diff --git a/library/core/src/num/imp/bignum.rs b/library/core/src/num/imp/bignum.rs
index 9e930ed15a234..95046e96fc8f2 100644
--- a/library/core/src/num/imp/bignum.rs
+++ b/library/core/src/num/imp/bignum.rs
@@ -1,7 +1,7 @@
 //! Custom arbitrary-precision number (bignum) implementation.
 //!
 //! This is designed to avoid the heap allocation at expense of stack memory.
-//! The most used bignum type, `Big32x40`, is limited by 32 × 40 = 1,280 bits
+//! The most used bignum type, `Big32x40` and  `Big64x20` you can choose, is limited by 32 × 40 = 1,280 bits
 //! and will take at most 160 bytes of stack memory. This is more than enough
 //! for round-tripping all possible finite `f64` values.
 //!
@@ -61,9 +61,10 @@ impl_full_ops! {
     u64: add(intrinsics::u64_add_with_overflow), mul/div(u128);
 }
 
-/// Table of powers of 5 representable in digits. Specifically, the largest {u8, u16, u32} value
+/// Table of powers of 5 representable in digits. Specifically, the largest {u8, u16, u32, u64} value
 /// that's a power of five, plus the corresponding exponent. Used in `mul_pow5`.
-const SMALL_POW5: [(u64, usize); 3] = [(125, 3), (15625, 6), (1_220_703_125, 13)];
+const SMALL_POW5: [(u64, usize); 4] =
+    [(125, 3), (15625, 6), (1_220_703_125, 13), (7_450_580_596_923_828_125, 27)];
 
 macro_rules! define_bignum {
     ($name:ident: type=$ty:ty, n=$n:expr) => {
@@ -100,7 +101,7 @@ macro_rules! define_bignum {
                 let mut sz = 0;
                 while v > 0 {
                     base[sz] = v as $ty;
-                    v >>= <$ty>::BITS;
+                    v = v.unbounded_shr(<$ty>::BITS);
                     sz += 1;
                 }
                 $name { size: sz, base }
@@ -382,11 +383,28 @@ macro_rules! define_bignum {
     };
 }
 
-/// The digit type for `Big32x40`.
+/// the digit type for `Big32x40`
 pub type Digit32 = u32;
 
+#[cfg(target_pointer_width = "32")]
 define_bignum!(Big32x40: type=Digit32, n=40);
 
+/// The digit type for `Big64x20`.
+pub type Digit64 = u64;
+
+#[cfg(target_pointer_width = "64")]
+define_bignum!(Big64x20: type=Digit64, n=20);
+
+#[cfg(target_pointer_width = "32")]
+pub type Big = Big32x40;
+#[cfg(target_pointer_width = "64")]
+pub type Big = Big64x20;
+
+#[cfg(target_pointer_width = "32")]
+pub type Digit = Digit32;
+#[cfg(target_pointer_width = "64")]
+pub type Digit = Digit64;
+
 // this one is used for testing only.
 #[doc(hidden)]
 pub mod tests {
diff --git a/library/core/src/num/imp/flt2dec/strategy/dragon.rs b/library/core/src/num/imp/flt2dec/strategy/dragon.rs
index de19ca6bb664d..4ae0d4b4ab0ba 100644
--- a/library/core/src/num/imp/flt2dec/strategy/dragon.rs
+++ b/library/core/src/num/imp/flt2dec/strategy/dragon.rs
@@ -9,24 +9,56 @@ use flt2dec::{Decoded, MAX_SIG_DIGITS, round_up};
 
 use crate::cmp::Ordering;
 use crate::mem::MaybeUninit;
-use crate::num::imp::bignum::{Big32x40 as Big, Digit32 as Digit};
+use crate::num::imp::bignum::{Big, Digit};
 use crate::num::imp::flt2dec;
 
