Auto merge of #110634 - saethlin:pointy-decoder, r=cjgillot

Rewrite MemDecoder around pointers not a slice

This is basically #109910 but I'm being a lot more aggressive. The pointer-based structure means that it makes a lot more sense to absorb more complexity into `MemDecoder`, most of the diff is just complexity moving from one place to another.

The primary argument for this structure is that we only incur a single bounds check when doing multi-byte reads from a `MemDecoder`. With the slice-based implementation we need to do those with `data[position..position + len]` , which needs to account for `position + len` wrapping. It would be possible to dodge the first bounds check if we stored a slice that starts at `position`, but that would require updating the pointer and length on every read.

This PR also embeds the failure path in a separate function, which means that this PR should subsume all the perf wins observed in #109867.

Author: bors

Diff for: compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -373,16 +373,6 @@ impl<'a, 'tcx> TyDecoder for DecodeContext<'a, 'tcx> {
         self.tcx()
     }
 
-    #[inline]
-    fn peek_byte(&self) -> u8 {
-        self.opaque.data[self.opaque.position()]
-    }
-
-    #[inline]
-    fn position(&self) -> usize {
-        self.opaque.position()
-    }
-
     fn cached_ty_for_shorthand<F>(&mut self, shorthand: usize, or_insert_with: F) -> Ty<'tcx>
     where
         F: FnOnce(&mut Self) -> Ty<'tcx>,
@@ -404,7 +394,7 @@ impl<'a, 'tcx> TyDecoder for DecodeContext<'a, 'tcx> {
     where
         F: FnOnce(&mut Self) -> R,
     {
-        let new_opaque = MemDecoder::new(self.opaque.data, pos);
+        let new_opaque = MemDecoder::new(self.opaque.data(), pos);
         let old_opaque = mem::replace(&mut self.opaque, new_opaque);
         let old_state = mem::replace(&mut self.lazy_state, LazyState::NoNode);
         let r = f(self);
@@ -625,17 +615,12 @@ impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for Symbol {
             SYMBOL_OFFSET => {
                 // read str offset
                 let pos = d.read_usize();
-                let old_pos = d.opaque.position();
 
                 // move to str offset and read
-                d.opaque.set_position(pos);
-                let s = d.read_str();
-                let sym = Symbol::intern(s);
-
-                // restore position
-                d.opaque.set_position(old_pos);
-
-                sym
+                d.opaque.with_position(pos, |d| {
+                    let s = d.read_str();
+                    Symbol::intern(s)
+                })
             }
             SYMBOL_PREINTERNED => {
                 let symbol_index = d.read_u32();

Diff for: compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs

@@ -45,9 +45,6 @@ impl<'a, 'tcx> Encodable<EncodeContext<'a, 'tcx>> for DefPathHashMapRef<'tcx> {
 
 impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for DefPathHashMapRef<'static> {
     fn decode(d: &mut DecodeContext<'a, 'tcx>) -> DefPathHashMapRef<'static> {
-        // Import TyDecoder so we can access the DecodeContext::position() method
-        use crate::rustc_middle::ty::codec::TyDecoder;
-
         let len = d.read_usize();
         let pos = d.position();
         let o = slice_owned(d.blob().clone(), |blob| &blob[pos..pos + len]);

Diff for: compiler/rustc_middle/src/ty/codec.rs

@@ -530,6 +530,16 @@ macro_rules! implement_ty_decoder {
                 fn read_raw_bytes(&mut self, len: usize) -> &[u8] {
                     self.opaque.read_raw_bytes(len)
                 }
+
+                #[inline]
+                fn position(&self) -> usize {
+                    self.opaque.position()
+                }
+
+                #[inline]
+                fn peek_byte(&self) -> u8 {
+                    self.opaque.peek_byte()
+                }
             }
         }
     }

Diff for: compiler/rustc_query_impl/src/on_disk_cache.rs

@@ -169,13 +169,12 @@ impl<'sess> rustc_middle::ty::OnDiskCache<'sess> for OnDiskCache<'sess> {
 
             // Decode the *position* of the footer, which can be found in the
             // last 8 bytes of the file.
-            decoder.set_position(data.len() - IntEncodedWithFixedSize::ENCODED_SIZE);
-            let footer_pos = IntEncodedWithFixedSize::decode(&mut decoder).0 as usize;
-
+            let footer_pos = decoder
+                .with_position(decoder.len() - IntEncodedWithFixedSize::ENCODED_SIZE, |decoder| {
+                    IntEncodedWithFixedSize::decode(decoder).0 as usize
+                });
             // Decode the file footer, which contains all the lookup tables, etc.
-            decoder.set_position(footer_pos);
-
-            decode_tagged(&mut decoder, TAG_FILE_FOOTER)
+            decoder.with_position(footer_pos, |decoder| decode_tagged(decoder, TAG_FILE_FOOTER))
         };
 
         Self {
@@ -522,29 +521,13 @@ impl<'a, 'tcx> CacheDecoder<'a, 'tcx> {
     }
 }
 
-trait DecoderWithPosition: Decoder {
-    fn position(&self) -> usize;
-}
-
-impl<'a> DecoderWithPosition for MemDecoder<'a> {
-    fn position(&self) -> usize {
-        self.position()
-    }
-}
-
-impl<'a, 'tcx> DecoderWithPosition for CacheDecoder<'a, 'tcx> {
-    fn position(&self) -> usize {
-        self.opaque.position()
-    }
-}
-
 // Decodes something that was encoded with `encode_tagged()` and verify that the
 // tag matches and the correct amount of bytes was read.
 fn decode_tagged<D, T, V>(decoder: &mut D, expected_tag: T) -> V
 where
     T: Decodable<D> + Eq + std::fmt::Debug,
     V: Decodable<D>,
-    D: DecoderWithPosition,
+    D: Decoder,
 {
     let start_pos = decoder.position();
 
