diff --git a/AK/FPControl.h b/AK/FPControl.h
index 6639338aba..b5cf575b1d 100644
--- a/AK/FPControl.h
+++ b/AK/FPControl.h
@@ -13,7 +13,7 @@ VALIDATE_IS_X86();
 
 namespace AK {
 
-enum class RoundingMode : u8 {
+enum class RoundingMode : u16 {
     NEAREST = 0b00,
     DOWN = 0b01,
     UP = 0b10,
diff --git a/AK/FloatingPoint.h b/AK/FloatingPoint.h
index 77ee165b2e..1f3346dcfa 100644
--- a/AK/FloatingPoint.h
+++ b/AK/FloatingPoint.h
@@ -22,16 +22,16 @@ union FloatExtractor<f128> {
     static constexpr int exponent_bias = 16383;
     static constexpr int exponent_bits = 15;
     static constexpr unsigned exponent_max = 32767;
-    struct {
+    struct [[gnu::packed]] {
         unsigned __int128 mantissa : 112;
-        unsigned exponent : 15;
-        unsigned sign : 1;
+        unsigned __int128 exponent : 15;
+        unsigned __int128 sign : 1;
     };
     f128 d;
 };
 // Validate that f128 and the FloatExtractor union are 128 bits.
-static_assert(sizeof(f128) == 16);
-static_assert(sizeof(FloatExtractor<f128>) == 16);
+static_assert(AssertSize<f128, 16>());
+static_assert(AssertSize<FloatExtractor<f128>, sizeof(f128)>());
 #endif
 
 #ifdef AK_HAS_FLOAT_80
@@ -42,13 +42,14 @@ union FloatExtractor<f80> {
     static constexpr int exponent_bias = 16383;
     static constexpr int exponent_bits = 15;
     static constexpr unsigned exponent_max = 32767;
-    struct {
+    struct [[gnu::packed]] {
         unsigned long long mantissa;
-        unsigned exponent : 15;
-        unsigned sign : 1;
+        unsigned long long exponent : 15;
+        unsigned long long sign : 1;
     };
     f80 d;
 };
+static_assert(AssertSize<FloatExtractor<f80>, sizeof(f80)>());
 #endif
 
 template<>
@@ -58,13 +59,21 @@ union FloatExtractor<f64> {
     static constexpr int exponent_bias = 1023;
     static constexpr int exponent_bits = 11;
     static constexpr unsigned exponent_max = 2047;
-    struct {
+    struct [[gnu::packed]] {
+        // FIXME: These types have to all be the same, otherwise this struct
+        //        goes from being a bitfield describing the layout of an f64
+        //        into being a multibyte mess on windows.
+        //        Technically, '-mno-ms-bitfields' is supposed to disable this
+        //        very intuitive and portable behaviour on windows, but it doesn't
+        //        work with the msvc ABI.
+        //        See <https://github.com/llvm/llvm-project/issues/24757>
         unsigned long long mantissa : 52;
-        unsigned exponent : 11;
-        unsigned sign : 1;
+        unsigned long long exponent : 11;
+        unsigned long long sign : 1;
     };
     f64 d;
 };
+static_assert(AssertSize<FloatExtractor<f64>, sizeof(f64)>());
 
 template<>
 union FloatExtractor<f32> {
@@ -73,13 +82,14 @@ union FloatExtractor<f32> {
     static constexpr int exponent_bias = 127;
     static constexpr int exponent_bits = 8;
     static constexpr unsigned exponent_max = 255;
-    struct {
-        unsigned long long mantissa : 23;
+    struct [[gnu::packed]] {
+        unsigned mantissa : 23;
         unsigned exponent : 8;
         unsigned sign : 1;
     };
     f32 d;
 };
+static_assert(AssertSize<FloatExtractor<f32>, sizeof(f32)>());
 
 template<size_t S, size_t E, size_t M>
 requires(S <= 1 && E >= 1 && M >= 1 && (S + E + M) <= 64) class FloatingPointBits final {