Remove WithCachedTypeInfo::stable_hash.

We store a stable hash value in the most common interned values (e.g.
types, predicates, regions). This is 16 bytes of data.

- In non-incremental builds it's a straightforward performance loss: the
  stable hash isn't computed or used, and the 16 bytes of space goes to
  waste (but it still gets hashed when interning).

- In incremental builds it avoids some hashing but doesn't seem to
  actually be a genuine performance win, and the complexity doesn't seem
  worth it.

Author: nnethercote

compiler/rustc_middle/src/ty/context.rs

@@ -18,13 +18,12 @@ use std::{fmt, iter, mem};
 use rustc_abi::{ExternAbi, FieldIdx, Layout, LayoutData, TargetDataLayout, VariantIdx};
 use rustc_ast as ast;
 use rustc_data_structures::defer;
-use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::intern::Interned;
 use rustc_data_structures::jobserver::Proxy;
 use rustc_data_structures::profiling::SelfProfilerRef;
 use rustc_data_structures::sharded::{IntoPointer, ShardedHashMap};
-use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::stable_hasher::HashStable;
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{
     self, DynSend, DynSync, FreezeReadGuard, Lock, RwLock, WorkerLocal,
@@ -47,9 +46,7 @@ use rustc_span::def_id::{CRATE_DEF_ID, DefPathHash, StableCrateId};
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw, sym};
 use rustc_type_ir::TyKind::*;
 pub use rustc_type_ir::lift::Lift;
-use rustc_type_ir::{
-    CollectAndApply, FnSigKind, TypeFlags, WithCachedTypeInfo, elaborate, search_graph,
-};
+use rustc_type_ir::{CollectAndApply, FnSigKind, WithCachedTypeInfo, elaborate, search_graph};
 use tracing::{debug, instrument};
 
 use crate::arena::Arena;
@@ -247,16 +244,13 @@ impl<'tcx> CtxtInterners<'tcx> {
     /// Interns a type. (Use `mk_*` functions instead, where possible.)
     #[allow(rustc::usage_of_ty_tykind)]
     #[inline(never)]
-    fn intern_ty(&self, kind: TyKind<'tcx>, sess: &Session, untracked: &Untracked) -> Ty<'tcx> {
+    fn intern_ty(&self, kind: TyKind<'tcx>) -> Ty<'tcx> {
         Ty(Interned::new_unchecked(
             self.type_
                 .intern(kind, |kind| {
                     let flags = ty::FlagComputation::<TyCtxt<'tcx>>::for_kind(&kind);
-                    let stable_hash = self.stable_hash(&flags, sess, untracked, &kind);
-
                     InternedInSet(self.arena.alloc(WithCachedTypeInfo {
                         internee: kind,
-                        stable_hash,
                         flags: flags.flags,
                         outer_exclusive_binder: flags.outer_exclusive_binder,
                     }))
@@ -268,21 +262,13 @@ impl<'tcx> CtxtInterners<'tcx> {
     /// Interns a const. (Use `mk_*` functions instead, where possible.)
     #[allow(rustc::usage_of_ty_tykind)]
     #[inline(never)]
-    fn intern_const(
-        &self,
-        kind: ty::ConstKind<'tcx>,
-        sess: &Session,
-        untracked: &Untracked,
-    ) -> Const<'tcx> {
+    fn intern_const(&self, kind: ty::ConstKind<'tcx>) -> Const<'tcx> {
         Const(Interned::new_unchecked(
             self.const_
                 .intern(kind, |kind: ty::ConstKind<'_>| {
                     let flags = ty::FlagComputation::<TyCtxt<'tcx>>::for_const_kind(&kind);
-                    let stable_hash = self.stable_hash(&flags, sess, untracked, &kind);
-
                     InternedInSet(self.arena.alloc(WithCachedTypeInfo {
                         internee: kind,
-                        stable_hash,
                         flags: flags.flags,
                         outer_exclusive_binder: flags.outer_exclusive_binder,
                     }))
@@ -291,43 +277,15 @@ impl<'tcx> CtxtInterners<'tcx> {
         ))
     }
 
