Auto merge of rust-lang#110083 - saethlin:encode-hashes-as-bytes, r=cjgillot

Encode hashes as bytes, not varint

In a few places, we store hashes as `u64` or `u128` and then apply `derive(Decodable, Encodable)` to the enclosing struct/enum. It is more efficient to encode hashes directly than try to apply some varint encoding. This PR adds two new types `Hash64` and `Hash128` which are produced by `StableHasher` and replace every use of storing a `u64` or `u128` that represents a hash.

Distribution of the byte lengths of leb128 encodings, from `x build --stage 2` with `incremental = true`

Before:
```
(  1) 373418203 (53.7%, 53.7%): 1
(  2) 196240113 (28.2%, 81.9%): 3
(  3) 108157958 (15.6%, 97.5%): 2
(  4)  17213120 ( 2.5%, 99.9%): 4
(  5)    223614 ( 0.0%,100.0%): 9
(  6)    216262 ( 0.0%,100.0%): 10
(  7)     15447 ( 0.0%,100.0%): 5
(  8)      3633 ( 0.0%,100.0%): 19
(  9)      3030 ( 0.0%,100.0%): 8
( 10)      1167 ( 0.0%,100.0%): 18
( 11)      1032 ( 0.0%,100.0%): 7
( 12)      1003 ( 0.0%,100.0%): 6
( 13)        10 ( 0.0%,100.0%): 16
( 14)        10 ( 0.0%,100.0%): 17
( 15)         5 ( 0.0%,100.0%): 12
( 16)         4 ( 0.0%,100.0%): 14
```

After:
```
(  1) 372939136 (53.7%, 53.7%): 1
(  2) 196240140 (28.3%, 82.0%): 3
(  3) 108014969 (15.6%, 97.5%): 2
(  4)  17192375 ( 2.5%,100.0%): 4
(  5)       435 ( 0.0%,100.0%): 5
(  6)        83 ( 0.0%,100.0%): 18
(  7)        79 ( 0.0%,100.0%): 10
(  8)        50 ( 0.0%,100.0%): 9
(  9)         6 ( 0.0%,100.0%): 19
```

The remaining 9 or 10 and 18 or 19 are `u64` and `u128` respectively that have the high bits set. As far as I can tell these are coming primarily from `SwitchTargets`.

Author: bors

compiler/rustc_abi/src/layout.rs

@@ -79,7 +79,8 @@ pub trait LayoutCalculator {
                 {
                     // `ReprOptions.layout_seed` is a deterministic seed that we can use to
                     // randomize field ordering with
-                    let mut rng = Xoshiro128StarStar::seed_from_u64(repr.field_shuffle_seed);
+                    let mut rng =
+                        Xoshiro128StarStar::seed_from_u64(repr.field_shuffle_seed.as_u64());
 
                     // Shuffle the ordering of the fields
                     optimizing.shuffle(&mut rng);

compiler/rustc_abi/src/lib.rs

@@ -9,6 +9,7 @@ use std::str::FromStr;
 
 use bitflags::bitflags;
 use rustc_data_structures::intern::Interned;
+use rustc_data_structures::stable_hasher::Hash64;
 #[cfg(feature = "nightly")]
 use rustc_data_structures::stable_hasher::StableOrd;
 use rustc_index::vec::{IndexSlice, IndexVec};
@@ -77,12 +78,12 @@ pub struct ReprOptions {
     pub flags: ReprFlags,
     /// The seed to be used for randomizing a type's layout
     ///
-    /// Note: This could technically be a `[u8; 16]` (a `u128`) which would
+    /// Note: This could technically be a `Hash128` which would
     /// be the "most accurate" hash as it'd encompass the item and crate
     /// hash without loss, but it does pay the price of being larger.
-    /// Everything's a tradeoff, a `u64` seed should be sufficient for our
+    /// Everything's a tradeoff, a 64-bit seed should be sufficient for our
     /// purposes (primarily `-Z randomize-layout`)
-    pub field_shuffle_seed: u64,
+    pub field_shuffle_seed: Hash64,
 }
 
