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//! Virtual registers. //! //! A virtual register is a set of related SSA values whose live ranges don't interfere. If all the //! values in a virtual register are assigned to the same location, fewer copies will result in the //! output. //! //! A virtual register is typically built by merging together SSA values that are "phi-related" - //! that is, one value is passed as a block argument to a branch and the other is the block parameter //! value itself. //! //! If any values in a virtual register are spilled, they will use the same stack slot. This avoids //! memory-to-memory copies when a spilled value is passed as a block argument. use crate::dbg::DisplayList; use crate::dominator_tree::DominatorTreePreorder; use crate::entity::entity_impl; use crate::entity::{EntityList, ListPool}; use crate::entity::{Keys, PrimaryMap, SecondaryMap}; use crate::ir::{Function, Value}; use crate::packed_option::PackedOption; use alloc::vec::Vec; use core::cmp::Ordering; use core::fmt; use core::slice; use smallvec::SmallVec; /// A virtual register reference. #[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)] pub struct VirtReg(u32); entity_impl!(VirtReg, "vreg"); type ValueList = EntityList<Value>; /// Collection of virtual registers. /// /// Each virtual register is a list of values. Also maintain a map from values to their unique /// virtual register, if any. pub struct VirtRegs { /// Memory pool for the value lists. pool: ListPool<Value>, /// The primary table of virtual registers. vregs: PrimaryMap<VirtReg, ValueList>, /// Allocated virtual register numbers that are no longer in use. unused_vregs: Vec<VirtReg>, /// Each value belongs to at most one virtual register. value_vregs: SecondaryMap<Value, PackedOption<VirtReg>>, /// Table used during the union-find phase while `vregs` is empty. union_find: SecondaryMap<Value, i32>, /// Values that have been activated in the `union_find` table, but not yet added to any virtual /// registers by the `finish_union_find()` function. pending_values: Vec<Value>, } impl VirtRegs { /// Create a new virtual register collection. pub fn new() -> Self { Self { pool: ListPool::new(), vregs: PrimaryMap::new(), unused_vregs: Vec::new(), value_vregs: SecondaryMap::new(), union_find: SecondaryMap::new(), pending_values: Vec::new(), } } /// Clear all virtual registers. pub fn clear(&mut self) { self.vregs.clear(); self.unused_vregs.clear(); self.value_vregs.clear(); self.pool.clear(); self.union_find.clear(); self.pending_values.clear(); } /// Get the virtual register containing `value`, if any. pub fn get(&self, value: Value) -> Option<VirtReg> { self.value_vregs[value].into() } /// Get the list of values in `vreg`. pub fn values(&self, vreg: VirtReg) -> &[Value] { self.vregs[vreg].as_slice(&self.pool) } /// Get an iterator over all virtual registers. pub fn all_virtregs(&self) -> Keys<VirtReg> { self.vregs.keys() } /// Get the congruence class of `value`. /// /// If `value` belongs to a virtual register, the congruence class is the values of the virtual /// register. Otherwise it is just the value itself. #[cfg_attr(feature = "cargo-clippy", allow(clippy::trivially_copy_pass_by_ref))] pub fn congruence_class<'a, 'b>(&'a self, value: &'b Value) -> &'b [Value] where 'a: 'b, { self.get(*value) .map_or_else(|| slice::from_ref(value), |vr| self.values(vr)) } /// Check if `a` and `b` belong to the same congruence class. pub fn same_class(&self, a: Value, b: Value) -> bool { match (self.get(a), self.get(b)) { (Some(va), Some(vb)) => va == vb, _ => a == b, } } /// Sort the values in `vreg` according to the dominator tree pre-order. /// /// Returns the slice of sorted values which `values(vreg)` will