Optimize VarInt decoding

[Matthias247]

Upfront: This isn't the most elegant change. But given VarInts sit in the
hot path it seems worth it.

The current VarInt decoding performs some duplicate bounds check.
VarInt::decode checks for available bytes in order to return an error.
And bytes::Buf performs additional bounds checks in various helper methods.

By using Buf::chunk and Buf::advance directly we can save some of those.
We could improve even more by only support decoding from contiguous buffers,
but that would change the API contract.

In addition it seemed like the u16 be->le conversion wasn't as free
as it should be. A 2byte LE buffer improves things here.

Overall this reduces CPU time spent in VarInt decoding from about 2% to around 1%.

[Matthias247]

For anyone who is more curious. Here is a baseline flamegraph. "Matched" is the amount of samples in VarInt::decode (2.6%).
Inside the method we can also see 0.68% time spent in swap_bytes.

Here is the changed version, with 1.2% of samples in VarInt::decode:

[Ralith]

Should we have a microbenchmark for this path?

[Ralith]

Does this specific change actually help? I wouldn't expect this to have any difference in bounds checking semantics.

[Matthias247]

There's an assert! in the get_u8, not just a rust array bounds check. I don't think those are removed. And since even without the "le" change this proved to be faster, I guess practical experience shows that it helps.

[djc]

Uh, left this sitting around for a long time. @Matthias247 what do you think?

[djc]

Could we do this without unsafe if we use <&[u8; 1]>::try_from() to extract an array out of the bounds check?

Along these lines:

diff --git a/quinn-proto/src/varint.rs b/quinn-proto/src/varint.rs
index 7f57bc3c..8f4b9b7c 100644
--- a/quinn-proto/src/varint.rs
+++ b/quinn-proto/src/varint.rs
@@ -1,4 +1,4 @@
-use std::{convert::TryInto, fmt};
+use std::{convert::{TryInto, TryFrom}, fmt};
 
 use bytes::{Buf, BufMut};
 use thiserror::Error;
@@ -146,38 +146,37 @@ pub struct VarIntBoundsExceeded;
 
 impl Codec for VarInt {
     fn decode<B: Buf>(r: &mut B) -> coding::Result<Self> {
-        if !r.has_remaining() {
-            return Err(UnexpectedEnd);
-        }
         let mut buf = [0; 8];
-        buf[0] = r.get_u8();
+        let input = r.chunk();
+        buf[0] = match input.get(0) {
+            Some(&b) => b,
+            None => return Err(UnexpectedEnd),
+        };
+
         let tag = buf[0] >> 6;
         buf[0] &= 0b0011_1111;
-        let x = match tag {
-            0b00 => u64::from(buf[0]),
+        let (x, len) = match tag {
+            0b00 => (u64::from(buf[0]), 1),
             0b01 => {
-                if r.remaining() < 1 {
-                    return Err(UnexpectedEnd);
-                }
-                r.copy_to_slice(&mut buf[1..2]);
-                u64::from(u16::from_be_bytes(buf[..2].try_into().unwrap()))
+                let arr = [0, buf[0]];
+                let read = <&[u8; 1]>::try_from(&input[1..]).map_err(|_| UnexpectedEnd)?;
+                let arr = [read[0], buf[0]];
+                (u64::from(u16::from_le_bytes(arr)), 2)
             }
             0b10 => {
-                if r.remaining() < 3 {
-                    return Err(UnexpectedEnd);
-                }
-                r.copy_to_slice(&mut buf[1..4]);
-                u64::from(u32::from_be_bytes(buf[..4].try_into().unwrap()))
+                let read = <&[u8; 3]>::try_from(&input[1..]).map_err(|_| UnexpectedEnd)?;
+                let arr = [read[2], read[1], read[0], buf[0]];
+                (u64::from(u32::from_le_bytes(arr)), 4)
             }
             0b11 => {
-                if r.remaining() < 7 {
-                    return Err(UnexpectedEnd);
-                }
-                r.copy_to_slice(&mut buf[1..8]);
-                u64::from_be_bytes(buf)
+                let read = <&[u8; 7]>::try_from(&input[1..]).map_err(|_| UnexpectedEnd)?;
+                let arr = [read[6], read[5], read[4], read[3], read[2], read[1], read[0], buf[0]];
+                (u64::from(u64::from_le_bytes(arr)), 8)
             }
             _ => unreachable!(),
         };
+
+        r.advance(len);
         Ok(VarInt(x))
     }