 static POW10: [Digit; 10] =
     [1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000];
 // precalculated arrays of `Digit`s for 5^(2^n).
+// FIXME(#162879): these tables have u64 and u32 versions, future versions may be generated by macro.
+#[cfg(target_pointer_width = "32")]
 static POW5TO16: [Digit; 2] = [0x86f26fc1, 0x23];
+#[cfg(target_pointer_width = "64")]
+static POW5TO16: [Digit; 1] = [0x2386f26fc1];
+
+#[cfg(target_pointer_width = "32")]
 static POW5TO32: [Digit; 3] = [0x85acef81, 0x2d6d415b, 0x4ee];
+#[cfg(target_pointer_width = "64")]
+static POW5TO32: [Digit; 2] = [0x2d6d415b85acef81, 0x4ee];
+
+#[cfg(target_pointer_width = "32")]
 static POW5TO64: [Digit; 5] = [0xbf6a1f01, 0x6e38ed64, 0xdaa797ed, 0xe93ff9f4, 0x184f03];
+#[cfg(target_pointer_width = "64")]
+static POW5TO64: [Digit; 3] = [0x6e38ed64bf6a1f01, 0xe93ff9f4daa797ed, 0x184f03];
+
+#[cfg(target_pointer_width = "32")]
 static POW5TO128: [Digit; 10] = [
     0x2e953e01, 0x3df9909, 0xf1538fd, 0x2374e42f, 0xd3cff5ec, 0xc404dc08, 0xbccdb0da, 0xa6337f19,
     0xe91f2603, 0x24e,
 ];
+#[cfg(target_pointer_width = "64")]
+static POW5TO128: [Digit; 5] =
+    [0x3df99092e953e01, 0x2374e42f0f1538fd, 0xc404dc08d3cff5ec, 0xa6337f19bccdb0da, 0x24ee91f2603];
+
+#[cfg(target_pointer_width = "32")]
 static POW5TO256: [Digit; 19] = [
     0x982e7c01, 0xbed3875b, 0xd8d99f72, 0x12152f87, 0x6bde50c6, 0xcf4a6e70, 0xd595d80f, 0x26b2716e,
     0xadc666b0, 0x1d153624, 0x3c42d35a, 0x63ff540e, 0xcc5573c0, 0x65f9ef17, 0x55bc28f2, 0x80dcc7f7,
     0xf46eeddc, 0x5fdcefce, 0x553f7,
 ];
+#[cfg(target_pointer_width = "64")]
+static POW5TO256: [Digit; 10] = [
+    0xbed3875b982e7c01,
+    0x12152f87d8d99f72,
+    0xcf4a6e706bde50c6,
+    0x26b2716ed595d80f,
+    0x1d153624adc666b0,
+    0x63ff540e3c42d35a,
+    0x65f9ef17cc5573c0,
+    0x80dcc7f755bc28f2,
+    0x5fdcefcef46eeddc,
+    0x553f7,
+];
 
 #[doc(hidden)]
 pub fn mul_pow10(x: &mut Big, n: usize) -> &mut Big {
diff --git a/library/coretests/tests/num/bignum.rs b/library/coretests/tests/num/bignum.rs
index 2b4c30cc261cb..cabc88821a907 100644
--- a/library/coretests/tests/num/bignum.rs
+++ b/library/coretests/tests/num/bignum.rs
@@ -1,4 +1,4 @@
-use core::num::imp::bignum::Big32x40;
+use core::num::imp::bignum::Big as BigFull;
 use core::num::imp::bignum::tests::Big8x3 as Big;
 
 #[test]
@@ -216,27 +216,27 @@ fn test_bit_length() {
 }
 
 #[test]
-fn test_bit_length_32x40() {
+fn test_bit_length_big() {
     for i in 0..32 * 40 {
         // 010000...000
-        assert_eq!(Big32x40::from_small(1).mul_pow2(i).bit_length(), i + 1);
+        assert_eq!(BigFull::from_small(1).mul_pow2(i).bit_length(), i + 1);
     }
     for i in 1..32 * 40 - 1 {
         // 010000...001
         assert_eq!(
-            Big32x40::from_small(1).mul_pow2(i).add(&Big32x40::from_small(1)).bit_length(),
+            BigFull::from_small(1).mul_pow2(i).add(&BigFull::from_small(1)).bit_length(),
             i + 1
         );
         // 110000...000
-        assert_eq!(Big32x40::from_small(3).mul_pow2(i).bit_length(), i + 2);
+        assert_eq!(BigFull::from_small(3).mul_pow2(i).bit_length(), i + 2);
     }
-    assert_eq!(Big32x40::from_small(0).bit_length(), 0);
-    assert_eq!(Big32x40::from_small(1).bit_length(), 1);
-    assert_eq!(Big32x40::from_small(5).bit_length(), 3);
-    assert_eq!(Big32x40::from_small(0x18).bit_length(), 5);
-    assert_eq!(Big32x40::from_u64(0x4073).bit_length(), 15);
-    assert_eq!(Big32x40::from_u64(0xffffff).bit_length(), 24);
-    assert_eq!(Big32x40::from_u64(0xffffffffffffffff).bit_length(), 64);
+    assert_eq!(BigFull::from_small(0).bit_length(), 0);
+    assert_eq!(BigFull::from_small(1).bit_length(), 1);
+    assert_eq!(BigFull::from_small(5).bit_length(), 3);
+    assert_eq!(BigFull::from_small(0x18).bit_length(), 5);
+    assert_eq!(BigFull::from_u64(0x4073).bit_length(), 15);
+    assert_eq!(BigFull::from_u64(0xffffff).bit_length(), 24);
+    assert_eq!(BigFull::from_u64(0xffffffffffffffff).bit_length(), 64);
 }
 
 #[test]
diff --git a/library/coretests/tests/num/flt2dec/strategy/dragon.rs b/library/coretests/tests/num/flt2dec/strategy/dragon.rs
index 886f50064a503..395f8e082e655 100644
--- a/library/coretests/tests/num/flt2dec/strategy/dragon.rs
+++ b/library/coretests/tests/num/flt2dec/strategy/dragon.rs
@@ -1,4 +1,4 @@
-use core::num::imp::bignum::Big32x40 as Big;
+use core::num::imp::bignum::Big;
 use core::num::imp::flt2dec::strategy::dragon::*;
 
 use super::super::*;