make InitMask lazy for fully init/uninit cases

Avoid materializing bits in the InitMask bitset when a single value
would be enough: when the mask represents a fully initialized or fully
uninitialized const allocation.

compiler/rustc_middle/src/mir/interpret/allocation/init_mask.rs

@@ -10,20 +10,185 @@ type Block = u64;
 
 /// A bitmask where each bit refers to the byte with the same index. If the bit is `true`, the byte
 /// is initialized. If it is `false` the byte is uninitialized.
-// Note: for performance reasons when interning, some of the `InitMask` fields can be partially
+/// The actual bits are only materialized when needed, and we try to keep this data lazy as long as
-// hashed. (see the `Hash` impl below for more details), so the impl is not derived.
+/// possible. Currently, if all the blocks have the same value, then the mask represents either a
-#[derive(Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable)]
+/// fully initialized or fully uninitialized const allocation, so we can only store that single
-#[derive(HashStable)]
+/// value.
+#[derive(Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
 pub struct InitMask {
-    blocks: Vec<Block>,
+    pub(crate) blocks: InitMaskBlocks,
     len: Size,
 }
 
+#[derive(Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub(crate) enum InitMaskBlocks {
+    Lazy {
+        /// Whether the lazy init mask is fully initialized or uninitialized.
+        state: bool,
+    },
+    Materialized(InitMaskMaterialized),
+}
+
+impl InitMask {
+    pub fn new(size: Size, state: bool) -> Self {
+        // Blocks start lazily allocated, until we have to materialize them.
+        let blocks = InitMaskBlocks::Lazy { state };
+        InitMask { len: size, blocks }
+    }
+
+    /// Checks whether the `range` is entirely initialized.
+    ///
+    /// Returns `Ok(())` if it's initialized. Otherwise returns a range of byte
+    /// indexes for the first contiguous span of the uninitialized access.
+    #[inline]
+    pub fn is_range_initialized(&self, range: AllocRange) -> Result<(), AllocRange> {
+        let end = range.end();
+        if end > self.len {
+            return Err(AllocRange::from(self.len..end));
+        }
+
+        match self.blocks {
+            InitMaskBlocks::Lazy { state } => {
+                // Lazily allocated blocks represent the full mask, and cover the requested range by
+                // definition.
+                if state { Ok(()) } else { Err(range) }
+            }
+            InitMaskBlocks::Materialized(ref blocks) => {
+                blocks.is_range_initialized(range.start, end)
+            }
+        }
+    }
+
+    /// Sets a specified range to a value. If the range is out-of-bounds, the mask will grow to
+    /// accomodate it entirely.
+    pub fn set_range(&mut self, range: AllocRange, new_state: bool) {
+        let start = range.start;
+        let end = range.end();
+
+        let is_full_overwrite = start == Size::ZERO && end >= self.len;
+
+        // Optimize the cases of a full init/uninit state, while handling growth if needed.
+        match self.blocks {
+            InitMaskBlocks::Lazy { ref mut state } if is_full_overwrite => {
+                // This is fully overwriting the mask, and we'll still have a single initialization
+                // state: the blocks can stay lazy.
+                *state = new_state;
+                self.len = end;
+            }
+            InitMaskBlocks::Materialized(_) if is_full_overwrite => {
+                // This is also fully overwriting materialized blocks with a single initialization
+                // state: we'll have no need for these blocks anymore and can make them lazy.
+                self.blocks = InitMaskBlocks::Lazy { state: new_state };
+                self.len = end;
+            }
+            InitMaskBlocks::Lazy { state } if state == new_state => {
+                // Here we're partially overwriting the mask but the initialization state doesn't
+                // change: the blocks can stay lazy.
+                if end > self.len {
+                    self.len = end;
+                }
+            }
+            _ => {
+                // Otherwise, we have a partial overwrite that can result in a mix of initialization
+                // states, so we'll need materialized blocks.
+                let len = self.len;
+                let blocks = self.materialize_blocks();
+
+                // There are 3 cases of interest here, if we have:
+                //
+                //         [--------]
+                //         ^        ^
+                //         0        len
+                //
+                // 1) the range to set can be in-bounds:
+                //
+                //            xxxx = [start, end]
