Optimize copying large ranges of undefmask blocks

src/librustc/mir/interpret/allocation.rs

@@ -100,8 +100,7 @@ impl AllocationExtra<(), ()> for () {
 impl<Tag, Extra> Allocation<Tag, Extra> {
     /// Creates a read-only allocation initialized by the given bytes
     pub fn from_bytes(slice: &[u8], align: Align, extra: Extra) -> Self {
-        let mut undef_mask = UndefMask::new(Size::ZERO);
-        undef_mask.grow(Size::from_bytes(slice.len() as u64), true);
+        let undef_mask = UndefMask::new(Size::from_bytes(slice.len() as u64), true);
         Self {
             bytes: slice.to_owned(),
             relocations: Relocations::new(),
@@ -121,7 +120,7 @@ impl<Tag, Extra> Allocation<Tag, Extra> {
         Allocation {
             bytes: vec![0; size.bytes() as usize],
             relocations: Relocations::new(),
-            undef_mask: UndefMask::new(size),
+            undef_mask: UndefMask::new(size, false),
             align,
             mutability: Mutability::Mutable,
             extra,
@@ -613,8 +612,9 @@ impl<Tag> DerefMut for Relocations<Tag> {
 ////////////////////////////////////////////////////////////////////////////////
 
 type Block = u64;
-const BLOCK_SIZE: u64 = 64;
 
+/// A bitmask where each bit refers to the byte with the same index. If the bit is `true`, the byte
+/// is defined. If it is `false` the byte is undefined.
 #[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash, RustcEncodable, RustcDecodable)]
 pub struct UndefMask {
     blocks: Vec<Block>,
@@ -624,12 +624,14 @@ pub struct UndefMask {
 impl_stable_hash_for!(struct mir::interpret::UndefMask{blocks, len});
 
 impl UndefMask {
-    pub fn new(size: Size) -> Self {
+    pub const BLOCK_SIZE: u64 = 64;
+
+    pub fn new(size: Size, state: bool) -> Self {
         let mut m = UndefMask {
             blocks: vec![],
             len: Size::ZERO,
         };
-        m.grow(size, false);
+        m.grow(size, state);
         m
     }
 
@@ -643,6 +645,7 @@ impl UndefMask {
             return Err(self.len);
         }
 
+        // FIXME(oli-obk): optimize this for allocations larger than a block.
         let idx = (start.bytes()..end.bytes())
             .map(|i| Size::from_bytes(i))
             .find(|&i| !self.get(i));
@@ -662,20 +665,63 @@ impl UndefMask {
     }
 
     pub fn set_range_inbounds(&mut self, start: Size, end: Size, new_state: bool) {
-        for i in start.bytes()..end.bytes() {
-            self.set(Size::from_bytes(i), new_state);
+        let (blocka, bita) = bit_index(start);
+        let (blockb, bitb) = bit_index(end);
+        if blocka == blockb {
+            // first set all bits but the first `bita`
+            // then unset the last `64 - bitb` bits
+            let range = if bitb == 0 {
+                u64::max_value() << bita
+            } else {
+                (u64::max_value() << bita) & (u64::max_value() >> (64 - bitb))
+            };
+            if new_state {
+                self.blocks[blocka] |= range;
+            } else {
+                self.blocks[blocka] &= !range;
+            }
+            return;
+        }
+        // across block boundaries
+        if new_state {
+            // set bita..64 to 1
+            self.blocks[blocka] |= u64::max_value() << bita;
+            // set 0..bitb to 1
+            if bitb != 0 {
+                self.blocks[blockb] |= u64::max_value() >> (64 - bitb);
+            }
+            // fill in all the other blocks (much faster than one bit at a time)
+            for block in (blocka + 1) .. blockb {
+                self.blocks[block] = u64::max_value();
+            }
+        } else {
+            // set bita..64 to 0
+            self.blocks[blocka] &= !(u64::max_value() << bita);
+            // set 0..bitb to 0
+            if bitb != 0 {
+                self.blocks[blockb] &= !(u64::max_value() >> (64 - bitb));
+            }
+            // fill in all the other blocks (much faster than one bit at a time)
+            for block in (blocka + 1) .. blockb {
+                self.blocks[block] = 0;
+            }
         }
     }
 
     #[inline]
     pub fn get(&self, i: Size) -> bool {
         let (block, bit) = bit_index(i);
-        (self.blocks[block] & 1 << bit) != 0
+        (self.blocks[block] & (1 << bit)) != 0
     }
 