@@ -568,16 +551,6 @@ impl<'a, 'tcx> TyDecoder for CacheDecoder<'a, 'tcx> {
         self.tcx
     }
 
-    #[inline]
-    fn position(&self) -> usize {
-        self.opaque.position()
-    }
-
-    #[inline]
-    fn peek_byte(&self) -> u8 {
-        self.opaque.data[self.opaque.position()]
-    }
-
     fn cached_ty_for_shorthand<F>(&mut self, shorthand: usize, or_insert_with: F) -> Ty<'tcx>
     where
         F: FnOnce(&mut Self) -> Ty<'tcx>,
@@ -600,9 +573,9 @@ impl<'a, 'tcx> TyDecoder for CacheDecoder<'a, 'tcx> {
     where
         F: FnOnce(&mut Self) -> R,
     {
-        debug_assert!(pos < self.opaque.data.len());
+        debug_assert!(pos < self.opaque.len());
 
-        let new_opaque = MemDecoder::new(self.opaque.data, pos);
+        let new_opaque = MemDecoder::new(self.opaque.data(), pos);
         let old_opaque = mem::replace(&mut self.opaque, new_opaque);
         let r = f(self);
         self.opaque = old_opaque;
@@ -743,17 +716,12 @@ impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for Symbol {
             SYMBOL_OFFSET => {
                 // read str offset
                 let pos = d.read_usize();
-                let old_pos = d.opaque.position();
 
                 // move to str offset and read
-                d.opaque.set_position(pos);
-                let s = d.read_str();
-                let sym = Symbol::intern(s);
-
-                // restore position
-                d.opaque.set_position(old_pos);
-
-                sym
+                d.opaque.with_position(pos, |d| {
+                    let s = d.read_str();
+                    Symbol::intern(s)
+                })
             }
             SYMBOL_PREINTERNED => {
                 let symbol_index = d.read_u32();

Diff for: compiler/rustc_query_system/src/dep_graph/serialized.rs

@@ -94,21 +94,19 @@ impl<'a, K: DepKind + Decodable<MemDecoder<'a>>> Decodable<MemDecoder<'a>>
 {
     #[instrument(level = "debug", skip(d))]
     fn decode(d: &mut MemDecoder<'a>) -> SerializedDepGraph<K> {
-        let start_position = d.position();
-
         // The last 16 bytes are the node count and edge count.
         debug!("position: {:?}", d.position());
-        d.set_position(d.data.len() - 2 * IntEncodedWithFixedSize::ENCODED_SIZE);
+        let (node_count, edge_count) =
+            d.with_position(d.len() - 2 * IntEncodedWithFixedSize::ENCODED_SIZE, |d| {
+                debug!("position: {:?}", d.position());
+                let node_count = IntEncodedWithFixedSize::decode(d).0 as usize;
+                let edge_count = IntEncodedWithFixedSize::decode(d).0 as usize;
+                (node_count, edge_count)
+            });
         debug!("position: {:?}", d.position());
 
-        let node_count = IntEncodedWithFixedSize::decode(d).0 as usize;
-        let edge_count = IntEncodedWithFixedSize::decode(d).0 as usize;
         debug!(?node_count, ?edge_count);
 
-        debug!("position: {:?}", d.position());
-        d.set_position(start_position);
-        debug!("position: {:?}", d.position());
-
         let mut nodes = IndexVec::with_capacity(node_count);
         let mut fingerprints = IndexVec::with_capacity(node_count);
         let mut edge_list_indices = IndexVec::with_capacity(node_count);

Diff for: compiler/rustc_serialize/src/leb128.rs

@@ -1,3 +1,6 @@
+use crate::opaque::MemDecoder;
+use crate::serialize::Decoder;
+
 /// Returns the length of the longest LEB128 encoding for `T`, assuming `T` is an integer type
 pub const fn max_leb128_len<T>() -> usize {
     // The longest LEB128 encoding for an integer uses 7 bits per byte.
@@ -50,21 +53,19 @@ 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 {
+        pub fn $fn_name(decoder: &mut MemDecoder<'_>) -> $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;
+            let byte = decoder.read_u8();
             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;
+                let byte = decoder.read_u8();
                 if (byte & 0x80) == 0 {
                     result |= (byte as $int_ty) << shift;
                     return result;
@@ -127,14 +128,13 @@ impl_write_signed_leb128!(write_isize_leb128, isize);
 macro_rules! impl_read_signed_leb128 {
     ($fn_name:ident, $int_ty:ty) => {
         #[inline]
-        pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
+        pub fn $fn_name(decoder: &mut MemDecoder<'_>) -> $int_ty {
             let mut result = 0;
             let mut shift = 0;
             let mut byte;
 
             loop {
-                byte = slice[*position];
-                *position += 1;
+                byte = decoder.read_u8();
                 result |= <$int_ty>::from(byte & 0x7F) << shift;
                 shift += 7;
 

Diff for: compiler/rustc_serialize/src/lib.rs

@@ -16,6 +16,7 @@ Core encoding and decoding interfaces.
 #![feature(maybe_uninit_slice)]
 #![feature(new_uninit)]
 #![feature(allocator_api)]
+#![feature(ptr_sub_ptr)]
 #![cfg_attr(test, feature(test))]
 #![allow(rustc::internal)]
 #![deny(rustc::untranslatable_diagnostic)]