u32 arithmetic check for special cases

src/gadgets/arithmetic_u32.rs

@@ -36,6 +36,37 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         self.assert_zero(x.0)
     }
 
+    /// Checks for special cases where the value of
+    /// `x * y + z`
+    /// can be determined without adding a `U32ArithmeticGate`.
+    pub fn arithmetic_u32_special_cases(
+        &mut self,
+        x: U32Target,
+        y: U32Target,
+        z: U32Target,
+    ) -> Option<(U32Target, U32Target)> {
+        let x_const = self.target_as_constant(x.0);
+        let y_const = self.target_as_constant(y.0);
+        let z_const = self.target_as_constant(z.0);
+
+        // If both terms are constant, return their (constant) sum.
+        let first_term_const = if let (Some(xx), Some(yy)) = (x_const, y_const) {
+            Some(xx * yy)
+        } else {
+            None
+        };
+
+        if let (Some(a), Some(b)) = (first_term_const, z_const) {
+            let sum_u64 = (a + b).to_canonical_u64();
+            let (low_u64, high_u64) = (sum_u64 % (1u64 << 32), sum_u64 >> 32);
+            let low = F::from_canonical_u64(low_u64);
+            let high = F::from_canonical_u64(high_u64);
+            return Some((self.constant_u32(low), self.constant_u32(high)));
+        }
+
+        None
+    }
+
     // Returns x * y + z.
     pub fn mul_add_u32(
         &mut self,
@@ -43,6 +74,10 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         y: U32Target,
         z: U32Target,
     ) -> (U32Target, U32Target) {
+        if let Some(result) = self.arithmetic_u32_special_cases(x, y, z) {
+            return result;
+        }
+
         let (gate_index, copy) = self.find_u32_arithmetic_gate();
 
         self.connect(