-    fn stable_hash<'a, T: HashStable<StableHashingContext<'a>>>(
-        &self,
-        flags: &ty::FlagComputation<TyCtxt<'tcx>>,
-        sess: &'a Session,
-        untracked: &'a Untracked,
-        val: &T,
-    ) -> Fingerprint {
-        // It's impossible to hash inference variables (and will ICE), so we don't need to try to cache them.
-        // Without incremental, we rarely stable-hash types, so let's not do it proactively.
-        if flags.flags.intersects(TypeFlags::HAS_INFER) || sess.opts.incremental.is_none() {
-            Fingerprint::ZERO
-        } else {
-            let mut hasher = StableHasher::new();
-            let mut hcx = StableHashingContext::new(sess, untracked);
-            val.hash_stable(&mut hcx, &mut hasher);
-            hasher.finish()
-        }
-    }
-
     /// Interns a predicate. (Use `mk_predicate` instead, where possible.)
     #[inline(never)]
-    fn intern_predicate(
-        &self,
-        kind: Binder<'tcx, PredicateKind<'tcx>>,
-        sess: &Session,
-        untracked: &Untracked,
-    ) -> Predicate<'tcx> {
+    fn intern_predicate(&self, kind: Binder<'tcx, PredicateKind<'tcx>>) -> Predicate<'tcx> {
         Predicate(Interned::new_unchecked(
             self.predicate
                 .intern(kind, |kind| {
                     let flags = ty::FlagComputation::<TyCtxt<'tcx>>::for_predicate(kind);
-
-                    let stable_hash = self.stable_hash(&flags, sess, untracked, &kind);
-
                     InternedInSet(self.arena.alloc(WithCachedTypeInfo {
                         internee: kind,
-                        stable_hash,
                         flags: flags.flags,
                         outer_exclusive_binder: flags.outer_exclusive_binder,
                     }))
@@ -463,12 +421,8 @@ pub struct CommonConsts<'tcx> {
 }
 
 impl<'tcx> CommonTypes<'tcx> {
-    fn new(
-        interners: &CtxtInterners<'tcx>,
-        sess: &Session,
-        untracked: &Untracked,
-    ) -> CommonTypes<'tcx> {
-        let mk = |ty| interners.intern_ty(ty, sess, untracked);
+    fn new(interners: &CtxtInterners<'tcx>) -> CommonTypes<'tcx> {
+        let mk = |ty| interners.intern_ty(ty);
 
         let ty_vars =
             (0..NUM_PREINTERNED_TY_VARS).map(|n| mk(Infer(ty::TyVar(TyVid::from(n))))).collect();
@@ -584,18 +538,8 @@ impl<'tcx> CommonLifetimes<'tcx> {
 }
 
 impl<'tcx> CommonConsts<'tcx> {
-    fn new(
-        interners: &CtxtInterners<'tcx>,
-        types: &CommonTypes<'tcx>,
-        sess: &Session,
-        untracked: &Untracked,
-    ) -> CommonConsts<'tcx> {
-        let mk_const = |c| {
-            interners.intern_const(
-                c, sess, // This is only used to create a stable hashing context.
-                untracked,
-            )
-        };
+    fn new(interners: &CtxtInterners<'tcx>, types: &CommonTypes<'tcx>) -> CommonConsts<'tcx> {
+        let mk_const = |c| interners.intern_const(c);
 
         let mk_valtree = |v| {
             ty::ValTree(Interned::new_unchecked(
@@ -1088,9 +1032,9 @@ impl<'tcx> TyCtxt<'tcx> {
             s.dcx().emit_fatal(err);
         });
         let interners = CtxtInterners::new(arena);
-        let common_types = CommonTypes::new(&interners, s, &untracked);
+        let common_types = CommonTypes::new(&interners);
         let common_lifetimes = CommonLifetimes::new(&interners);
-        let common_consts = CommonConsts::new(&interners, &common_types, s, &untracked);
+        let common_consts = CommonConsts::new(&interners, &common_types);
 
         let gcx = gcx_cell.get_or_init(|| GlobalCtxt {
             sess: s,
@@ -2221,12 +2165,7 @@ impl<'tcx> TyCtxt<'tcx> {
 