 impl ReprOptions {

compiler/rustc_codegen_llvm/src/common.rs

@@ -10,7 +10,7 @@ use crate::value::Value;
 use rustc_ast::Mutability;
 use rustc_codegen_ssa::mir::place::PlaceRef;
 use rustc_codegen_ssa::traits::*;
-use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::stable_hasher::{Hash128, HashStable, StableHasher};
 use rustc_hir::def_id::DefId;
 use rustc_middle::bug;
 use rustc_middle::mir::interpret::{ConstAllocation, GlobalAlloc, Scalar};
@@ -261,7 +261,7 @@ impl<'ll, 'tcx> ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
                             let hash = self.tcx.with_stable_hashing_context(|mut hcx| {
                                 let mut hasher = StableHasher::new();
                                 alloc.hash_stable(&mut hcx, &mut hasher);
-                                hasher.finish::<u128>()
+                                hasher.finish::<Hash128>()
                             });
                             llvm::set_value_name(value, format!("alloc_{hash:032x}").as_bytes());
                         }

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -21,6 +21,7 @@ use rustc_codegen_ssa::debuginfo::type_names;
 use rustc_codegen_ssa::mir::debuginfo::{DebugScope, FunctionDebugContext, VariableKind};
 use rustc_codegen_ssa::traits::*;
 use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::stable_hasher::Hash128;
 use rustc_data_structures::sync::Lrc;
 use rustc_hir::def_id::{DefId, DefIdMap};
 use rustc_index::vec::IndexVec;
@@ -61,7 +62,7 @@ pub struct CodegenUnitDebugContext<'ll, 'tcx> {
     llcontext: &'ll llvm::Context,
     llmod: &'ll llvm::Module,
     builder: &'ll mut DIBuilder<'ll>,
-    created_files: RefCell<FxHashMap<Option<(u128, SourceFileHash)>, &'ll DIFile>>,
+    created_files: RefCell<FxHashMap<Option<(Hash128, SourceFileHash)>, &'ll DIFile>>,
 
     type_map: metadata::TypeMap<'ll, 'tcx>,
     namespace_map: RefCell<DefIdMap<&'ll DIScope>>,

compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs

@@ -12,7 +12,7 @@
 // * `"` is treated as the start of a string.
 
 use rustc_data_structures::fx::FxHashSet;
-use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::stable_hasher::{Hash64, HashStable, StableHasher};
 use rustc_hir::def_id::DefId;
 use rustc_hir::definitions::{DefPathData, DefPathDataName, DisambiguatedDefPathData};
 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Mutability};
@@ -675,8 +675,7 @@ fn push_const_param<'tcx>(tcx: TyCtxt<'tcx>, ct: ty::Const<'tcx>, output: &mut S
                     hcx.while_hashing_spans(false, |hcx| {
                         ct.to_valtree().hash_stable(hcx, &mut hasher)
                     });
-                    let hash: u64 = hasher.finish();
-                    hash
+                    hasher.finish::<Hash64>()
                 });
 
                 if cpp_like_debuginfo(tcx) {

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -75,7 +75,7 @@ pub(crate) fn eval_nullary_intrinsic<'tcx>(
         }
         sym::type_id => {
             ensure_monomorphic_enough(tcx, tp_ty)?;
-            ConstValue::from_u64(tcx.type_id_hash(tp_ty))
+            ConstValue::from_u64(tcx.type_id_hash(tp_ty).as_u64())
         }
         sym::variant_count => match tp_ty.kind() {
             // Correctly handles non-monomorphic calls, so there is no need for ensure_monomorphic_enough.

compiler/rustc_data_structures/src/fingerprint.rs

@@ -1,4 +1,4 @@
-use crate::stable_hasher;
+use crate::stable_hasher::{Hash64, StableHasher, StableHasherResult};
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use std::hash::{Hash, Hasher};
 
@@ -9,32 +9,49 @@ mod tests;
 #[repr(C)]
 pub struct Fingerprint(u64, u64);
 