also return from now on. pub fn sort_values( &mut self, vreg: VirtReg, func: &Function, preorder: &DominatorTreePreorder, ) -> &[Value] { let s = self.vregs[vreg].as_mut_slice(&mut self.pool); s.sort_unstable_by(|&a, &b| preorder.pre_cmp_def(a, b, func)); s } /// Insert a single value into a sorted virtual register. /// /// It is assumed that the virtual register containing `big` is already sorted by /// `sort_values()`, and that `single` does not already belong to a virtual register. /// /// If `big` is not part of a virtual register, one will be created. pub fn insert_single( &mut self, big: Value, single: Value, func: &Function, preorder: &DominatorTreePreorder, ) -> VirtReg { debug_assert_eq!(self.get(single), None, "Expected singleton {}", single); // Make sure `big` has a vreg. let vreg = self.get(big).unwrap_or_else(|| { let vr = self.alloc(); self.vregs[vr].push(big, &mut self.pool); self.value_vregs[big] = vr.into(); vr }); // Determine the insertion position for `single`. let index = match self .values(vreg) .binary_search_by(|&v| preorder.pre_cmp_def(v, single, func)) { Ok(_) => panic!("{} already in {}", single, vreg), Err(i) => i, }; self.vregs[vreg].insert(index, single, &mut self.pool); self.value_vregs[single] = vreg.into(); vreg } /// Remove a virtual register. /// /// The values in `vreg` become singletons, and the virtual register number may be reused in /// the future. pub fn remove(&mut self, vreg: VirtReg) { // Start by reassigning all the values. for &v in self.vregs[vreg].as_slice(&self.pool) { let old = self.value_vregs[v].take(); debug_assert_eq!(old, Some(vreg)); } self.vregs[vreg].clear(&mut self.pool); self.unused_vregs.push(vreg); } /// Allocate a new empty virtual register. fn alloc(&mut self) -> VirtReg { self.unused_vregs .pop() .unwrap_or_else(|| self.vregs.push(Default::default())) } /// Unify `values` into a single virtual register. /// /// The values in the slice can be singletons or they can belong to a virtual register already. /// If a value belongs to a virtual register, all of the values in that register must be /// present. /// /// The values are assumed to already be in topological order. pub fn unify(&mut self, values: &[Value]) -> VirtReg { // Start by clearing all virtual registers involved. let mut singletons = 0; let mut cleared = 0; for &val in values { match self.get(val) { None => singletons += 1, Some(vreg) => { if !self.vregs[vreg].is_empty() { cleared += self.vregs[vreg].len(&self.pool); self.vregs[vreg].clear(&mut self.pool); self.unused_vregs.push(vreg); } } } } debug_assert_eq!( values.len(), singletons + cleared, "Can't unify partial virtual registers" ); let vreg = self.alloc(); self.vregs[vreg].extend(values.iter().cloned(), &mut self.pool); for &v in values { self.value_vregs[v] = vreg.into(); } vreg } } impl fmt::Display for VirtRegs { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { for vreg in self.all_virtregs() { write!(f, "\n{} = {}", vreg, DisplayList(self.values(vreg)))?; } Ok(()) } } /// Expanded version of a union-find table entry. enum UFEntry { /// This value is a a set leader. The embedded number is the set's rank. Rank(u32), /// This value belongs to the same set as the linked value. Link(Value), } /// The `union_find` table contains `i32` entries that are interpreted as follows: /// /// x = 0: The value belongs to its own singleton set. /// x > 0: The value is the leader of a set with rank x. /// x < 0: The value belongs to the same set as the value numbered !x. /// /// The rank of a set is an upper bound on the number of links that must be followed from a member /// of the set to the set leader. /// /// A singleton set is the same as a set with rank 0. It contains only the leader value. impl UFEntry { /// Decode a table entry. fn decode(x: i32) -> Self { if x < 0 { Self::Link(Value::from_u32((!x) as u32)) } else { Self::Rank(x as u32) } } /// Encode a link entry. fn encode_link(v: Value) -> i32 { !