Unroll small Brillig loops

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

@@ -209,8 +209,9 @@ impl Loops {
         let mut unroll_errors = vec![];
         while let Some(next_loop) = self.yet_to_unroll.pop() {
             if function.runtime().is_brillig() {
-                // TODO (#6470): Decide whether to unroll this loop.
-                continue;
+                if !next_loop.is_small_loop(function, &self.cfg) {
+                    continue;
+                }
             }
             // If we've previously modified a block in this loop we need to refresh the context.
             // This happens any time we have nested loops.
@@ -593,21 +594,54 @@ impl Loop {
     /// of unrolled instructions times the number of iterations would result in smaller bytecode
     /// than if we keep the loops with their overheads.
     fn is_small_loop(&self, function: &Function, cfg: &ControlFlowGraph) -> bool {
+        self.boilerplate_stats(function, cfg).map(|s| s.is_small()).unwrap_or_default()
+    }
+
+    /// Collect boilerplate stats if we can figure out the upper and lower bounds of the loop.
+    fn boilerplate_stats(
+        &self,
+        function: &Function,
+        cfg: &ControlFlowGraph,
+    ) -> Option<BoilerplateStats> {
         let Ok(Some((lower, upper))) = self.get_const_bounds(function, cfg) else {
-            return false;
+            return None;
         };
         let Some(lower) = lower.try_to_u64() else {
-            return false;
+            return None;
         };
         let Some(upper) = upper.try_to_u64() else {
-            return false;
+            return None;
         };
-        let num_iterations = (upper - lower) as usize;
         let refs = self.find_pre_header_reference_values(function, cfg);
         let (loads, stores) = self.count_loads_and_stores(function, &refs);
         let all_instructions = self.count_all_instructions(function);
         let useful_instructions = all_instructions - loads - stores - LOOP_BOILERPLATE_COUNT;
-        useful_instructions * num_iterations < all_instructions
+        Some(BoilerplateStats {
+            iterations: (upper - lower) as usize,
+            loads,
+            stores,
+            all_instructions,
+            useful_instructions,
+        })
+    }
+}
+
+#[derive(Debug)]
+struct BoilerplateStats {
+    iterations: usize,
+    loads: usize,
+    stores: usize,
+    all_instructions: usize,
+    useful_instructions: usize,
+}
+
+impl BoilerplateStats {
+    /// A small loop is where if we unroll it into the pre-header then considering the
+    /// number of iterations we still end up with a smaller bytecode than if we leave
+    /// the blocks in tact with all the boilerplate involved in jumping, and the extra
+    /// reference access instructions.
+    fn is_small(&self) -> bool {
+        self.useful_instructions * self.iterations < self.all_instructions
     }
 }
 
@@ -1014,6 +1048,105 @@ mod tests {
         assert!(loop0.is_small_loop(function, &loops.cfg));
     }
 
