Auto merge of rust-lang#145737 - cjgillot:gvn-valueset, r=saethlin

bors · bors · commit e10aa8891182 · 2025-09-19T12:15:03.000Z
GVN: stop hashing opaque values GVN generates values that are not meant to be unified with any other. For instance `Opaque` (aka we don't know anything), non-deterministic constants and borrows. The current algorithm generates a unique index, so the generated `Value` will be different from all the existing. This is wasteful, as we should not hash that `Value` at all. This PR proposes to do this. This involves partially reimplementing a `FxIndexSet`, but yields a small but consistent perf improvement (rust-lang#145737 (comment)).
diff --git a/Cargo.lock b/Cargo.lock
@@ -4235,6 +4235,7 @@ name = "rustc_mir_transform"
 version = "0.0.0"
 dependencies = [
  "either",
+ "hashbrown",
  "itertools",
  "rustc_abi",
  "rustc_arena",
diff --git a/compiler/rustc_mir_transform/Cargo.toml b/compiler/rustc_mir_transform/Cargo.toml
@@ -6,6 +6,7 @@ edition = "2024"
 [dependencies]
 # tidy-alphabetical-start
 either = "1"
+hashbrown = "0.15"
 itertools = "0.12"
 rustc_abi = { path = "../rustc_abi" }
 rustc_arena = { path = "../rustc_arena" }
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
@@ -85,16 +85,18 @@
 //! that contain `AllocId`s.
 
 use std::borrow::Cow;
+use std::hash::{Hash, Hasher};
 
 use either::Either;
+use hashbrown::hash_table::{Entry, HashTable};
 use itertools::Itertools as _;
 use rustc_abi::{self as abi, BackendRepr, FIRST_VARIANT, FieldIdx, Primitive, Size, VariantIdx};
 use rustc_const_eval::const_eval::DummyMachine;
 use rustc_const_eval::interpret::{
     ImmTy, Immediate, InterpCx, MemPlaceMeta, MemoryKind, OpTy, Projectable, Scalar,
     intern_const_alloc_for_constprop,
 };
-use rustc_data_structures::fx::{FxIndexSet, MutableValues};
+use rustc_data_structures::fx::FxHasher;
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_hir::def::DefKind;
 use rustc_index::bit_set::DenseBitSet;
@@ -152,9 +154,29 @@ impl<'tcx> crate::MirPass<'tcx> for GVN {
 }
 
 newtype_index! {
+    /// This represents a `Value` in the symbolic execution.
+    #[debug_format = "_v{}"]
     struct VnIndex {}
 }
 
+/// Marker type to forbid hashing and comparing opaque values.
+/// This struct should only be constructed by `ValueSet::insert_unique` to ensure we use that
+/// method to create non-unifiable values. It will ICE if used in `ValueSet::insert`.
+#[derive(Copy, Clone, Debug, Eq)]
+struct VnOpaque;
+impl PartialEq for VnOpaque {
+    fn eq(&self, _: &VnOpaque) -> bool {
+        // ICE if we try to compare unique values
+        unreachable!()
+    }
+}
+impl Hash for VnOpaque {
+    fn hash<T: Hasher>(&self, _: &mut T) {
+        // ICE if we try to hash unique values
+        unreachable!()
+    }
+}
+
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 enum AddressKind {
     Ref(BorrowKind),
@@ -166,15 +188,17 @@ enum Value<'tcx> {
     // Root values.
     /// Used to represent values we know nothing about.
     /// The `usize` is a counter incremented by `new_opaque`.
-    Opaque(usize),
+    Opaque(VnOpaque),
     /// Evaluated or unevaluated constant value.
     Constant {
         value: Const<'tcx>,
         /// Some constants do not have a deterministic value. To avoid merging two instances of the
         /// same `Const`, we assign them an additional integer index.
-        // `disambiguator` is 0 iff the constant is deterministic.
-        disambiguator: usize,
+        // `disambiguator` is `None` iff the constant is deterministic.
+        disambiguator: Option<VnOpaque>,
     },
+
+    // Aggregates.
     /// An aggregate value, either tuple/closure/struct/enum.
     /// This does not contain unions, as we cannot reason with the value.
     Aggregate(VariantIdx, Vec<VnIndex>),
@@ -192,7 +216,7 @@ enum Value<'tcx> {
         place: Place<'tcx>,
         kind: AddressKind,
         /// Give each borrow and pointer a different provenance, so we don't merge them.
-        provenance: usize,
+        provenance: VnOpaque,
     },
 
