feat(ssa): Perform dead instruction elimination on intrinsic functions

crates/nargo_cli/tests/compile_success/intrinsic_die/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "intrinsic_die"
+type = "bin"
+authors = [""]
+compiler_version = "0.6.0"
+
+[dependencies]

crates/nargo_cli/tests/compile_success/intrinsic_die/src/main.nr

@@ -0,0 +1,9 @@
+use dep::std;
+
+// This test checks that we perform dead-instruction-elimination on intrinsic functions.
+
+fn main(x: Field) {
+    let bytes = x.to_be_bytes(32);
+
+    let hash = std::hash::pedersen([x]);
+}

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

@@ -68,6 +68,27 @@ impl std::fmt::Display for Intrinsic {
 }
 
 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.
+    pub(crate) fn has_side_effects(&self) -> bool {
+        match self {
+            Intrinsic::Println => true,
+
+            Intrinsic::Sort
+            | Intrinsic::ArrayLen
+            | Intrinsic::SlicePushBack
+            | Intrinsic::SlicePushFront
+            | Intrinsic::SlicePopBack
+            | Intrinsic::SlicePopFront
+            | Intrinsic::SliceInsert
+            | Intrinsic::SliceRemove
+            | Intrinsic::ToBits(_)
+            | Intrinsic::ToRadix(_)
+            | Intrinsic::BlackBox(_) => false,
+        }
+    }
+
     /// 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> {

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

@@ -92,12 +92,26 @@ impl Context {
 
         let instruction = &function.dfg[instruction_id];
 
-        // These instruction types cannot be removed
-        if matches!(
-            instruction,
-            Constrain(_) | Call { .. } | Store { .. } | EnableSideEffects { .. }
-        ) {
-            return false;
+        match instruction {
+            // These instruction types can be safely removed
+            Binary(_)
+            | Cast(_, _)
+            | Not(_)
+            | Truncate { .. }
+            | Allocate
+            | Load { .. }
+            | ArrayGet { .. }
+            | ArraySet { .. } => (),
+
+            // These instruction types cannot be removed
+            Constrain(_) | Store { .. } | EnableSideEffects { .. } => return false,
+
+            Call { func, .. } => match function.dfg[*func] {
+                Value::Intrinsic(intrinsic) if !intrinsic.has_side_effects() => {
+                    // Intrinsics without side-effects can be safely removed.
+                }
+                _ => return false,
+            },
         }
 
         let results = function.dfg.instruction_results(instruction_id);