+                //         [--------]
+                //         ^        ^
+                //         0        len
+                //
+                // Here, we'll simply set the single `start` to `end` range.
+                //
+                // 2) the range to set can be partially out-of-bounds:
+                //
+                //                xxxx = [start, end]
+                //         [--------]
+                //         ^        ^
+                //         0        len
+                //
+                // We have 2 subranges to handle:
+                // - we'll set the existing `start` to `len` range.
+                // - we'll grow and set the `len` to `end` range.
+                //
+                // 3) the range to set can be fully out-of-bounds:
+                //
+                //                   ---xxxx = [start, end]
+                //         [--------]
+                //         ^        ^
+                //         0        len
+                //
+                // Since we're growing the mask to a single `new_state` value, we consider the gap
+                // from `len` to `start` to be part of the range, and have a single subrange to
+                // handle: we'll grow and set the `len` to `end` range.
+                //
+                // Note that we have to materialize, set blocks, and grow the mask. We could
+                // therefore slightly optimize things in situations where these writes overlap.
+                // However, as of writing this, growing the mask doesn't happen in practice yet, so
+                // we don't do this micro-optimization.
+
+                if end <= len {
+                    // Handle case 1.
+                    blocks.set_range_inbounds(start, end, new_state);
+                } else {
+                    if start < len {
+                        // Handle the first subrange of case 2.
+                        blocks.set_range_inbounds(start, len, new_state);
+                    }
+
+                    // Handle the second subrange of case 2, and case 3.
+                    blocks.grow(len, end - len, new_state); // `Size` operation
+                    self.len = end;
+                }
+            }
+        }
+    }
+
+    /// Materializes this mask's blocks when the mask is lazy.
+    #[inline]
+    fn materialize_blocks(&mut self) -> &mut InitMaskMaterialized {
+        if let InitMaskBlocks::Lazy { state } = self.blocks {
+            self.blocks = InitMaskBlocks::Materialized(InitMaskMaterialized::new(self.len, state));
+        }
+
+        let InitMaskBlocks::Materialized(ref mut blocks) = self.blocks else {
+            bug!("initmask blocks must be materialized here")
+        };
+        blocks
+    }
+
+    /// Returns the initialization state at the specified in-bounds index.
+    #[inline]
+    pub fn get(&self, idx: Size) -> bool {
+        match self.blocks {
+            InitMaskBlocks::Lazy { state } => state,
+            InitMaskBlocks::Materialized(ref blocks) => blocks.get(idx),
+        }
+    }
+}
+
+/// The actual materialized blocks of the bitmask, when we can't keep the `InitMask` lazy.
+// Note: for performance reasons when interning, some of the fields can be partially
+// hashed. (see the `Hash` impl below for more details), so the impl is not derived.
+#[derive(Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, HashStable)]
+pub(crate) struct InitMaskMaterialized {
+    pub(crate) blocks: Vec<Block>,
+}
+
 // Const allocations are only hashed for interning. However, they can be large, making the hashing
 // expensive especially since it uses `FxHash`: it's better suited to short keys, not potentially
 // big buffers like the allocation's init mask. We can partially hash some fields when they're
 // large.
-impl hash::Hash for InitMask {
+impl hash::Hash for InitMaskMaterialized {
     fn hash<H: hash::Hasher>(&self, state: &mut H) {
         const MAX_BLOCKS_TO_HASH: usize = super::MAX_BYTES_TO_HASH / std::mem::size_of::<Block>();
         const MAX_BLOCKS_LEN: usize = super::MAX_HASHED_BUFFER_LEN / std::mem::size_of::<Block>();
@@ -41,18 +206,15 @@ impl hash::Hash for InitMask {
         } else {
             self.blocks.hash(state);
         }
-
-        // Hash the other fields as usual.
-        self.len.hash(state);
     }
 }
 
-impl InitMask {
+impl InitMaskMaterialized {
     pub const BLOCK_SIZE: u64 = 64;
 
-    pub fn new(size: Size, state: bool) -> Self {
+    fn new(size: Size, state: bool) -> Self {
-        let mut m = InitMask { blocks: vec![], len: Size::ZERO };
+        let mut m = InitMaskMaterialized { blocks: vec![] };
-        m.grow(size, state);
+        m.grow(Size::ZERO, size, state);
         m
     }
 
