regalloc2/src/ion/spill.rs

/*
 * This file was initially derived from the files
 * `js/src/jit/BacktrackingAllocator.h` and
 * `js/src/jit/BacktrackingAllocator.cpp` in Mozilla Firefox, and was
 * originally licensed under the Mozilla Public License 2.0. We
 * subsequently relicensed it to Apache-2.0 WITH LLVM-exception (see
 * https://github.com/bytecodealliance/regalloc2/issues/7).
 *
 * Since the initial port, the design has been substantially evolved
 * and optimized.
 */

//! Spillslot allocation.

use super::{
    AllocRegResult, Env, LiveRangeKey, LiveRangeSet, PReg, PRegIndex, RegTraversalIter,
    SpillSetIndex, SpillSlotData, SpillSlotIndex, SpillSlotList,
};
use crate::{Allocation, Function, SpillSlot};
use smallvec::smallvec;

impl<'a, F: Function> Env<'a, F> {
    pub fn try_allocating_regs_for_spilled_bundles(&mut self) {
        trace!("allocating regs for spilled bundles");
        for i in 0..self.spilled_bundles.len() {
            let bundle = self.spilled_bundles[i]; // don't borrow self

            if self.bundles[bundle.index()].ranges.is_empty() {
                continue;
            }

            let class = self.spillsets[self.bundles[bundle.index()].spillset.index()].class;
            let hint = self.spillsets[self.bundles[bundle.index()].spillset.index()].reg_hint;

            // This may be an empty-range bundle whose ranges are not
            // sorted; sort all range-lists again here.
            self.bundles[bundle.index()]
                .ranges
                .sort_unstable_by_key(|entry| entry.range.from);

            let mut success = false;
            self.stats.spill_bundle_reg_probes += 1;
            for preg in
                RegTraversalIter::new(self.env, class, hint, PReg::invalid(), bundle.index(), None)
            {
                trace!("trying bundle {:?} to preg {:?}", bundle, preg);
                let preg_idx = PRegIndex::new(preg.index());
                if let AllocRegResult::Allocated(_) =
                    self.try_to_allocate_bundle_to_reg(bundle, preg_idx, None)
                {
                    self.stats.spill_bundle_reg_success += 1;
                    success = true;
                    break;
                }
            }
            if !success {
                trace!(
                    "spilling bundle {:?}: marking spillset {:?} as required",
                    bundle,
                    self.bundles[bundle.index()].spillset
                );
                self.spillsets[self.bundles[bundle.index()].spillset.index()].required = true;
            }
        }
    }

    pub fn spillslot_can_fit_spillset(
        &mut self,
        spillslot: SpillSlotIndex,
        spillset: SpillSetIndex,
    ) -> bool {
        for &vreg in &self.spillsets[spillset.index()].vregs {
            for entry in &self.vregs[vreg.index()].ranges {
                if self.spillslots[spillslot.index()]
                    .ranges
                    .btree
                    .contains_key(&LiveRangeKey::from_range(&entry.range))
                {
                    return false;
                }
            }
        }
        true
    }

    pub fn allocate_spillset_to_spillslot(
        &mut self,
        spillset: SpillSetIndex,
        spillslot: SpillSlotIndex,
    ) {
        self.spillsets[spillset.index()].slot = spillslot;

        for vreg in &self.spillsets[spillset.index()].vregs {
            trace!(
                "spillslot {:?} alloc'ed to spillset {:?}: vreg {:?}",
                spillslot,
                spillset,
                vreg,
            );
            for entry in &self.vregs[vreg.index()].ranges {
                trace!(
                    "spillslot {:?} getting range {:?} from LR {:?} from vreg {:?}",
                    spillslot,
                    entry.range,
                    entry.index,
                    vreg,
                );
                self.spillslots[spillslot.index()]
                    .ranges
                    .btree
                    .insert(LiveRangeKey::from_range(&entry.range), entry.index);
            }
        }
    }

    pub fn allocate_spillslots(&mut self) {
        const MAX_ATTEMPTS: usize = 10;

        for spillset in 0..self.spillsets.len() {
            trace!("allocate spillslot: {}", spillset);
            let spillset = SpillSetIndex::new(spillset);
            if !self.spillsets[spillset.index()].required {
                continue;
            }
            // Get or create the spillslot list for this size.
            let size = self.spillsets[spillset.index()].size as usize;
            if size >= self.slots_by_size.len() {
                self.slots_by_size.resize(
                    size + 1,
                    SpillSlotList {
                        slots: smallvec![],
                        probe_start: 0,
                    },
                );
            }
            // Try a few existing spillslots.
            let mut i = self.slots_by_size[size].probe_start;
            let mut success = false;
            // Never probe the same element more than once: limit the
            // attempt count to the number of slots in existence.
            for _attempt in 0..std::cmp::min(self.slots_by_size[size].slots.len(), MAX_ATTEMPTS) {
                // Note: this indexing of `slots` is always valid
                // because either the `slots` list is empty and the
                // iteration limit above consequently means we don't
                // run this loop at all, or else `probe_start` is
                // in-bounds (because it is made so below when we add
                // a slot, and it always takes on the last index `i`
                // after this loop).
                let spillslot = self.slots_by_size[size].slots[i];

                if self.spillslot_can_fit_spillset(spillslot, spillset) {
                    self.allocate_spillset_to_spillslot(spillset, spillslot);
                    success = true;
                    self.slots_by_size[size].probe_start = i;
                    break;
                }

                i = self.slots_by_size[size].next_index(i);
            }

            if !success {
                // Allocate a new spillslot.
                let spillslot = SpillSlotIndex::new(self.spillslots.len());
                self.spillslots.push(SpillSlotData {
                    ranges: LiveRangeSet::new(),
                    alloc: Allocation::none(),
                    slots: size as u32,
                });
                self.slots_by_size[size].slots.push(spillslot);
                self.slots_by_size[size].probe_start = self.slots_by_size[size].slots.len() - 1;

                self.allocate_spillset_to_spillslot(spillset, spillslot);
            }
        }

        // Assign actual slot indices to spillslots.
        for i in 0..self.spillslots.len() {
            self.spillslots[i].alloc = self.allocate_spillslot(self.spillslots[i].slots);
        }

        trace!("spillslot allocator done");
    }

    pub fn allocate_spillslot(&mut self, size: u32) -> Allocation {
        let mut offset = self.num_spillslots;
        // Align up to `size`.
        debug_assert!(size.is_power_of_two());
        offset = (offset + size - 1) & !(size - 1);
        let slot = if self.func.multi_spillslot_named_by_last_slot() {
            offset + size - 1
        } else {
            offset
        };
        offset += size;
        self.num_spillslots = offset;
        Allocation::stack(SpillSlot::new(slot as usize))
    }
}