     // Extractions.
@@ -211,6 +235,107 @@ enum Value<'tcx> {
     },
 }
 
+/// Stores and deduplicates pairs of `(Value, Ty)` into in `VnIndex` numbered values.
+///
+/// This data structure is mostly a partial reimplementation of `FxIndexMap<VnIndex, (Value, Ty)>`.
+/// We do not use a regular `FxIndexMap` to skip hashing values that are unique by construction,
+/// like opaque values, address with provenance and non-deterministic constants.
+struct ValueSet<'tcx> {
+    indices: HashTable<VnIndex>,
+    hashes: IndexVec<VnIndex, u64>,
+    values: IndexVec<VnIndex, Value<'tcx>>,
+    types: IndexVec<VnIndex, Ty<'tcx>>,
+}
+
+impl<'tcx> ValueSet<'tcx> {
+    fn new(num_values: usize) -> ValueSet<'tcx> {
+        ValueSet {
+            indices: HashTable::with_capacity(num_values),
+            hashes: IndexVec::with_capacity(num_values),
+            values: IndexVec::with_capacity(num_values),
+            types: IndexVec::with_capacity(num_values),
+        }
+    }
+
+    /// Insert a `(Value, Ty)` pair without hashing or deduplication.
+    /// This always creates a new `VnIndex`.
+    #[inline]
+    fn insert_unique(
+        &mut self,
+        ty: Ty<'tcx>,
+        value: impl FnOnce(VnOpaque) -> Value<'tcx>,
+    ) -> VnIndex {
+        let value = value(VnOpaque);
+
+        debug_assert!(match value {
+            Value::Opaque(_) | Value::Address { .. } => true,
+            Value::Constant { disambiguator, .. } => disambiguator.is_some(),
+            _ => false,
+        });
+
+        let index = self.hashes.push(0);
+        let _index = self.types.push(ty);
+        debug_assert_eq!(index, _index);
+        let _index = self.values.push(value);
+        debug_assert_eq!(index, _index);
+        index
+    }
+
+    /// Insert a `(Value, Ty)` pair to be deduplicated.
+    /// Returns `true` as second tuple field if this value did not exist previously.
+    #[allow(rustc::pass_by_value)] // closures take `&VnIndex`
+    fn insert(&mut self, ty: Ty<'tcx>, value: Value<'tcx>) -> (VnIndex, bool) {
+        debug_assert!(match value {
+            Value::Opaque(_) | Value::Address { .. } => false,
+            Value::Constant { disambiguator, .. } => disambiguator.is_none(),
+            _ => true,
+        });
+
+        let hash: u64 = {
+            let mut h = FxHasher::default();
+            value.hash(&mut h);
+            ty.hash(&mut h);
+            h.finish()
+        };
+
+        let eq = |index: &VnIndex| self.values[*index] == value && self.types[*index] == ty;
+        let hasher = |index: &VnIndex| self.hashes[*index];
+        match self.indices.entry(hash, eq, hasher) {
+            Entry::Occupied(entry) => {
+                let index = *entry.get();
+                (index, false)
+            }
+            Entry::Vacant(entry) => {
+                let index = self.hashes.push(hash);
+                entry.insert(index);
+                let _index = self.values.push(value);
+                debug_assert_eq!(index, _index);
+                let _index = self.types.push(ty);
+                debug_assert_eq!(index, _index);
+                (index, true)
+            }
+        }
+    }
+
+    /// Return the `Value` associated with the given `VnIndex`.
+    #[inline]
+    fn value(&self, index: VnIndex) -> &Value<'tcx> {
+        &self.values[index]
+    }
+
+    /// Return the type associated with the given `VnIndex`.
+    #[inline]
+    fn ty(&self, index: VnIndex) -> Ty<'tcx> {
+        self.types[index]
+    }
+
+    /// Replace the value associated with `index` with an opaque value.
+    #[inline]
+    fn forget(&mut self, index: VnIndex) {
+        self.values[index] = Value::Opaque(VnOpaque);
+    }
+}
+
 struct VnState<'body, 'tcx> {
     tcx: TyCtxt<'tcx>,
     ecx: InterpCx<'tcx, DummyMachine>,
@@ -221,11 +346,9 @@ struct VnState<'body, 'tcx> {
     /// Locals that are assigned that value.
     // This vector does not hold all the values of `VnIndex` that we create.
     rev_locals: IndexVec<VnIndex, SmallVec<[Local; 1]>>,
-    values: FxIndexSet<(Value<'tcx>, Ty<'tcx>)>,
+    values: ValueSet<'tcx>,
     /// Values evaluated as constants if possible.
     evaluated: IndexVec<VnIndex, Option<OpTy<'tcx>>>,
-    /// Counter to generate different values.
-    next_opaque: usize,
     /// Cache the deref values.
     derefs: Vec<VnIndex>,
     ssa: &'body SsaLocals,
@@ -256,9 +379,8 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             is_coroutine: body.coroutine.is_some(),
             locals: IndexVec::from_elem(None, local_decls),
             rev_locals: IndexVec::with_capacity(num_values),
-            values: FxIndexSet::with_capacity_and_hasher(num_values, Default::default()),
+            values: ValueSet::new(num_values),
             evaluated: IndexVec::with_capacity(num_values),
-            next_opaque: 1,
             derefs: Vec::new(),
             ssa,
             dominators,
@@ -272,8 +394,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
 
