the loop
+//! - Constant
+//! - Already marked as loop invariants
+//!
+//! We also check that we are not hoisting instructions with side effects.
+use fxhash::FxHashSet as HashSet;
+
+use crate::ssa::{
+    ir::{
+        basic_block::BasicBlockId,
+        function::{Function, RuntimeType},
+        function_inserter::FunctionInserter,
+        instruction::InstructionId,
+        value::ValueId,
+    },
+    Ssa,
+};
+
+use super::unrolling::{Loop, Loops};
+
+impl Ssa {
+    #[tracing::instrument(level = "trace", skip(self))]
+    pub(crate) fn loop_invariant_code_motion(mut self) -> Ssa {
+        let brillig_functions = self
+            .functions
+            .iter_mut()
+            .filter(|(_, func)| matches!(func.runtime(), RuntimeType::Brillig(_)));
+
+        for (_, function) in brillig_functions {
+            function.loop_invariant_code_motion();
+        }
+
+        self
+    }
+}
+
+impl Function {
+    fn loop_invariant_code_motion(&mut self) {
+        Loops::find_all(self).hoist_loop_invariants(self);
+    }
+}
+
+impl Loops {
+    fn hoist_loop_invariants(self, function: &mut Function) {
+        let mut context = LoopInvariantContext::new(function);
+
+        for loop_ in self.yet_to_unroll.iter() {
+            let Ok(pre_header) = loop_.get_pre_header(context.inserter.function, &self.cfg) else {
+                // If the loop does not have a preheader we skip hoisting loop invariants for this loop
+                continue;
+            };
+            context.hoist_loop_invariants(loop_, pre_header);
+        }
+
+        context.map_dependent_instructions();
+    }
+}
+
+struct LoopInvariantContext<'f> {
+    inserter: FunctionInserter<'f>,
+    defined_in_loop: HashSet<ValueId>,
+    loop_invariants: HashSet<ValueId>,
+}
+
+impl<'f> LoopInvariantContext<'f> {
+    fn new(function: &'f mut Function) -> Self {
+        Self {
+            inserter: FunctionInserter::new(function),
+            defined_in_loop: HashSet::default(),
+            loop_invariants: HashSet::default(),
+        }
+    }
+
+    fn hoist_loop_invariants(&mut self, loop_: &Loop, pre_header: BasicBlockId) {
+        self.set_values_defined_in_loop(loop_);
+
+        for block in loop_.blocks.iter() {
+            for instruction_id in self.inserter.function.dfg[*block].take_instructions() {
+                let hoist_invariant = self.can_hoist_invariant(instruction_id);
+
+                if hoist_invariant {
+                    self.inserter.push_instruction(instruction_id, pre_header);
+                } else {
+                    self.inserter.push_instruction(instruction_id, *block);
+                }
+
+                self.update_values_defined_in_loop_and_invariants(instruction_id, hoist_invariant);
+            }
+        }
+    }
+
+    /// Gather the variables declared within the loop
+    fn set_values_defined_in_loop(&mut self, loop_: &Loop) {
+        for block in loop_.blocks.iter() {
+            let params = self.inserter.function.dfg.block_parameters(*block);
+            self.defined_in_loop.extend(params);
+            for instruction_id in self.inserter.function.dfg[*block].instructions() {
+                let results = self.inserter.function.dfg.instruction_results(*instruction_id);
+                self.defined_in_loop.extend(results);
+            }
+        }
+    }
+
+    /// Update any values defined in the loop and loop invariants after a
+    /// analyzing and re-inserting a loop's instruction.
+    fn update_values_defined_in_loop_and_invariants(
+        &mut self,
+        instruction_id: InstructionId,
+        hoist_invariant: bool,
+    ) {
+        let results = self.inserter.function.dfg.instruction_results(instruction_id).to_vec();
+        // We will have new IDs after pushing instructions.
+        // We should mark the resolved result IDs as also being defined within the loop.
