feat(ssa): Deduplicate intrinsics with predicates

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

@@ -45,8 +45,7 @@ pub(crate) type InstructionId = Id<Instruction>;
 /// - Opcodes which the IR knows the target machine has
 /// special support for. (LowLevel)
 /// - Opcodes which have no function definition in the
-/// source code and must be processed by the IR. An example
-/// of this is println.
+/// source code and must be processed by the IR.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, Serialize, Deserialize)]
 pub(crate) enum Intrinsic {
     ArrayLen,
@@ -112,6 +111,9 @@ impl Intrinsic {
     /// Returns whether the `Intrinsic` has side effects.
     ///
     /// If there are no side effects then the `Intrinsic` can be removed if the result is unused.
+    ///
+    /// An obvious example of a side effect is pushing an item to a slice, but functions which
+    /// can fail due to implicit constraints are also considered to have a side effect.
     pub(crate) fn has_side_effects(&self) -> bool {
         match self {
             Intrinsic::AssertConstant
@@ -152,6 +154,34 @@ impl Intrinsic {
         }
     }
 
+    /// Intrinsics which only have a side effect due to the chance that
+    /// they can fail a constraint can be deduplicated.
+    pub(crate) fn can_be_deduplicated(&self, deduplicate_with_predicate: bool) -> bool {
+        match self {
+            // These apply a constraint in the form of ACIR opcodes, but they can be deduplicated
+            // if the inputs are the same. If they depend on a side effect variable (e.g. because
+            // they were in an if-then-else) then `handle_instruction_side_effects` in `flatten_cfg`
+            // will have attached the condition variable to their inputs directly, so they don't
+            // directly depend on the corresponding `enable_side_effect` instruction any more.
+            // However, to conform with the expectations of `Instruction::can_be_deduplicated` and
+            // `constant_folding` we only use this information if the caller shows interest in it.
+            Intrinsic::ToBits(_)
+            | Intrinsic::ToRadix(_)
+            | Intrinsic::BlackBox(
+                BlackBoxFunc::MultiScalarMul
+                | BlackBoxFunc::EmbeddedCurveAdd
+                | BlackBoxFunc::RecursiveAggregation,
+            ) => deduplicate_with_predicate,
+
+            Intrinsic::AssertConstant
+            | Intrinsic::StaticAssert
+            | Intrinsic::ApplyRangeConstraint
+            | Intrinsic::AsWitness => deduplicate_with_predicate,
+
+            _ => !self.has_side_effects(),
+        }
+    }
+
     /// Lookup an Intrinsic by name and return it if found.
     /// If there is no such intrinsic by that name, None is returned.
     pub(crate) fn lookup(name: &str) -> Option<Intrinsic> {
@@ -245,7 +275,7 @@ pub(crate) enum Instruction {
     /// - `code1` will have side effects iff `condition1` evaluates to `true`
     ///
     /// This instruction is only emitted after the cfg flattening pass, and is used to annotate
-    /// instruction regions with an condition that corresponds to their position in the CFG's
+    /// instruction regions with a condition that corresponds to their position in the CFG's
     /// if-branching structure.
     EnableSideEffectsIf { condition: ValueId },
 
@@ -331,6 +361,11 @@ impl Instruction {
     /// If `deduplicate_with_predicate` is set, we assume we're deduplicating with the instruction
     /// and its predicate, rather than just the instruction. Setting this means instructions that
     /// rely on predicates can be deduplicated as well.
+    ///
+    /// Some instructions get the predicate attached to their inputs by `handle_instruction_side_effects` in `flatten_cfg`.
+    /// These can be deduplicated because they implicitly depend on the predicate, not only when the caller uses the
+    /// predicate variable as a key to cache results. However, to avoid tight coupling between passes, we make the deduplication
+    /// conditional on whether the caller wants the predicate to be taken into account or not.
     pub(crate) fn can_be_deduplicated(
         &self,
         dfg: &DataFlowGraph,
@@ -348,7 +383,9 @@ impl Instruction {
             | DecrementRc { .. } => false,
 
             Call { func, .. } => match dfg[*func] {
-                Value::Intrinsic(intrinsic) => !intrinsic.has_side_effects(),
+                Value::Intrinsic(intrinsic) => {
+                    intrinsic.can_be_deduplicated(deduplicate_with_predicate)
+                }
                 _ => false,
             },
 
@@ -421,6 +458,7 @@ impl Instruction {
                 // Explicitly allows removal of unused ec operations, even if they can fail
                 Value::Intrinsic(Intrinsic::BlackBox(BlackBoxFunc::MultiScalarMul))
                 | Value::Intrinsic(Intrinsic::BlackBox(BlackBoxFunc::EmbeddedCurveAdd)) => true,
+
                 Value::Intrinsic(intrinsic) => !intrinsic.has_side_effects(),
 
                 // All foreign functions are treated as having side effects.
@@ -436,7 +474,7 @@ impl Instruction {
         }
     }
 
-    /// If true the instruction will depends on enable_side_effects context during acir-gen
+    /// If true the instruction will depend on `enable_side_effects` context during acir-gen.
     pub(crate) fn requires_acir_gen_predicate(&self, dfg: &DataFlowGraph) -> bool {
         match self {
             Instruction::Binary(binary)