     #[instrument(level = "trace", skip(self), ret)]
     fn insert(&mut self, ty: Ty<'tcx>, value: Value<'tcx>) -> VnIndex {
-        let (index, new) = self.values.insert_full((value, ty));
-        let index = VnIndex::from_usize(index);
+        let (index, new) = self.values.insert(ty, value);
         if new {
             // Grow `evaluated` and `rev_locals` here to amortize the allocations.
             let evaluated = self.eval_to_const(index);
@@ -285,18 +406,16 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
         index
     }
 
-    fn next_opaque(&mut self) -> usize {
-        let next_opaque = self.next_opaque;
-        self.next_opaque += 1;
-        next_opaque
-    }
-
     /// Create a new `Value` for which we have no information at all, except that it is distinct
     /// from all the others.
     #[instrument(level = "trace", skip(self), ret)]
     fn new_opaque(&mut self, ty: Ty<'tcx>) -> VnIndex {
-        let value = Value::Opaque(self.next_opaque());
-        self.insert(ty, value)
+        let index = self.values.insert_unique(ty, Value::Opaque);
+        let _index = self.evaluated.push(None);
+        debug_assert_eq!(index, _index);
+        let _index = self.rev_locals.push(SmallVec::new());
+        debug_assert_eq!(index, _index);
+        index
     }
 