-impl Fingerprint {
-    pub const ZERO: Fingerprint = Fingerprint(0, 0);
+pub trait FingerprintComponent {
+    fn as_u64(&self) -> u64;
+}
 
+impl FingerprintComponent for Hash64 {
     #[inline]
-    pub fn new(_0: u64, _1: u64) -> Fingerprint {
-        Fingerprint(_0, _1)
+    fn as_u64(&self) -> u64 {
+        Hash64::as_u64(*self)
+    }
+}
+
+impl FingerprintComponent for u64 {
+    #[inline]
+    fn as_u64(&self) -> u64 {
+        *self
     }
+}
+
+impl Fingerprint {
+    pub const ZERO: Fingerprint = Fingerprint(0, 0);
 
     #[inline]
-    pub fn from_smaller_hash(hash: u64) -> Fingerprint {
-        Fingerprint(hash, hash)
+    pub fn new<A, B>(_0: A, _1: B) -> Fingerprint
+    where
+        A: FingerprintComponent,
+        B: FingerprintComponent,
+    {
+        Fingerprint(_0.as_u64(), _1.as_u64())
     }
 
     #[inline]
-    pub fn to_smaller_hash(&self) -> u64 {
+    pub fn to_smaller_hash(&self) -> Hash64 {
         // Even though both halves of the fingerprint are expected to be good
         // quality hash values, let's still combine the two values because the
         // Fingerprints in DefPathHash have the StableCrateId portion which is
         // the same for all DefPathHashes from the same crate. Combining the
         // two halves makes sure we get a good quality hash in such cases too.
-        self.0.wrapping_mul(3).wrapping_add(self.1)
+        Hash64::new(self.0.wrapping_mul(3).wrapping_add(self.1))
     }
 
     #[inline]
-    pub fn as_value(&self) -> (u64, u64) {
-        (self.0, self.1)
+    pub fn split(&self) -> (Hash64, Hash64) {
+        (Hash64::new(self.0), Hash64::new(self.1))
     }
 
     #[inline]
@@ -131,9 +148,9 @@ impl FingerprintHasher for crate::unhash::Unhasher {
     }
 }
 
-impl stable_hasher::StableHasherResult for Fingerprint {
+impl StableHasherResult for Fingerprint {
     #[inline]
-    fn finish(hasher: stable_hasher::StableHasher) -> Self {
+    fn finish(hasher: StableHasher) -> Self {
         let (_0, _1) = hasher.finalize();
         Fingerprint(_0, _1)
     }

compiler/rustc_data_structures/src/fingerprint/tests.rs

@@ -1,11 +1,12 @@
 use super::*;
+use crate::stable_hasher::Hash64;
 
 // Check that `combine_commutative` is order independent.
 #[test]
 fn combine_commutative_is_order_independent() {
-    let a = Fingerprint::new(0xf6622fb349898b06, 0x70be9377b2f9c610);
-    let b = Fingerprint::new(0xa9562bf5a2a5303c, 0x67d9b6c82034f13d);
-    let c = Fingerprint::new(0x0d013a27811dbbc3, 0x9a3f7b3d9142ec43);
+    let a = Fingerprint::new(Hash64::new(0xf6622fb349898b06), Hash64::new(0x70be9377b2f9c610));
+    let b = Fingerprint::new(Hash64::new(0xa9562bf5a2a5303c), Hash64::new(0x67d9b6c82034f13d));
+    let c = Fingerprint::new(Hash64::new(0x0d013a27811dbbc3), Hash64::new(0x9a3f7b3d9142ec43));
     let permutations = [(a, b, c), (a, c, b), (b, a, c), (b, c, a), (c, a, b), (c, b, a)];
     let f = a.combine_commutative(b).combine_commutative(c);
     for p in &permutations {