+    #[test]
+    fn test_brillig_unroll_small_loop() {
+        let ssa = brillig_unroll_test_case();
+
+        // Example taken from an equivalent ACIR program (ie. remove the `unconstrained`) and run
+        // `cargo run -q -p nargo_cli --  --program-dir . compile --show-ssa`
+        let expected = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32):
+            v1 = allocate -> &mut u32
+            store u32 0 at v1
+            v3 = load v1 -> u32
+            store v3 at v1
+            v4 = load v1 -> u32
+            v6 = add v4, u32 1
+            store v6 at v1
+            v7 = load v1 -> u32
+            v9 = add v7, u32 2
+            store v9 at v1
+            v10 = load v1 -> u32
+            v12 = add v10, u32 3
+            store v12 at v1
+            jmp b1()
+          b1():
+            v13 = load v1 -> u32
+            v14 = eq v13, v0
+            constrain v13 == v0
+            return
+        }
+        ";
+
+        let (ssa, errors) = ssa.try_unroll_loops();
+        assert_eq!(errors.len(), 0, "Unroll should have no errors");
+        assert_eq!(ssa.main().reachable_blocks().len(), 2, "The loop should be unrolled");
+
+        assert_normalized_ssa_equals(ssa, expected);
+    }
+
+    #[test]
+    fn test_brillig_unroll_6470_small() {
+        // Few enough iterations so that we can perform the unroll.
+        let ssa = brillig_unroll_test_case_6470(3);
+        let (ssa, errors) = ssa.try_unroll_loops();
+        assert_eq!(errors.len(), 0, "Unroll should have no errors");
+        assert_eq!(ssa.main().reachable_blocks().len(), 2, "The loop should be unrolled");
+
+        // The IDs are shifted by one compared to what the ACIR version printed.
+        let expected = "
+        brillig(inline) fn __validate_gt_remainder f0 {
+          b0(v0: [u64; 6]):
+            inc_rc v0
+            inc_rc [u64 0, u64 0, u64 0, u64 0, u64 0, u64 0] of u64
+            v3 = allocate -> &mut [u64; 6]
+            store [u64 0, u64 0, u64 0, u64 0, u64 0, u64 0] of u64 at v3
+            v5 = load v3 -> [u64; 6]
+            v7 = array_get v0, index u32 0 -> u64
+            v9 = add v7, u64 1
+            v10 = array_set v5, index u32 0, value v9
+            store v10 at v3
+            v11 = load v3 -> [u64; 6]
+            v13 = array_get v0, index u32 1 -> u64
+            v14 = add v13, u64 1
+            v15 = array_set v11, index u32 1, value v14
+            store v15 at v3
+            v16 = load v3 -> [u64; 6]
+            v18 = array_get v0, index u32 2 -> u64
+            v19 = add v18, u64 1
+            v20 = array_set v16, index u32 2, value v19
+            store v20 at v3
+            jmp b1()
+          b1():
+            v21 = load v3 -> [u64; 6]
+            dec_rc v0
+            return v21
+        }
+        ";
+        assert_normalized_ssa_equals(ssa, expected);
+    }
+
+    #[test]
+    fn test_brillig_unroll_6470_large() {
+        // More iterations than it can unroll
+        let ssa = brillig_unroll_test_case_6470(6);
+
+        let function = ssa.main();
+        let mut loops = Loops::find_all(function);
+        let loop0 = loops.yet_to_unroll.pop().unwrap();
+        let stats = loop0.boilerplate_stats(function, &loops.cfg).unwrap();
+        assert_eq!(stats.is_small(), false);
+
+        let (ssa, errors) = ssa.try_unroll_loops();
+        assert_eq!(errors.len(), 0, "Unroll should have no errors");
+        assert_eq!(
+            ssa.main().reachable_blocks().len(),
+            4,
+            "The loop should be considered too costly to unroll"
+        );
+    }
+
     /// Simple test loop:
     /// ```text
     /// unconstrained fn main(sum: u32) {
@@ -1054,4 +1187,50 @@ mod tests {
         ";
         Ssa::from_str(src).unwrap()
     }
+
+    /// Test case from #6470:
+    /// ```text
+    /// unconstrained fn __validate_gt_remainder(a_u60: [u64; 6]) -> [u64; 6] {
+    ///     let mut result_u60: [u64; 6] = [0; 6];
+    ///
+    ///     for i in 0..6 {
+    ///         result_u60[i] = a_u60[i] + 1;
+    ///     }
+    ///
+    ///     result_u60
+    /// }
+    /// ```
+    /// The `num_iterations` parameter can be used to make it more costly to inline.
+    fn brillig_unroll_test_case_6470(num_iterations: usize) -> Ssa {
+        let src = format!(
+            "
+        // After `static_assert` and `assert_constant`:
+        brillig(inline) fn __validate_gt_remainder f0 {{
+          b0(v0: [u64; 6]):
+            inc_rc v0
+            inc_rc [u64 0, u64 0, u64 0, u64 0, u64 0, u64 0] of u64
+            v4 = allocate -> &mut [u64; 6]
+            store [u64 0, u64 0, u64 0, u64 0, u64 0, u64 0] of u64 at v4
+            jmp b1(u32 0)
+          b1(v1: u32):
+            v7 = lt v1, u32 {num_iterations}
+            jmpif v7 then: b3, else: b2
+          b3():
+            v9 = load v4 -> [u64; 6]
+            v10 = array_get v0, index v1 -> u64
+            v12 = add v10, u64 1
+            v13 = array_set v9, index v1, value v12
+            v15 = add v1, u32 1
+            store v13 at v4
+            v16 = add v1, u32 1
+            jmp b1(v16)
+          b2():
+            v8 = load v4 -> [u64; 6]
+            dec_rc v0
+            return v8
+        }}
+        "
+        );
+        Ssa::from_str(&src).unwrap()
+    }
 }