     /// Create a new `Value::Address` distinct from all the others.
@@ -309,18 +428,49 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
             AddressKind::Address(mutbl) => Ty::new_ptr(self.tcx, pty, mutbl.to_mutbl_lossy()),
         };
-        let value = Value::Address { place, kind, provenance: self.next_opaque() };
-        self.insert(ty, value)
+        let index =
+            self.values.insert_unique(ty, |provenance| Value::Address { place, kind, provenance });
+        let evaluated = self.eval_to_const(index);
+        let _index = self.evaluated.push(evaluated);
+        debug_assert_eq!(index, _index);
+        let _index = self.rev_locals.push(SmallVec::new());
+        debug_assert_eq!(index, _index);
+        index
+    }
+
+    #[instrument(level = "trace", skip(self), ret)]
+    fn insert_constant(&mut self, value: Const<'tcx>) -> VnIndex {
+        let (index, new) = if value.is_deterministic() {
+            // The constant is deterministic, no need to disambiguate.
+            let constant = Value::Constant { value, disambiguator: None };
+            self.values.insert(value.ty(), constant)
+        } else {
+            // Multiple mentions of this constant will yield different values,
+            // so assign a different `disambiguator` to ensure they do not get the same `VnIndex`.
+            let index = self.values.insert_unique(value.ty(), |disambiguator| Value::Constant {
+                value,
+                disambiguator: Some(disambiguator),
+            });
+            (index, true)
+        };
+        if new {
+            let evaluated = self.eval_to_const(index);
+            let _index = self.evaluated.push(evaluated);
+            debug_assert_eq!(index, _index);
+            let _index = self.rev_locals.push(SmallVec::new());
+            debug_assert_eq!(index, _index);
+        }
+        index
     }
 
     #[inline]
     fn get(&self, index: VnIndex) -> &Value<'tcx> {
-        &self.values.get_index(index.as_usize()).unwrap().0
+        self.values.value(index)
     }
 
     #[inline]
     fn ty(&self, index: VnIndex) -> Ty<'tcx> {
-        self.values.get_index(index.as_usize()).unwrap().1
+        self.values.ty(index)
     }
 
     /// Record that `local` is assigned `value`. `local` must be SSA.
@@ -331,33 +481,18 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
         self.rev_locals[value].push(local);
     }
 
-    fn insert_constant(&mut self, value: Const<'tcx>) -> VnIndex {
-        let disambiguator = if value.is_deterministic() {
-            // The constant is deterministic, no need to disambiguate.
-            0
-        } else {
-            // Multiple mentions of this constant will yield different values,
-            // so assign a different `disambiguator` to ensure they do not get the same `VnIndex`.
-            let disambiguator = self.next_opaque();
-            // `disambiguator: 0` means deterministic.
-            debug_assert_ne!(disambiguator, 0);
-            disambiguator
-        };
-        self.insert(value.ty(), Value::Constant { value, disambiguator })
-    }
-
     fn insert_bool(&mut self, flag: bool) -> VnIndex {
         // Booleans are deterministic.
         let value = Const::from_bool(self.tcx, flag);
         debug_assert!(value.is_deterministic());
-        self.insert(self.tcx.types.bool, Value::Constant { value, disambiguator: 0 })
+        self.insert(self.tcx.types.bool, Value::Constant { value, disambiguator: None })
     }
 
     fn insert_scalar(&mut self, ty: Ty<'tcx>, scalar: Scalar) -> VnIndex {
         // Scalars are deterministic.
         let value = Const::from_scalar(self.tcx, scalar, ty);
         debug_assert!(value.is_deterministic());
-        self.insert(ty, Value::Constant { value, disambiguator: 0 })
+        self.insert(ty, Value::Constant { value, disambiguator: None })
     }
 
     fn insert_tuple(&mut self, ty: Ty<'tcx>, values: Vec<VnIndex>) -> VnIndex {
@@ -372,8 +507,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
 
     fn invalidate_derefs(&mut self) {
         for deref in std::mem::take(&mut self.derefs) {
-            let opaque = self.next_opaque();
-            self.values.get_index_mut2(deref.index()).unwrap().0 = Value::Opaque(opaque);
+            self.values.forget(deref);
         }
     }
 
@@ -1572,7 +1706,7 @@ impl<'tcx> VnState<'_, 'tcx> {
         // This was already constant in MIR, do not change it. If the constant is not
         // deterministic, adding an additional mention of it in MIR will not give the same value as
         // the former mention.
-        if let Value::Constant { value, disambiguator: 0 } = *self.get(index) {
+        if let Value::Constant { value, disambiguator: None } = *self.get(index) {
             debug_assert!(value.is_deterministic());
             return Some(ConstOperand { span: DUMMY_SP, user_ty: None, const_: value });
         }