(v.as_u32() as i32) } } /// Union-find algorithm for building virtual registers. /// /// Before values are added to virtual registers, it is possible to use a union-find algorithm to /// construct virtual registers efficiently. This support implemented here is used as follows: /// /// 1. Repeatedly call the `union(a, b)` method to request that `a` and `b` are placed in the same /// virtual register. /// 2. When done, call `finish_union_find()` to construct the virtual register sets based on the /// `union()` calls. /// /// The values that were passed to `union(a, b)` must not belong to any existing virtual registers /// by the time `finish_union_find()` is called. /// /// For more information on the algorithm implemented here, see Chapter 21 "Data Structures for /// Disjoint Sets" of Cormen, Leiserson, Rivest, Stein, "Introduction to algorithms", 3rd Ed. /// /// The [Wikipedia entry on disjoint-set data /// structures](https://en.wikipedia.org/wiki/Disjoint-set_data_structure) is also good. impl VirtRegs { /// Find the leader value and rank of the set containing `v`. /// Compress the path if needed. fn find(&mut self, mut val: Value) -> (Value, u32) { let mut val_stack = SmallVec::<[Value; 8]>::new(); let found = loop { match UFEntry::decode(self.union_find[val]) { UFEntry::Rank(rank) => break (val, rank), UFEntry::Link(parent) => { val_stack.push(val); val = parent; } } }; // Compress the path while let Some(val) = val_stack.pop() { self.union_find[val] = UFEntry::encode_link(found.0); } found } /// Union the two sets containing `a` and `b`. /// /// This ensures that `a` and `b` will belong to the same virtual register after calling /// `finish_union_find()`. pub fn union(&mut self, a: Value, b: Value) { let (leader_a, rank_a) = self.find(a); let (leader_b, rank_b) = self.find(b); if leader_a == leader_b { return; } // The first time we see a value, its rank will be 0. Add it to the list of pending values. if rank_a == 0 { debug_assert_eq!(a, leader_a); self.pending_values.push(a); } if rank_b == 0 { debug_assert_eq!(b, leader_b); self.pending_values.push(b); } // Merge into the set with the greater rank. This preserves the invariant that the rank is // an upper bound on the number of links to the leader. match rank_a.cmp(&rank_b) { Ordering::Less => { self.union_find[leader_a] = UFEntry::encode_link(leader_b); } Ordering::Greater => { self.union_find[leader_b] = UFEntry::encode_link(leader_a); } Ordering::Equal => { // When the two sets have the same rank, we arbitrarily pick the a-set to preserve. // We need to increase the rank by one since the elements in the b-set are now one // link further away from the leader. self.union_find[leader_a] += 1; self.union_find[leader_b] = UFEntry::encode_link(leader_a); } } } /// Compute virtual registers based on previous calls to `union(a, b)`. /// /// This terminates the union-find algorithm, so the next time `union()` is called, it is for a /// new independent batch of values. /// /// The values in each virtual register will be ordered according to when they were first /// passed to `union()`, but backwards. It is expected that `sort_values()` will be used to /// create a more sensible value order. /// /// The new virtual registers will be appended to `new_vregs`, if present. pub fn finish_union_find(&mut self, mut new_vregs: Option<&mut Vec<VirtReg>>) { debug_assert_eq!