     #[inline]
     pub fn mk_predicate(self, binder: Binder<'tcx, PredicateKind<'tcx>>) -> Predicate<'tcx> {
-        self.interners.intern_predicate(
-            binder,
-            self.sess,
-            // This is only used to create a stable hashing context.
-            &self.untracked,
-        )
+        self.interners.intern_predicate(binder)
     }
 
     #[inline]
@@ -2347,24 +2286,14 @@ impl<'tcx> TyCtxt<'tcx> {
 
     #[inline]
     pub fn mk_ct_from_kind(self, kind: ty::ConstKind<'tcx>) -> Const<'tcx> {
-        self.interners.intern_const(
-            kind,
-            self.sess,
-            // This is only used to create a stable hashing context.
-            &self.untracked,
-        )
+        self.interners.intern_const(kind)
     }
 
     // Avoid this in favour of more specific `Ty::new_*` methods, where possible.
     #[allow(rustc::usage_of_ty_tykind)]
     #[inline]
     pub fn mk_ty_from_kind(self, st: TyKind<'tcx>) -> Ty<'tcx> {
-        self.interners.intern_ty(
-            st,
-            self.sess,
-            // This is only used to create a stable hashing context.
-            &self.untracked,
-        )
+        self.interners.intern_ty(st)
     }
 
     pub fn mk_param_from_def(self, param: &ty::GenericParamDef) -> GenericArg<'tcx> {

compiler/rustc_middle/src/ty/predicate.rs

@@ -644,6 +644,6 @@ mod size_asserts {
     use super::*;
     // tidy-alphabetical-start
     static_assert_size!(PredicateKind<'_>, 40);
-    static_assert_size!(WithCachedTypeInfo<PredicateKind<'_>>, 64);
+    static_assert_size!(WithCachedTypeInfo<PredicateKind<'_>>, 48);
     // tidy-alphabetical-end
 }

compiler/rustc_middle/src/ty/region.rs

@@ -460,6 +460,6 @@ mod size_asserts {
     use super::*;
     // tidy-alphabetical-start
     static_assert_size!(RegionKind<'_>, 20);
-    static_assert_size!(ty::WithCachedTypeInfo<RegionKind<'_>>, 48);
+    static_assert_size!(ty::WithCachedTypeInfo<RegionKind<'_>>, 28);
     // tidy-alphabetical-end
 }

compiler/rustc_middle/src/ty/sty.rs

@@ -2167,6 +2167,6 @@ mod size_asserts {
     use super::*;
     // tidy-alphabetical-start
     static_assert_size!(TyKind<'_>, 32);
-    static_assert_size!(ty::WithCachedTypeInfo<TyKind<'_>>, 56);
+    static_assert_size!(ty::WithCachedTypeInfo<TyKind<'_>>, 40);
     // tidy-alphabetical-end
 }

compiler/rustc_type_ir/src/ty_info.rs

@@ -2,28 +2,19 @@ use std::cmp::Ordering;
 use std::hash::{Hash, Hasher};
 use std::ops::Deref;
 
-#[cfg(feature = "nightly")]
-use rustc_data_structures::fingerprint::Fingerprint;
 #[cfg(feature = "nightly")]
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_type_ir_macros::GenericTypeVisitable;
 
 use crate::{DebruijnIndex, TypeFlags};
 
 /// A helper type that you can wrap round your own type in order to automatically
-/// cache the stable hash, type flags and debruijn index on creation and
-/// not recompute it whenever the information is needed.
-/// This is only done in incremental mode. You can also opt out of caching by using
-/// StableHash::ZERO for the hash, in which case the hash gets computed each time.
-/// This is useful if you have values that you intern but never (can?) use for stable
-/// hashing.
+/// cache the type flags and debruijn index on creation and not recompute it
+/// whenever the information is needed.
 #[derive(Copy, Clone, GenericTypeVisitable)]
 pub struct WithCachedTypeInfo<T> {
     pub internee: T,
 
