Make MAX_DEGREE_LOG configurable (#1677)

Pulled out of #1676, where it helps making the tests run faster.

I think this could possibly be removed by #1667 again.

.github/workflows/pr-tests.yml

@@ -11,6 +11,7 @@ on:
 env:
   CARGO_TERM_COLOR: always
   POWDR_GENERATE_PROOFS: "true"
+  MAX_DEGREE_LOG: "10"
 
 jobs:
   build:

backend/src/composite/split.rs

@@ -99,7 +99,7 @@ pub(crate) fn machine_fixed_columns<F: FieldElement>(
         );
         match machine_degrees.iter().next() {
             Some(&degree) => iter::once(degree as usize).collect(),
-            None => (MIN_DEGREE_LOG..=MAX_DEGREE_LOG)
+            None => (MIN_DEGREE_LOG..=*MAX_DEGREE_LOG)
                 .map(|log_size| 1 << log_size)
                 .collect(),
         }

executor/src/constant_evaluator/mod.rs

@@ -1,10 +1,12 @@
 use std::{
     collections::{BTreeMap, HashMap},
+    env,
     sync::{Arc, RwLock},
 };
 
 pub use data_structures::{get_uniquely_sized, get_uniquely_sized_cloned, VariablySizedColumn};
 use itertools::Itertools;
+use lazy_static::lazy_static;
 use powdr_ast::{
     analyzed::{Analyzed, FunctionValueDefinition, Symbol, TypedExpression},
     parsed::{
@@ -19,7 +21,22 @@ use rayon::prelude::{IntoParallelIterator, ParallelIterator};
 mod data_structures;
 
 pub const MIN_DEGREE_LOG: usize = 5;
-pub const MAX_DEGREE_LOG: usize = 22;
+lazy_static! {
+    // The maximum degree can add a significant cost during setup, because
+    // the fixed columns need to be committed to in all sizes up to the max degree.
+    // This gives the user the possibility to overwrite the default value.
+    pub static ref MAX_DEGREE_LOG: usize = {
+        let default_max_degree_log = 22;
+
+        let max_degree_log = match env::var("MAX_DEGREE_LOG") {
+            Ok(val) => val.parse::<usize>().unwrap(),
+            Err(_) => default_max_degree_log,
+        };
+        log::info!("For variably-sized machine, the maximum degree is 2^{max_degree_log}. \
+            You can set the environment variable MAX_DEGREE_LOG to change this value.");
+        max_degree_log
+    };
+}
 
 /// Generates the fixed column values for all fixed columns that are defined
 /// (and not just declared).
@@ -37,7 +54,7 @@ pub fn generate<T: FieldElement>(analyzed: &Analyzed<T>) -> Vec<(String, Variabl
                 let values = if let Some(degree) = poly.degree {
                     generate_values(analyzed, degree, &name, value, index).into()
                 } else {
-                    (MIN_DEGREE_LOG..=MAX_DEGREE_LOG)
+                    (MIN_DEGREE_LOG..=*MAX_DEGREE_LOG)
                         .map(|degree_log| {
                             let degree = 1 << degree_log;
                             generate_values(analyzed, degree, &name, value, index)

executor/src/witgen/mod.rs

@@ -350,7 +350,7 @@ impl<'a, T: FieldElement> FixedData<'a, T> {
     }
 
     fn common_degree<'b>(&self, ids: impl IntoIterator<Item = &'b PolyID>) -> DegreeType {
-        self.common_set_degree(ids).unwrap_or(1 << MAX_DEGREE_LOG)
+        self.common_set_degree(ids).unwrap_or(1 << *MAX_DEGREE_LOG)
     }
 
     fn is_variable_size<'b>(&self, ids: impl IntoIterator<Item = &'b PolyID>) -> bool {