( self.pending_values.iter().find(|&&v| self.get(v).is_some()), None, "Values participating in union-find must not belong to existing virtual registers" ); while let Some(val) = self.pending_values.pop() { let (leader, _) = self.find(val); // Get the vreg for `leader`, or create it. let vreg = self.get(leader).unwrap_or_else(|| { // Allocate a vreg for `leader`, but leave it empty. let vr = self.alloc(); if let Some(ref mut vec) = new_vregs { vec.push(vr); } self.value_vregs[leader] = vr.into(); vr }); // Push values in `pending_values` order, including when `v == leader`. self.vregs[vreg].push(val, &mut self.pool); self.value_vregs[val] = vreg.into(); // Clear the entry in the union-find table. The `find(val)` call may still look at this // entry in a future iteration, but that it ok. It will return a rank 0 leader that has // already been assigned to the correct virtual register. self.union_find[val] = 0; } // We do *not* call `union_find.clear()` table here because re-initializing the table for // sparse use takes time linear in the number of values in the function. Instead we reset // the entries that are known to be non-zero in the loop above. } } #[cfg(test)] mod tests { use super::*; use crate::entity::EntityRef; use crate::ir::Value; #[test] fn empty_union_find() { let mut vregs = VirtRegs::new(); vregs.finish_union_find(None); assert_eq!(vregs.all_virtregs().count(), 0); } #[test] fn union_self() { let mut vregs = VirtRegs::new(); let v1 = Value::new(1); vregs.union(v1, v1); vregs.finish_union_find(None); assert_eq!(vregs.get(v1), None); assert_eq!(vregs.all_virtregs().count(), 0); } #[test] fn union_pair() { let mut vregs = VirtRegs::new(); let v1 = Value::new(1); let v2 = Value::new(2); vregs.union(v1, v2); vregs.finish_union_find(None); assert_eq!(vregs.congruence_class(&v1), &[v2, v1]); assert_eq!(vregs.congruence_class(&v2), &[v2, v1]); assert_eq!(vregs.all_virtregs().count(), 1); } #[test] fn union_pair_backwards() { let mut vregs = VirtRegs::new(); let v1 = Value::new(1); let v2 = Value::new(2); vregs.union(v2, v1); vregs.finish_union_find(None); assert_eq!(vregs.congruence_class(&v1), &[v1, v2]); assert_eq!(vregs.congruence_class(&v2), &[v1, v2]); assert_eq!(vregs.all_virtregs().count(), 1); } #[test] fn union_tree() { let mut vregs = VirtRegs::new(); let v1 = Value::new(1); let v2 = Value::new(2); let v3 = Value::new(3); let v4 = Value::new(4); vregs.union(v2, v4); vregs.union(v3, v1); // Leaders: v2, v3 vregs.union(v4, v1); vregs.finish_union_find(None); assert_eq!(vregs.congruence_class(&v1), &[v1, v3, v4, v2]); assert_eq!(vregs.congruence_class(&v2), &[v1, v3, v4, v2]); assert_eq!(vregs.congruence_class(&v3), &[v1, v3, v4, v2]); assert_eq!(vregs.congruence_class(&v4), &[v1, v3, v4, v2]); assert_eq!(vregs.all_virtregs().count(), 1); } #[test] fn union_two() { let mut vregs = VirtRegs::new(); let v1 = Value::new(1); let v2 = Value::new(2); let v3 = Value::new(3); let v4 = Value::new(4); vregs.union(v2, v4); vregs.union(v3, v1); // Leaders: v2, v3 vregs.finish_union_find(None); assert_eq!(vregs.congruence_class(&v1), &[v1, v3]); assert_eq!(vregs.congruence_class(&v2), &[v4, v2]); assert_eq!(vregs.congruence_class(&v3), &[v1, v3]); assert_eq!(vregs.congruence_class(&v4), &[v4, v2]); assert_eq!(vregs.all_virtregs().count(), 2); } #[test] fn union_uneven() { let mut vregs = VirtRegs::new(); let v1 = Value::new(1); let v2 = Value::new(2); let v3 = Value::new(3); let v4 = Value::new(4); vregs.union(v2, v4); // Rank 0-0 vregs.union(v3, v2); // Rank 0-1 vregs.union(v2, v1); // Rank 1-0 vregs.finish_union_find(None); assert_eq!(vregs.congruence_class(&v1), &[v1, v3, v4, v2]); assert_eq!(vregs.congruence_class(&v2), &[v1, v3, v4, v2]); assert_eq!(vregs.congruence_class(&v3), &[v1, v3, v4, v2]); assert_eq!(vregs.congruence_class(&v4), &[v1, v3, v4, v2]); assert_eq!(vregs.all_virtregs().count(), 1); } }