@@ -62,8 +224,8 @@ impl InitMask {
         // Each bit in a `Block` represents the initialization state of one byte of an allocation,
         // so we use `.bytes()` here.
         let bits = bits.bytes();
-        let a = bits / InitMask::BLOCK_SIZE;
+        let a = bits / Self::BLOCK_SIZE;
-        let b = bits % InitMask::BLOCK_SIZE;
+        let b = bits % Self::BLOCK_SIZE;
         (usize::try_from(a).unwrap(), usize::try_from(b).unwrap())
     }
 
@@ -71,7 +233,7 @@ impl InitMask {
     fn size_from_bit_index(block: impl TryInto<u64>, bit: impl TryInto<u64>) -> Size {
         let block = block.try_into().ok().unwrap();
         let bit = bit.try_into().ok().unwrap();
-        Size::from_bytes(block * InitMask::BLOCK_SIZE + bit)
+        Size::from_bytes(block * Self::BLOCK_SIZE + bit)
     }
 
     /// Checks whether the `range` is entirely initialized.
@@ -79,13 +241,8 @@ impl InitMask {
     /// Returns `Ok(())` if it's initialized. Otherwise returns a range of byte
     /// indexes for the first contiguous span of the uninitialized access.
     #[inline]
-    pub fn is_range_initialized(&self, range: AllocRange) -> Result<(), AllocRange> {
+    fn is_range_initialized(&self, start: Size, end: Size) -> Result<(), AllocRange> {
-        let end = range.end();
+        let uninit_start = self.find_bit(start, end, false);
-        if end > self.len {
-            return Err(AllocRange::from(self.len..end));
-        }
-
-        let uninit_start = self.find_bit(range.start, end, false);
 
         match uninit_start {
             Some(uninit_start) => {
@@ -96,15 +253,6 @@ impl InitMask {
         }
     }
 
-    pub fn set_range(&mut self, range: AllocRange, new_state: bool) {
-        let end = range.end();
-        let len = self.len;
-        if end > len {
-            self.grow(end - len, new_state);
-        }
-        self.set_range_inbounds(range.start, end, new_state);
-    }
-
     fn set_range_inbounds(&mut self, start: Size, end: Size, new_state: bool) {
         let (blocka, bita) = Self::bit_index(start);
         let (blockb, bitb) = Self::bit_index(end);
@@ -150,27 +298,35 @@ impl InitMask {
     }
 
     #[inline]
-    pub fn get(&self, i: Size) -> bool {
+    fn get(&self, i: Size) -> bool {
         let (block, bit) = Self::bit_index(i);
         (self.blocks[block] & (1 << bit)) != 0
     }
 
-    fn grow(&mut self, amount: Size, new_state: bool) {
+    fn grow(&mut self, len: Size, amount: Size, new_state: bool) {
         if amount.bytes() == 0 {
             return;
         }
         let unused_trailing_bits =
-            u64::try_from(self.blocks.len()).unwrap() * Self::BLOCK_SIZE - self.len.bytes();
+            u64::try_from(self.blocks.len()).unwrap() * Self::BLOCK_SIZE - len.bytes();
+
+        // If there's not enough capacity in the currently allocated blocks, allocate some more.
         if amount.bytes() > unused_trailing_bits {
             let additional_blocks = amount.bytes() / Self::BLOCK_SIZE + 1;
-            self.blocks.extend(
+
-                // FIXME(oli-obk): optimize this by repeating `new_state as Block`.
+            // We allocate the blocks to the correct value for the requested init state, so we won't
-                iter::repeat(0).take(usize::try_from(additional_blocks).unwrap()),
+            // have to manually set them with another write.
-            );
+            let block = if new_state { u64::MAX } else { 0 };
+            self.blocks
+                .extend(iter::repeat(block).take(usize::try_from(additional_blocks).unwrap()));
+        }
+
+        // New blocks have already been set here, so we only need to set the unused trailing bits,
+        // if any.
+        if unused_trailing_bits > 0 {
+            let in_bounds_tail = Size::from_bytes(unused_trailing_bits);
+            self.set_range_inbounds(len, len + in_bounds_tail, new_state); // `Size` operation
         }
-        let start = self.len;
-        self.len += amount;
-        self.set_range_inbounds(start, start + amount, new_state); // `Size` operation
     }
 
