optimize: pow if exp is statically known

Copy the optimization that unrolls the loop from `pow` to `checked_pow`
and `overflowing_pow`.

Author: Kmeakin

library/core/src/num/uint_macros.rs

@@ -2091,6 +2091,22 @@ macro_rules! uint_impl {
             let mut base = self;
             let mut acc: Self = 1;
 
+            if intrinsics::is_val_statically_known(exp) {
+                while exp > 1 {
+                    if (exp & 1) == 1 {
+                        acc = try_opt!(acc.checked_mul(base));
+                    }
+                    exp /= 2;
+                    base = try_opt!(base.checked_mul(base));
+                }
+
+                // since exp!=0, finally the exp must be 1.
+                // Deal with the final bit of the exponent separately, since
+                // squaring the base afterwards is not necessary and may cause a
+                // needless overflow.
+                return acc.checked_mul(base);
+            }
+
             loop {
                 if (exp & 1) == 1 {
                     acc = try_opt!(acc.checked_mul(base));
@@ -3247,6 +3263,26 @@ macro_rules! uint_impl {
             let mut overflow = false;
             let mut tmp_overflow;
 
+            if intrinsics::is_val_statically_known(exp) {
+                while exp > 1 {
+                    if (exp & 1) == 1 {
+                        (acc, tmp_overflow) = acc.overflowing_mul(base);
+                        overflow |= tmp_overflow;
+                    }
+                    exp /= 2;
+                    (base, tmp_overflow) = base.overflowing_mul(base);
+                    overflow |= tmp_overflow;
+                }
+
+                // since exp!=0, finally the exp must be 1.
+                // Deal with the final bit of the exponent separately, since
+                // squaring the base afterwards is not necessary and may cause a
+                // needless overflow.
+                (acc, tmp_overflow) = acc.overflowing_mul(base);
+                overflow |= tmp_overflow;
+                return (acc, overflow);
+            }
+
             loop {
                 if (exp & 1) == 1 {
                     (acc, tmp_overflow) = acc.overflowing_mul(base);