Improve widening mul operations' performance on wasm (#828)

Co-authored-by: Marcin <[email protected]>

ff/src/biginteger/arithmetic.rs

@@ -82,12 +82,34 @@ pub fn sbb_for_sub_with_borrow(a: &mut u64, b: u64, borrow: u8) -> u8 {
     }
 }
 
+#[inline(always)]
+#[doc(hidden)]
+pub const fn widening_mul(a: u64, b: u64) -> u128 {
+    #[cfg(not(target_family = "wasm"))]
+    {
+        a as u128 * b as u128
+    }
+    #[cfg(target_family = "wasm")]
+    {
+        let a_lo = a as u32 as u64;
+        let a_hi = a >> 32;
+        let b_lo = b as u32 as u64;
+        let b_hi = b >> 32;
+
+        let lolo = (a_lo * b_lo) as u128;
+        let lohi = ((a_lo * b_hi) as u128) << 32;
+        let hilo = ((a_hi * b_lo) as u128) << 32;
+        let hihi = ((a_hi * b_hi) as u128) << 64;
+        (lolo | hihi) + (lohi + hilo)
+    }
+}
+
 /// Calculate a + b * c, returning the lower 64 bits of the result and setting
 /// `carry` to the upper 64 bits.
 #[inline(always)]
 #[doc(hidden)]
 pub fn mac(a: u64, b: u64, c: u64, carry: &mut u64) -> u64 {
-    let tmp = (a as u128) + (b as u128 * c as u128);
+    let tmp = (a as u128) + widening_mul(b, c);
     *carry = (tmp >> 64) as u64;
     tmp as u64
 }
@@ -97,21 +119,22 @@ pub fn mac(a: u64, b: u64, c: u64, carry: &mut u64) -> u64 {
 #[inline(always)]
 #[doc(hidden)]
 pub fn mac_discard(a: u64, b: u64, c: u64, carry: &mut u64) {
-    let tmp = (a as u128) + (b as u128 * c as u128);
+    let tmp = (a as u128) + widening_mul(b, c);
     *carry = (tmp >> 64) as u64;
 }
 
 macro_rules! mac_with_carry {
     ($a:expr, $b:expr, $c:expr, &mut $carry:expr$(,)?) => {{
-        let tmp = ($a as u128) + ($b as u128 * $c as u128) + ($carry as u128);
+        let tmp =
+            ($a as u128) + $crate::biginteger::arithmetic::widening_mul($b, $c) + ($carry as u128);
         $carry = (tmp >> 64) as u64;
         tmp as u64
     }};
 }
 
 macro_rules! mac {
     ($a:expr, $b:expr, $c:expr, &mut $carry:expr$(,)?) => {{
-        let tmp = ($a as u128) + ($b as u128 * $c as u128);
+        let tmp = ($a as u128) + $crate::biginteger::arithmetic::widening_mul($b, $c);
         $carry = (tmp >> 64) as u64;
         tmp as u64
     }};
@@ -122,7 +145,7 @@ macro_rules! mac {
 #[inline(always)]
 #[doc(hidden)]
 pub fn mac_with_carry(a: u64, b: u64, c: u64, carry: &mut u64) -> u64 {
-    let tmp = (a as u128) + (b as u128 * c as u128) + (*carry as u128);
+    let tmp = (a as u128) + widening_mul(b, c) + (*carry as u128);
     *carry = (tmp >> 64) as u64;
     tmp as u64
 }