-    #[cfg(feature = "nightly")]
-    pub stable_hash: Fingerprint,
-
     /// This field provides fast access to information that is also contained
     /// in `kind`.
     ///
@@ -87,39 +78,13 @@ impl<T> Deref for WithCachedTypeInfo<T> {
 impl<T: Hash> Hash for WithCachedTypeInfo<T> {
     #[inline]
     fn hash<H: Hasher>(&self, s: &mut H) {
-        #[cfg(feature = "nightly")]
-        if self.stable_hash != Fingerprint::ZERO {
-            return self.stable_hash.hash(s);
-        }
-
         self.internee.hash(s)
     }
 }
 
 #[cfg(feature = "nightly")]
 impl<T: HashStable<Hcx>, Hcx> HashStable<Hcx> for WithCachedTypeInfo<T> {
     fn hash_stable(&self, hcx: &mut Hcx, hasher: &mut StableHasher) {
-        if self.stable_hash == Fingerprint::ZERO || cfg!(debug_assertions) {
-            // No cached hash available. This can only mean that incremental is disabled.
-            // We don't cache stable hashes in non-incremental mode, because they are used
-            // so rarely that the performance actually suffers.
-
-            // We need to build the hash as if we cached it and then hash that hash, as
-            // otherwise the hashes will differ between cached and non-cached mode.
-            let stable_hash: Fingerprint = {
-                let mut hasher = StableHasher::new();
-                self.internee.hash_stable(hcx, &mut hasher);
-                hasher.finish()
-            };
-            if cfg!(debug_assertions) && self.stable_hash != Fingerprint::ZERO {
-                assert_eq!(
-                    stable_hash, self.stable_hash,
-                    "cached stable hash does not match freshly computed stable hash"
-                );
-            }
-            stable_hash.hash_stable(hcx, hasher);
-        } else {
-            self.stable_hash.hash_stable(hcx, hasher);
-        }
+        self.internee.hash_stable(hcx, hasher);
     }
 }

tests/debuginfo/function-names.rs

@@ -39,7 +39,7 @@
 // Const generic parameter
 //@ gdb-command:info functions -q function_names::const_generic_fn.*
 //@ gdb-check:[...]static fn function_names::const_generic_fn_bool<false>();
-//@ gdb-check:[...]static fn function_names::const_generic_fn_non_int<{CONST#6dd80cc0c950c171}>();
+//@ gdb-check:[...]static fn function_names::const_generic_fn_non_int<{CONST#68cf9e429c783d36}>();
 //@ gdb-check:[...]static fn function_names::const_generic_fn_signed_int<-7>();
 //@ gdb-check:[...]static fn function_names::const_generic_fn_unsigned_int<14>();
 

tests/ui/symbol-names/basic.legacy.stderr

@@ -1,10 +1,10 @@
-error: symbol-name(_ZN5basic4main17h947b7a9ed2b2bf56E)
+error: symbol-name(_ZN5basic4main17h27f62b2d0b2beac6E)
   --> $DIR/basic.rs:8:1
    |
 LL | #[rustc_dump_symbol_name]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: demangling(basic::main::h947b7a9ed2b2bf56)
+error: demangling(basic::main::h27f62b2d0b2beac6)
   --> $DIR/basic.rs:8:1
    |
 LL | #[rustc_dump_symbol_name]

tests/ui/symbol-names/issue-60925.legacy.stderr

@@ -1,10 +1,10 @@
-error: symbol-name(_ZN11issue_609253foo37Foo$LT$issue_60925..llv$u6d$..Foo$GT$3foo17hba5ac046b858f549E)
+error: symbol-name(_ZN11issue_609253foo37Foo$LT$issue_60925..llv$u6d$..Foo$GT$3foo17h1eb769490ff06e77E)
   --> $DIR/issue-60925.rs:21:9
    |
 LL |         #[rustc_dump_symbol_name]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: demangling(issue_60925::foo::Foo<issue_60925::llvm::Foo>::foo::hba5ac046b858f549)
+error: demangling(issue_60925::foo::Foo<issue_60925::llvm::Foo>::foo::h1eb769490ff06e77)
   --> $DIR/issue-60925.rs:21:9
    |
 LL |         #[rustc_dump_symbol_name]