Merge pull request #907 from Zokrates/cache-branch-conditions

Avoid creating empty if-else statements, cache conditions when flattening

changelogs/unreleased/907-schaeff

@@ -0,0 +1 @@
+Reduce the cost of conditionals

zokrates_core/src/flatten/mod.rs

@@ -35,6 +35,8 @@ pub struct Flattener<'ast, T: Field> {
     layout: HashMap<Identifier<'ast>, FlatVariable>,
     /// Cached bit decompositions to avoid re-generating them
     bits_cache: HashMap<FlatExpression<T>, Vec<FlatExpression<T>>>,
+    /// Cached flattened conditions for branches
+    condition_cache: HashMap<BooleanExpression<'ast, T>, FlatVariable>,
 }
 
 trait FlattenOutput<T: Field>: Sized {
@@ -159,6 +161,7 @@ impl<'ast, T: Field> Flattener<'ast, T> {
             next_var_idx: 0,
             layout: HashMap::new(),
             bits_cache: HashMap::new(),
+            condition_cache: HashMap::new(),
         }
     }
 
@@ -413,6 +416,7 @@ impl<'ast, T: Field> Flattener<'ast, T> {
                         name_x_or_y.into(),
                         T::one().into(),
                     ));
+
                     output
                 }
                 s => vec![s],
@@ -437,10 +441,13 @@ impl<'ast, T: Field> Flattener<'ast, T> {
         consequence: U,
         alternative: U,
     ) -> FlatUExpression<T> {
-        let condition = self.flatten_boolean_expression(statements_flattened, condition);
+        let condition_flat =
+            self.flatten_boolean_expression(statements_flattened, condition.clone());
 
         let condition_id = self.use_sym();
-        statements_flattened.push(FlatStatement::Definition(condition_id, condition));
+        statements_flattened.push(FlatStatement::Definition(condition_id, condition_flat));
+
+        self.condition_cache.insert(condition, condition_id);
 
         let (consequence, alternative) = if self.config.isolate_branches {
             let mut consequence_statements = vec![];
@@ -636,7 +643,11 @@ impl<'ast, T: Field> Flattener<'ast, T> {
         statements_flattened: &mut FlatStatements<T>,
         expression: BooleanExpression<'ast, T>,
     ) -> FlatExpression<T> {
-        // those will be booleans in the future
+        // check the cache
+        if let Some(c) = self.condition_cache.get(&expression) {
+            return (*c).into();
+        }
+
         match expression {
             BooleanExpression::Identifier(x) => {
                 FlatExpression::Identifier(*self.layout.get(&x).unwrap())
@@ -2184,7 +2195,13 @@ impl<'ast, T: Field> Flattener<'ast, T> {
                 }));
             }
             ZirStatement::IfElse(condition, consequence, alternative) => {
-                let condition = self.flatten_boolean_expression(statements_flattened, condition);
+                let condition_flat =
+                    self.flatten_boolean_expression(statements_flattened, condition.clone());
+
+                let condition_id = self.use_sym();
+                statements_flattened.push(FlatStatement::Definition(condition_id, condition_flat));
+
+                self.condition_cache.insert(condition, condition_id);
 
                 if self.config.isolate_branches {
                     let mut consequence_statements = vec![];
@@ -2198,10 +2215,13 @@ impl<'ast, T: Field> Flattener<'ast, T> {
                         .for_each(|s| self.flatten_statement(&mut alternative_statements, s));
 
                     let consequence_statements =
-                        self.make_conditional(consequence_statements, condition.clone());
+                        self.make_conditional(consequence_statements, condition_id.into());
                     let alternative_statements = self.make_conditional(
                         alternative_statements,
-                        FlatExpression::Sub(box FlatExpression::Number(T::one()), box condition),
+                        FlatExpression::Sub(
+                            box FlatExpression::Number(T::one()),
+                            box condition_id.into(),
+                        ),
                     );
 
                     statements_flattened.extend(consequence_statements);

zokrates_core/src/static_analysis/flatten_complex_types.rs

@@ -601,11 +601,13 @@ fn fold_if_else_expression<'ast, T: Field, E: Flatten<'ast, T>>(
 
     assert_eq!(consequence.len(), alternative.len());
 
-    statements_buffer.push(zir::ZirStatement::IfElse(
-        condition.clone(),
-        consequence_statements,
-        alternative_statements,
-    ));
+    if !consequence_statements.is_empty() || !alternative_statements.is_empty() {
+        statements_buffer.push(zir::ZirStatement::IfElse(
+            condition.clone(),
+            consequence_statements,
+            alternative_statements,
+        ));
+    }
 
     use crate::zir::IfElse;
 

zokrates_core_test/tests/tests/cached_condition.json

@@ -0,0 +1,4 @@
+{
+	"entry_point": "./tests/tests/cached_condition.zok",
+    "max_constraint_count": 2015
+}

zokrates_core_test/tests/tests/cached_condition.zok

@@ -0,0 +1,4 @@
+// `a < b` should be flattened a single time, even if 1000 elements are assigned conditionally
+
+def main(field a, field b) -> field[1000]:
+    return if a < b then [0f; 1000] else [1; 1000] fi