+        let results =
+            results.into_iter().map(|value| self.inserter.resolve(value)).collect::<Vec<_>>();
+        self.defined_in_loop.extend(results.iter());
+
+        // We also want the update result IDs when we are marking loop invariants as we may not
+        // be going through the blocks of the loop in execution order
+        if hoist_invariant {
+            // Track already found loop invariants
+            self.loop_invariants.extend(results.iter());
+        }
+    }
+
+    fn can_hoist_invariant(&mut self, instruction_id: InstructionId) -> bool {
+        let mut is_loop_invariant = true;
+        // The list of blocks for a nested loop contain any inner loops as well.
+        // We may have already re-inserted new instructions if two loops share blocks
+        // so we need to map all the values in the instruction which we want to check.
+        let (instruction, _) = self.inserter.map_instruction(instruction_id);
+        instruction.for_each_value(|value| {
+            // If an instruction value is defined in the loop and not already a loop invariant
+            // the instruction results are not loop invariants.
+            //
+            // We are implicitly checking whether the values are constant as well.
+            // The set of values defined in the loop only contains instruction results and block parameters
+            // which cannot be constants.
+            is_loop_invariant &=
+                !self.defined_in_loop.contains(&value) || self.loop_invariants.contains(&value);
+        });
+        is_loop_invariant && instruction.can_be_deduplicated(&self.inserter.function.dfg, false)
+    }
+
+    fn map_dependent_instructions(&mut self) {
+        let blocks = self.inserter.function.reachable_blocks();
+        for block in blocks {
+            for instruction_id in self.inserter.function.dfg[block].take_instructions() {
+                self.inserter.push_instruction(instruction_id, block);
+            }
+            self.inserter.map_terminator_in_place(block);
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::ssa::opt::assert_normalized_ssa_equals;
+    use crate::ssa::Ssa;
+
+    #[test]
+    fn simple_loop_invariant_code_motion() {
+        let src = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32, v1: u32):
+              jmp b1(u32 0)
+          b1(v2: u32):
+              v5 = lt v2, u32 4
+              jmpif v5 then: b3, else: b2
+          b3():
+              v6 = mul v0, v1
+              constrain v6 == u32 6
+              v8 = add v2, u32 1
+              jmp b1(v8)
+          b2():
+              return
+        }
+        ";
+
+        let mut ssa = Ssa::from_str(src).unwrap();
+        let main = ssa.main_mut();
+
+        let instructions = main.dfg[main.entry_block()].instructions();
+        assert_eq!(instructions.len(), 0); // The final return is not counted
+
+        // `v6 = mul v0, v1` in b3 should now be `v3 = mul v0, v1` in b0
+        let expected = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32, v1: u32):
+            v3 = mul v0, v1
+            jmp b1(u32 0)
+          b1(v2: u32):
+            v6 = lt v2, u32 4
+            jmpif v6 then: b3, else: b2
+          b3():
+            constrain v3 == u32 6
+            v9 = add v2, u32 1
+            jmp b1(v9)
+          b2():
+            return
+        }
+        ";
+
+        let ssa = ssa.loop_invariant_code_motion();
+        assert_normalized_ssa_equals(ssa, expected);
+    }
+
+    #[test]
+    fn nested_loop_invariant_code_motion() {
+        // Check that a loop invariant in the inner loop of a nested loop
+        // is hoisted to the parent loop's pre-header block.
+        let src = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32, v1: u32):
+            jmp b1(u32 0)
+          b1(v2: u32):
+            v6 = lt v2, u32 4
+            jmpif v6 then: b3, else: b2
+          b3():
+            jmp b4(u32 0)
+          b4(v3: u32):
+            v7 = lt v3, u32 4
+            jmpif v7 then: b6, else: b5
+          b6():
+            v10 = mul v0, v1
+            constrain v10 == u32 6
+            v12 = add v3, u32 1
+            jmp b4(v12)
+          b5():
+            v9 = add v2, u32 1
+            jmp b1(v9)
+          b2():
+            return
+        }
+        ";
+
+        let mut ssa = Ssa::from_str(src).unwrap();
+        let main = ssa.main_mut();
+
+        let instructions = main.dfg[main.entry_block()].instructions();
+        assert_eq!(instructions.len(), 0); // The final return is not counted
+
+        // `v10 = mul v0, v1` in b6 should now be `v4 = mul v0, v1` in b0
+        let expected = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32, v1: u32):
+            v4 = mul v0, v1
+            jmp b1(u32 0)
+          b1(v2: u32):
+            v7 = lt v2, u32 4
+            jmpif v7 then: b3, else: b2
+          b3():
+            jmp b4(u32 0)
+          b4(v3: u32):
+            v8 = lt v3, u32 4
+            jmpif v8 then: b6, else: b5
+          b6():
+            constrain v4 == u32 6
+            v12 = add v3, u32 1
+            jmp b4(v12)
+          b5():
+            v10 = add v2, u32 1
+            jmp b1(v10)
+          b2():
+            return
+        }
+        ";
+
+        let ssa = ssa.loop_invariant_code_motion();
+        assert_normalized_ssa_equals(ssa, expected);
+    }
+
+    #[test]
+    fn hoist_invariant_with_invariant_as_argument() {
+        // Check that an instruction which has arguments defined in the loop
+        // but which are already marked loop invariants is still hoisted to the preheader.
