1 files changed, 145 insertions, 0 deletions
diff --git a/compiler/rustc_passes/src/liveness/rwu_table.rs b/compiler/rustc_passes/src/liveness/rwu_table.rs
new file mode 100644
index 000000000..6d5983f53
--- /dev/null
+++ b/compiler/rustc_passes/src/liveness/rwu_table.rs
@@ -0,0 +1,145 @@
+use crate::liveness::{LiveNode, Variable};
+use std::iter;
+
+#[derive(Clone, Copy)]
+pub(super) struct RWU {
+    pub(super) reader: bool,
+    pub(super) writer: bool,
+    pub(super) used: bool,
+}
+
+/// Conceptually, this is like a `Vec<Vec<RWU>>`. But the number of
+/// RWU`s can get very large, so it uses a more compact representation.
+pub(super) struct RWUTable {
+    /// Total number of live nodes.
+    live_nodes: usize,
+    /// Total number of variables.
+    vars: usize,
+
+    /// A compressed representation of `RWU`s.
+    ///
+    /// Each word represents 2 different `RWU`s packed together. Each packed RWU
+    /// is stored in 4 bits: a reader bit, a writer bit, a used bit and a
+    /// padding bit.
+    ///
+    /// The data for each live node is contiguous and starts at a word boundary,
+    /// so there might be an unused space left.
+    words: Vec<u8>,
+    /// Number of words per each live node.
+    live_node_words: usize,
+}
+
+impl RWUTable {
+    const RWU_READER: u8 = 0b0001;
+    const RWU_WRITER: u8 = 0b0010;
+    const RWU_USED: u8 = 0b0100;
+    const RWU_MASK: u8 = 0b1111;
+
+    /// Size of packed RWU in bits.
+    const RWU_BITS: usize = 4;
+    /// Size of a word in bits.
+    const WORD_BITS: usize = std::mem::size_of::<u8>() * 8;
+    /// Number of packed RWUs that fit into a single word.
+    const WORD_RWU_COUNT: usize = Self::WORD_BITS / Self::RWU_BITS;
+
+    pub(super) fn new(live_nodes: usize, vars: usize) -> RWUTable {
+        let live_node_words = (vars + Self::WORD_RWU_COUNT - 1) / Self::WORD_RWU_COUNT;
+        Self { live_nodes, vars, live_node_words, words: vec![0u8; live_node_words * live_nodes] }
+    }
+
+    fn word_and_shift(&self, ln: LiveNode, var: Variable) -> (usize, u32) {
+        assert!(ln.index() < self.live_nodes);
+        assert!(var.index() < self.vars);
+
+        let var = var.index();
+        let word = var / Self::WORD_RWU_COUNT;
+        let shift = Self::RWU_BITS * (var % Self::WORD_RWU_COUNT);
+        (ln.index() * self.live_node_words + word, shift as u32)
+    }
+
+    fn pick2_rows_mut(&mut self, a: LiveNode, b: LiveNode) -> (&mut [u8], &mut [u8]) {
+        assert!(a.index() < self.live_nodes);
+        assert!(b.index() < self.live_nodes);
+        assert!(a != b);
+
+        let a_start = a.index() * self.live_node_words;
+        let b_start = b.index() * self.live_node_words;
+
+        unsafe {
+            let ptr = self.words.as_mut_ptr();
+            (
+                std::slice::from_raw_parts_mut(ptr.add(a_start), self.live_node_words),
+                std::slice::from_raw_parts_mut(ptr.add(b_start), self.live_node_words),
+            )
+        }
+    }
+
+    pub(super) fn copy(&mut self, dst: LiveNode, src: LiveNode) {
+        if dst == src {
+            return;
+        }
+
+        let (dst_row, src_row) = self.pick2_rows_mut(dst, src);
+        dst_row.copy_from_slice(src_row);
+    }
+
+    /// Sets `dst` to the union of `dst` and `src`, returns true if `dst` was
+    /// changed.
+    pub(super) fn union(&mut self, dst: LiveNode, src: LiveNode) -> bool {
+        if dst == src {
+            return false;
+        }
+
+        let mut changed = false;
+        let (dst_row, src_row) = self.pick2_rows_mut(dst, src);
+        for (dst_word, src_word) in iter::zip(dst_row, &*src_row) {
+            let old = *dst_word;
+            let new = *dst_word | src_word;
+            *dst_word = new;
+            changed |= old != new;
+        }
+        changed
+    }
+
+    pub(super) fn get_reader(&self, ln: LiveNode, var: Variable) -> bool {
+        let (word, shift) = self.word_and_shift(ln, var);
+        (self.words[word] >> shift) & Self::RWU_READER != 0
+    }
+
+    pub(super) fn get_writer(&self, ln: LiveNode, var: Variable) -> bool {
+        let (word, shift) = self.word_and_shift(ln, var);
+        (self.words[word] >> shift) & Self::RWU_WRITER != 0
+    }
+
+    pub(super) fn get_used(&self, ln: LiveNode, var: Variable) -> bool {
+        let (word, shift) = self.word_and_shift(ln, var);
+        (self.words[word] >> shift) & Self::RWU_USED != 0
+    }
+
+    pub(super) fn get(&self, ln: LiveNode, var: Variable) -> RWU {
+        let (word, shift) = self.word_and_shift(ln, var);
+        let rwu_packed = self.words[word] >> shift;
+        RWU {
+            reader: rwu_packed & Self::RWU_READER != 0,
+            writer: rwu_packed & Self::RWU_WRITER != 0,
+            used: rwu_packed & Self::RWU_USED != 0,
+        }
+    }
+
+    pub(super) fn set(&mut self, ln: LiveNode, var: Variable, rwu: RWU) {
+        let mut packed = 0;
+        if rwu.reader {
+            packed |= Self::RWU_READER;
+        }
+        if rwu.writer {
+            packed |= Self::RWU_WRITER;
+        }
+        if rwu.used {
+            packed |= Self::RWU_USED;
+        }
+
+        let (word, shift) = self.word_and_shift(ln, var);
+        let word = &mut self.words[word];
+        *word = (*word & !(Self::RWU_MASK << shift)) | (packed << shift)
+    }
+}