compiler/rustc_data_structures/src/hashes.rs

@@ -0,0 +1,132 @@
+//! rustc encodes a lot of hashes. If hashes are stored as `u64` or `u128`, a `derive(Encodable)`
+//! will apply varint encoding to the hashes, which is less efficient than directly encoding the 8
+//! or 16 bytes of the hash.
+//!
+//! The types in this module represent 64-bit or 128-bit hashes produced by a `StableHasher`.
+//! `Hash64` and `Hash128` expose some utilty functions to encourage users to not extract the inner
+//! hash value as an integer type and accidentally apply varint encoding to it.
+//!
+//! In contrast with `Fingerprint`, users of these types cannot and should not attempt to construct
+//! and decompose these types into constitutent pieces. The point of these types is only to
+//! connect the fact that they can only be produced by a `StableHasher` to their
+//! `Encode`/`Decode` impls.
+
+use crate::stable_hasher::{StableHasher, StableHasherResult};
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+use std::fmt;
+use std::ops::BitXorAssign;
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Default)]
+pub struct Hash64 {
+    inner: u64,
+}
+
+impl Hash64 {
+    pub const ZERO: Hash64 = Hash64 { inner: 0 };
+
+    #[inline]
+    pub(crate) fn new(n: u64) -> Self {
+        Self { inner: n }
+    }
+
+    #[inline]
+    pub fn as_u64(self) -> u64 {
+        self.inner
+    }
+}
+
+impl BitXorAssign<u64> for Hash64 {
+    #[inline]
+    fn bitxor_assign(&mut self, rhs: u64) {
+        self.inner ^= rhs;
+    }
+}
+
+impl<S: Encoder> Encodable<S> for Hash64 {
+    #[inline]
+    fn encode(&self, s: &mut S) {
+        s.emit_raw_bytes(&self.inner.to_le_bytes());
+    }
+}
+
+impl<D: Decoder> Decodable<D> for Hash64 {
+    #[inline]
+    fn decode(d: &mut D) -> Self {
+        Self { inner: u64::from_le_bytes(d.read_raw_bytes(8).try_into().unwrap()) }
+    }
+}
+
+impl StableHasherResult for Hash64 {
+    #[inline]
+    fn finish(hasher: StableHasher) -> Self {
+        Self { inner: hasher.finalize().0 }
+    }
+}
+
+impl fmt::Debug for Hash64 {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.inner.fmt(f)
+    }
+}
+
+impl fmt::LowerHex for Hash64 {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::LowerHex::fmt(&self.inner, f)
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Default)]
+pub struct Hash128 {
+    inner: u128,
+}
+
+impl Hash128 {
+    #[inline]
+    pub fn truncate(self) -> Hash64 {
+        Hash64 { inner: self.inner as u64 }
+    }
+
+    #[inline]
+    pub fn wrapping_add(self, other: Self) -> Self {
+        Self { inner: self.inner.wrapping_add(other.inner) }
+    }
+
+    #[inline]
+    pub fn as_u128(self) -> u128 {
+        self.inner
+    }
+}
+
+impl<S: Encoder> Encodable<S> for Hash128 {
+    #[inline]
+    fn encode(&self, s: &mut S) {
+        s.emit_raw_bytes(&self.inner.to_le_bytes());
+    }
+}
+
+impl<D: Decoder> Decodable<D> for Hash128 {
+    #[inline]
+    fn decode(d: &mut D) -> Self {
+        Self { inner: u128::from_le_bytes(d.read_raw_bytes(16).try_into().unwrap()) }
+    }
+}
+
+impl StableHasherResult for Hash128 {
+    #[inline]
+    fn finish(hasher: StableHasher) -> Self {
+        let (_0, _1) = hasher.finalize();
+        Self { inner: u128::from(_0) | (u128::from(_1) << 64) }
+    }
+}
+
+impl fmt::Debug for Hash128 {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.inner.fmt(f)
+    }
+}
+
+impl fmt::LowerHex for Hash128 {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::LowerHex::fmt(&self.inner, f)
+    }
+}

compiler/rustc_data_structures/src/lib.rs

@@ -86,6 +86,7 @@ pub mod work_queue;
 pub use atomic_ref::AtomicRef;
 pub mod aligned;
 pub mod frozen;
+mod hashes;
 pub mod owned_slice;
 pub mod sso;
 pub mod steal;