     /// Returns the index of the first bit in `start..end` (end-exclusive) that is equal to is_init.
@@ -188,7 +344,7 @@ impl InitMask {
         /// ```
         /// Also, if not stated, assume that `is_init = true`, that is, we are searching for the first 1 bit.
         fn find_bit_fast(
-            init_mask: &InitMask,
+            init_mask: &InitMaskMaterialized,
             start: Size,
             end: Size,
             is_init: bool,
@@ -223,7 +379,7 @@ impl InitMask {
                     None
                 } else {
                     let bit = bits.trailing_zeros();
-                    Some(InitMask::size_from_bit_index(block, bit))
+                    Some(InitMaskMaterialized::size_from_bit_index(block, bit))
                 }
             }
 
@@ -253,9 +409,9 @@ impl InitMask {
             // This provides the desired behavior of searching blocks 0 and 1 for (a),
             // and searching only block 0 for (b).
             // There is no concern of overflows since we checked for `start >= end` above.
-            let (start_block, start_bit) = InitMask::bit_index(start);
+            let (start_block, start_bit) = InitMaskMaterialized::bit_index(start);
             let end_inclusive = Size::from_bytes(end.bytes() - 1);
-            let (end_block_inclusive, _) = InitMask::bit_index(end_inclusive);
+            let (end_block_inclusive, _) = InitMaskMaterialized::bit_index(end_inclusive);
 
             // Handle first block: need to skip `start_bit` bits.
             //
@@ -340,7 +496,7 @@ impl InitMask {
 
         #[cfg_attr(not(debug_assertions), allow(dead_code))]
         fn find_bit_slow(
-            init_mask: &InitMask,
+            init_mask: &InitMaskMaterialized,
             start: Size,
             end: Size,
             is_init: bool,
@@ -436,10 +592,19 @@ impl<'a> Iterator for InitChunkIter<'a> {
             return None;
         }
 
-        let end_of_chunk =
+        let end_of_chunk = match self.init_mask.blocks {
-            self.init_mask.find_bit(self.start, self.end, !self.is_init).unwrap_or(self.end);
+            InitMaskBlocks::Lazy { .. } => {
+                // If we're iterating over the chunks of lazy blocks, we just emit a single
+                // full-size chunk.
+                self.end
+            }
+            InitMaskBlocks::Materialized(ref blocks) => {
+                let end_of_chunk =
+                    blocks.find_bit(self.start, self.end, !self.is_init).unwrap_or(self.end);
+                end_of_chunk
+            }
+        };
         let range = self.start..end_of_chunk;
-
         let ret =
             Some(if self.is_init { InitChunk::Init(range) } else { InitChunk::Uninit(range) });
 
@@ -504,17 +669,19 @@ impl InitMask {
 
     /// Applies multiple instances of the run-length encoding to the initialization mask.
     pub fn apply_copy(&mut self, defined: InitCopy, range: AllocRange, repeat: u64) {
-        // An optimization where we can just overwrite an entire range of initialization
+        // An optimization where we can just overwrite an entire range of initialization bits if
-        // bits if they are going to be uniformly `1` or `0`.
+        // they are going to be uniformly `1` or `0`. If this happens to be a full-range overwrite,
+        // we won't need materialized blocks either.
         if defined.ranges.len() <= 1 {
-            self.set_range_inbounds(
+            let start = range.start;
-                range.start,
+            let end = range.start + range.size * repeat; // `Size` operations
-                range.start + range.size * repeat, // `Size` operations
+            self.set_range(AllocRange::from(start..end), defined.initial);
-                defined.initial,
-            );
             return;
         }
 
+        // We're about to do one or more partial writes, so we ensure the blocks are materialized.
+        let blocks = self.materialize_blocks();
+
         for mut j in 0..repeat {
             j *= range.size.bytes();
             j += range.start.bytes();
@@ -522,7 +689,7 @@ impl InitMask {
             for range in &defined.ranges {
                 let old_j = j;
                 j += range;
-                self.set_range_inbounds(Size::from_bytes(old_j), Size::from_bytes(j), cur);
+                blocks.set_range_inbounds(Size::from_bytes(old_j), Size::from_bytes(j), cur);
                 cur = !cur;
             }
         }