dev: optimized bitshifts by using a lookup table for powers of two

kkrt-labs · Oct 2, 2024 · 96dea68 · 96dea68
1 parent 7048c0d
commit 96dea68
Showing 1 changed file with 24 additions and 0 deletions.
diff --git a/crates/utils/src/math.cairo b/crates/utils/src/math.cairo
@@ -2,6 +2,7 @@ use core::integer::{u512};
 use core::num::traits::{Zero, One, BitSize, OverflowingAdd, OverflowingMul, Bounded};
 use core::panic_with_felt252;
 use core::traits::{BitAnd};
+use utils::constants::POW_2_256;
 
 // === Exponentiation ===
 
@@ -245,6 +246,14 @@ impl BitshiftImpl<
         if shift > BitSize::<T>::bits() - One::one() {
             panic_with_felt252('mul Overflow');
         }
+        // if the shift is within the bit size of u256 (<= 255 bits),
+        // use the POW_2 lookup table to get 2^shift for efficient multiplication
+        if shift <= BitSize::<u256>::bits() - One::<u32>::one() {
+            // In case the pow2 is greater than the max value of T, we have an overflow
+            // so we can panic
+            return self * (*POW_2_256.span().at(shift)).try_into().expect('mul Overflow');
+        }
+        // for shifts greater than 255 bits, perform the shift manually
         let two = One::one() + One::one();
         self * two.pow(shift.try_into().expect('mul Overflow'))
     }
@@ -254,6 +263,10 @@ impl BitshiftImpl<
         if shift > BitSize::<T>::bits() - One::one() {
             panic_with_felt252('mul Overflow');
         }
+        // use the POW_2 lookup table when the bit size
+        if shift <= BitSize::<u256>::bits() - One::<u32>::one() {
+            return self / (*POW_2_256.span().at(shift)).try_into().expect('mul Overflow');
+        }
         let two = One::one() + One::one();
         self / two.pow(shift.try_into().expect('mul Overflow'))
     }
@@ -309,12 +322,23 @@ pub impl WrappingBitshiftImpl<
     +Into<T, u256>
 > of WrappingBitshift<T> {
     fn wrapping_shl(self: T, shift: usize) -> T {
+        if shift <= BitSize::<u256>::bits() - One::<u32>::one() {
+            let pow_2: u256 = (*POW_2_256.span().at(shift));
+            let pow2_mod_t: u256 = pow_2 % Bounded::<T>::MAX.into();
+            let (result, _) = self.overflowing_mul(pow2_mod_t.try_into().unwrap());
+            return result;
+        }
         let two = One::<T>::one() + One::<T>::one();
         let (result, _) = self.overflowing_mul(two.wrapping_pow(shift.try_into().unwrap()));
         result
     }
 
     fn wrapping_shr(self: T, shift: usize) -> T {
+        if shift <= BitSize::<u256>::bits() - One::<u32>::one() {
+            let pow_2: u256 = (*POW_2_256.span().at(shift));
+            let pow2_mod_t: u256 = pow_2 % Bounded::<T>::MAX.into();
+            return self / pow2_mod_t.try_into().unwrap();
+        }
         let two = One::<T>::one() + One::<T>::one();
 
         if shift > BitSize::<T>::bits() - One::one() {