+        //
+        // For example, in b3 we have the following instructions:
+        // ```text
+        // v6 = mul v0, v1
+        // v7 = mul v6, v0
+        // ```
+        // `v6` should be marked a loop invariants as `v0` and `v1` are both declared outside of the loop.
+        // As we will be hoisting `v6 = mul v0, v1` to the loop preheader we know that we can also
+        // hoist `v7 = mul v6, v0`.
+        let src = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32, v1: u32):
+            jmp b1(u32 0)
+          b1(v2: u32):
+            v5 = lt v2, u32 4
+            jmpif v5 then: b3, else: b2
+          b3():
+            v6 = mul v0, v1
+            v7 = mul v6, v0
+            v8 = eq v7, u32 12
+            constrain v7 == u32 12
+            v9 = add v2, u32 1
+            jmp b1(v9)
+          b2():
+            return
+        }
+        ";
+
+        let mut ssa = Ssa::from_str(src).unwrap();
+        let main = ssa.main_mut();
+
+        let instructions = main.dfg[main.entry_block()].instructions();
+        assert_eq!(instructions.len(), 0); // The final return is not counted
+
+        let expected = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32, v1: u32):
+            v3 = mul v0, v1
+            v4 = mul v3, v0
+            v6 = eq v4, u32 12
+            jmp b1(u32 0)
+          b1(v2: u32):
+            v9 = lt v2, u32 4
+            jmpif v9 then: b3, else: b2
+          b3():
+            constrain v4 == u32 12
+            v11 = add v2, u32 1
+            jmp b1(v11)
+          b2():
+            return
+        }
+        ";
+
+        let ssa = ssa.loop_invariant_code_motion();
+        assert_normalized_ssa_equals(ssa, expected);
+    }
+
+    #[test]
+    fn do_not_hoist_instructions_with_side_effects() {
+        // In `v12 = load v5` in `b3`, `v5` is defined outside the loop.
+        // However, as the instruction has side effects, we want to make sure
+        // we do not hoist the instruction to the loop preheader.
+        let src = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32, v1: u32):
+            v4 = make_array [u32 0, u32 0, u32 0, u32 0, u32 0] : [u32; 5]
+            inc_rc v4
+            v5 = allocate -> &mut [u32; 5]
+            store v4 at v5
+            jmp b1(u32 0)
+          b1(v2: u32):
+            v7 = lt v2, u32 4
+            jmpif v7 then: b3, else: b2
+          b3():
+            v12 = load v5 -> [u32; 5]
+            v13 = array_set v12, index v0, value v1
+            store v13 at v5
+            v15 = add v2, u32 1
+            jmp b1(v15)
+          b2():
+            v8 = load v5 -> [u32; 5]
+            v10 = array_get v8, index u32 2 -> u32
+            constrain v10 == u32 3
+            return
+        }
+        ";
+
+        let mut ssa = Ssa::from_str(src).unwrap();
+        let main = ssa.main_mut();
+
+        let instructions = main.dfg[main.entry_block()].instructions();
+        assert_eq!(instructions.len(), 4); // The final return is not counted
+
+        let ssa = ssa.loop_invariant_code_motion();
+        // The code should be unchanged
+        assert_normalized_ssa_equals(ssa, src);
+    }
+}