1 files changed, 163 insertions, 0 deletions
diff --git a/compiler/rustc_serialize/src/leb128.rs b/compiler/rustc_serialize/src/leb128.rs
new file mode 100644
index 000000000..08b3c0542
--- /dev/null
+++ b/compiler/rustc_serialize/src/leb128.rs
@@ -0,0 +1,163 @@
+#![macro_use]
+
+macro_rules! max_leb128_len {
+    ($int_ty:ty) => {
+        // The longest LEB128 encoding for an integer uses 7 bits per byte.
+        (std::mem::size_of::<$int_ty>() * 8 + 6) / 7
+    };
+}
+
+// Returns the longest LEB128 encoding of all supported integer types.
+pub const fn max_leb128_len() -> usize {
+    max_leb128_len!(u128)
+}
+
+macro_rules! impl_write_unsigned_leb128 {
+    ($fn_name:ident, $int_ty:ty) => {
+        #[inline]
+        pub fn $fn_name(
+            out: &mut [::std::mem::MaybeUninit<u8>; max_leb128_len!($int_ty)],
+            mut value: $int_ty,
+        ) -> &[u8] {
+            let mut i = 0;
+
+            loop {
+                if value < 0x80 {
+                    unsafe {
+                        *out.get_unchecked_mut(i).as_mut_ptr() = value as u8;
+                    }
+
+                    i += 1;
+                    break;
+                } else {
+                    unsafe {
+                        *out.get_unchecked_mut(i).as_mut_ptr() = ((value & 0x7f) | 0x80) as u8;
+                    }
+
+                    value >>= 7;
+                    i += 1;
+                }
+            }
+
+            unsafe { ::std::mem::MaybeUninit::slice_assume_init_ref(&out.get_unchecked(..i)) }
+        }
+    };
+}
+
+impl_write_unsigned_leb128!(write_u16_leb128, u16);
+impl_write_unsigned_leb128!(write_u32_leb128, u32);
+impl_write_unsigned_leb128!(write_u64_leb128, u64);
+impl_write_unsigned_leb128!(write_u128_leb128, u128);
+impl_write_unsigned_leb128!(write_usize_leb128, usize);
+
+macro_rules! impl_read_unsigned_leb128 {
+    ($fn_name:ident, $int_ty:ty) => {
+        #[inline]
+        pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
+            // The first iteration of this loop is unpeeled. This is a
+            // performance win because this code is hot and integer values less
+            // than 128 are very common, typically occurring 50-80% or more of
+            // the time, even for u64 and u128.
+            let byte = slice[*position];
+            *position += 1;
+            if (byte & 0x80) == 0 {
+                return byte as $int_ty;
+            }
+            let mut result = (byte & 0x7F) as $int_ty;
+            let mut shift = 7;
+            loop {
+                let byte = slice[*position];
+                *position += 1;
+                if (byte & 0x80) == 0 {
+                    result |= (byte as $int_ty) << shift;
+                    return result;
+                } else {
+                    result |= ((byte & 0x7F) as $int_ty) << shift;
+                }
+                shift += 7;
+            }
+        }
+    };
+}
+
+impl_read_unsigned_leb128!(read_u16_leb128, u16);
+impl_read_unsigned_leb128!(read_u32_leb128, u32);
+impl_read_unsigned_leb128!(read_u64_leb128, u64);
+impl_read_unsigned_leb128!(read_u128_leb128, u128);
+impl_read_unsigned_leb128!(read_usize_leb128, usize);
+
+macro_rules! impl_write_signed_leb128 {
+    ($fn_name:ident, $int_ty:ty) => {
+        #[inline]
+        pub fn $fn_name(
+            out: &mut [::std::mem::MaybeUninit<u8>; max_leb128_len!($int_ty)],
+            mut value: $int_ty,
+        ) -> &[u8] {
+            let mut i = 0;
+
+            loop {
+                let mut byte = (value as u8) & 0x7f;
+                value >>= 7;
+                let more = !(((value == 0) && ((byte & 0x40) == 0))
+                    || ((value == -1) && ((byte & 0x40) != 0)));
+
+                if more {
+                    byte |= 0x80; // Mark this byte to show that more bytes will follow.
+                }
+
+                unsafe {
+                    *out.get_unchecked_mut(i).as_mut_ptr() = byte;
+                }
+
+                i += 1;
+
+                if !more {
+                    break;
+                }
+            }
+
+            unsafe { ::std::mem::MaybeUninit::slice_assume_init_ref(&out.get_unchecked(..i)) }
+        }
+    };
+}
+
+impl_write_signed_leb128!(write_i16_leb128, i16);
+impl_write_signed_leb128!(write_i32_leb128, i32);
+impl_write_signed_leb128!(write_i64_leb128, i64);
+impl_write_signed_leb128!(write_i128_leb128, i128);
+impl_write_signed_leb128!(write_isize_leb128, isize);
+
+macro_rules! impl_read_signed_leb128 {
+    ($fn_name:ident, $int_ty:ty) => {
+        #[inline]
+        pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
+            let mut result = 0;
+            let mut shift = 0;
+            let mut byte;
+
+            loop {
+                byte = slice[*position];
+                *position += 1;
+                result |= <$int_ty>::from(byte & 0x7F) << shift;
+                shift += 7;
+
+                if (byte & 0x80) == 0 {
+                    break;
+                }
+            }
+
+            if (shift < <$int_ty>::BITS) && ((byte & 0x40) != 0) {
+                // sign extend
+                result |= (!0 << shift);
+            }
+
+            result
+        }
+    };
+}
+
+impl_read_signed_leb128!(read_i16_leb128, i16);
+impl_read_signed_leb128!(read_i32_leb128, i32);
+impl_read_signed_leb128!(read_i64_leb128, i64);
+impl_read_signed_leb128!(read_i128_leb128, i128);
+impl_read_signed_leb128!(read_isize_leb128, isize);