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

@@ -6,7 +6,7 @@
 //!   by the [`DataFlowGraph`] automatically as new instructions are pushed.
 //! - Check whether any input values have been constrained to be equal to a value of a simpler form
 //!   by a [constrain instruction][Instruction::Constrain]. If so, replace the input value with the simpler form.
-//! - Check whether the instruction [can_be_replaced][Instruction::can_be_replaced()]
+//! - Check whether the instruction [can_be_deduplicated][Instruction::can_be_deduplicated()]
 //!   by duplicate instruction earlier in the same block.
 //!
 //! These operations are done in parallel so that they can each benefit from each other
@@ -54,6 +54,9 @@ use fxhash::FxHashMap as HashMap;
 impl Ssa {
     /// Performs constant folding on each instruction.
     ///
+    /// It will not look at constraints to inform simplifications
+    /// based on the stated equivalence of two instructions.
+    ///
     /// See [`constant_folding`][self] module for more information.
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn fold_constants(mut self) -> Ssa {
@@ -188,9 +191,12 @@ pub(crate) struct BrilligInfo<'a> {
     brillig_functions: &'a BTreeMap<FunctionId, Function>,
 }
 
-/// HashMap from (Instruction, side_effects_enabled_var) to the results of the instruction.
+/// HashMap from `(Instruction, side_effects_enabled_var)` to the results of the instruction.
 /// Stored as a two-level map to avoid cloning Instructions during the `.get` call.
 ///
+/// The `side_effects_enabled_var` is optional because we only use them when `Instruction::requires_acir_gen_predicate`
+/// is true _and_ the constraint information is also taken into account.
+///
 /// In addition to each result, the original BasicBlockId is stored as well. This allows us
 /// to deduplicate instructions across blocks as long as the new block dominates the original.
 type InstructionResultCache = HashMap<Instruction, HashMap<Option<ValueId>, ResultCache>>;
@@ -223,6 +229,7 @@ impl<'brillig> Context<'brillig> {
     fn fold_constants_in_block(&mut self, function: &mut Function, block: BasicBlockId) {
         let instructions = function.dfg[block].take_instructions();
 
+        // Default side effect condition variable with an enabled state.
         let mut side_effects_enabled_var =
             function.dfg.make_constant(FieldElement::one(), Type::bool());
 
@@ -403,6 +410,7 @@ impl<'brillig> Context<'brillig> {
 
         // If the instruction doesn't have side-effects and if it won't interact with enable_side_effects during acir_gen,
         // we cache the results so we can reuse them if the same instruction appears again later in the block.
+        // Others have side effects representing failure, which are implicit in the ACIR code and can also be deduplicated.
         if instruction.can_be_deduplicated(dfg, self.use_constraint_info) {
             let use_predicate =
                 self.use_constraint_info && instruction.requires_acir_gen_predicate(dfg);
@@ -417,6 +425,8 @@ impl<'brillig> Context<'brillig> {
         }
     }
 
+    /// Get the simplification mapping from complex to simpler instructions,
+    /// which all depend on the same side effect condition variable.
     fn get_constraint_map(
         &mut self,
         side_effects_enabled_var: ValueId,
@@ -1341,4 +1351,43 @@ mod test {
         let ssa = ssa.fold_constants_with_brillig(&brillig);
         assert_normalized_ssa_equals(ssa, expected);
     }
+
+    #[test]
+    fn deduplicates_side_effecting_intrinsics() {
+        let src = "
+        // After EnableSideEffectsIf removal:
+        acir(inline) fn main f0 {
+          b0(v0: Field, v1: Field, v2: u1):
+            v4 = call is_unconstrained() -> u1
+            v7 = call to_be_radix(v0, u32 256) -> [u8; 1]    // `a.to_be_radix(256)`;
+            inc_rc v7
+            v8 = call to_be_radix(v0, u32 256) -> [u8; 1]    // duplicate load of `a`
+            inc_rc v8
+            v9 = cast v2 as Field                            // `if c { a.to_be_radix(256) }`
+            v10 = mul v0, v9                                 // attaching `c` to `a`
+            v11 = call to_be_radix(v10, u32 256) -> [u8; 1]  // calling `to_radix(c * a)`
+            inc_rc v11
+            enable_side_effects v2                           // side effect var for `c` shifted down by removal
+            return
+        }
+        ";
+        let ssa = Ssa::from_str(src).unwrap();
+        let expected = "
+        acir(inline) fn main f0 {
+          b0(v0: Field, v1: Field, v2: u1):
+            v4 = call is_unconstrained() -> u1
+            v7 = call to_be_radix(v0, u32 256) -> [u8; 1]
+            inc_rc v7
+            inc_rc v7
+            v8 = cast v2 as Field
+            v9 = mul v0, v8
+            v10 = call to_be_radix(v9, u32 256) -> [u8; 1]
+            inc_rc v10
+            enable_side_effects v2
+            return
+        }
+        ";
+        let ssa = ssa.fold_constants_using_constraints();
+        assert_normalized_ssa_equals(ssa, expected);
+    }
 }