     #[inline]
     pub fn set(&mut self, i: Size, new_state: bool) {
         let (block, bit) = bit_index(i);
+        self.set_bit(block, bit, new_state);
+    }
+
+    #[inline]
+    fn set_bit(&mut self, block: usize, bit: usize, new_state: bool) {
         if new_state {
             self.blocks[block] |= 1 << bit;
         } else {
@@ -684,11 +730,15 @@ impl UndefMask {
     }
 
     pub fn grow(&mut self, amount: Size, new_state: bool) {
-        let unused_trailing_bits = self.blocks.len() as u64 * BLOCK_SIZE - self.len.bytes();
+        if amount.bytes() == 0 {
+            return;
+        }
+        let unused_trailing_bits = self.blocks.len() as u64 * Self::BLOCK_SIZE - self.len.bytes();
         if amount.bytes() > unused_trailing_bits {
-            let additional_blocks = amount.bytes() / BLOCK_SIZE + 1;
+            let additional_blocks = amount.bytes() / Self::BLOCK_SIZE + 1;
             assert_eq!(additional_blocks as usize as u64, additional_blocks);
             self.blocks.extend(
+                // FIXME(oli-obk): optimize this by repeating `new_state as Block`
                 iter::repeat(0).take(additional_blocks as usize),
             );
         }
@@ -701,8 +751,8 @@ impl UndefMask {
 #[inline]
 fn bit_index(bits: Size) -> (usize, usize) {
     let bits = bits.bytes();
-    let a = bits / BLOCK_SIZE;
-    let b = bits % BLOCK_SIZE;
+    let a = bits / UndefMask::BLOCK_SIZE;
+    let b = bits % UndefMask::BLOCK_SIZE;
     assert_eq!(a as usize as u64, a);
     assert_eq!(b as usize as u64, b);
     (a as usize, b as usize)

src/librustc_mir/interpret/memory.rs

@@ -700,24 +700,29 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
         // relocations overlapping the edges; those would not be handled correctly).
         let relocations = {
             let relocations = self.get(src.alloc_id)?.relocations(self, src, size);
-            let mut new_relocations = Vec::with_capacity(relocations.len() * (length as usize));
-            for i in 0..length {
-                new_relocations.extend(
-                    relocations
-                    .iter()
-                    .map(|&(offset, reloc)| {
-                        // compute offset for current repetition
-                        let dest_offset = dest.offset + (i * size);
-                        (
-                            // shift offsets from source allocation to destination allocation
-                            offset + dest_offset - src.offset,
-                            reloc,
-                        )
-                    })
-                );
-            }
+            if relocations.is_empty() {
+                // nothing to copy, ignore even the `length` loop
+                Vec::new()
+            } else {
+                let mut new_relocations = Vec::with_capacity(relocations.len() * (length as usize));
+                for i in 0..length {
+                    new_relocations.extend(
+                        relocations
+                        .iter()
+                        .map(|&(offset, reloc)| {
+                            // compute offset for current repetition
+                            let dest_offset = dest.offset + (i * size);
+                            (
+                                // shift offsets from source allocation to destination allocation
+                                offset + dest_offset - src.offset,
+                                reloc,
+                            )
+                        })
+                    );
+                }
 
-            new_relocations
+                new_relocations
+            }
         };
 
         let tcx = self.tcx.tcx;
@@ -784,20 +789,65 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
         // The bits have to be saved locally before writing to dest in case src and dest overlap.
         assert_eq!(size.bytes() as usize as u64, size.bytes());
 
-        let undef_mask = self.get(src.alloc_id)?.undef_mask.clone();
-        let dest_allocation = self.get_mut(dest.alloc_id)?;
+        let undef_mask = &self.get(src.alloc_id)?.undef_mask;
+
+        // Since we are copying `size` bytes from `src` to `dest + i * size` (`for i in 0..repeat`),
+        // a naive undef mask copying algorithm would repeatedly have to read the undef mask from
+        // the source and write it to the destination. Even if we optimized the memory accesses,
+        // we'd be doing all of this `repeat` times.
+        // Therefor we precompute a compressed version of the undef mask of the source value and
+        // then write it back `repeat` times without computing any more information from the source.
+
+        // a precomputed cache for ranges of defined/undefined bits
+        // 0000010010001110 will become
+        // [5, 1, 2, 1, 3, 3, 1]
+        // where each element toggles the state
+        let mut ranges = smallvec::SmallVec::<[u64; 1]>::new();
+        let first = undef_mask.get(src.offset);
+        let mut cur_len = 1;
+        let mut cur = first;
+        for i in 1..size.bytes() {
+            // FIXME: optimize to bitshift the current undef block's bits and read the top bit
+            if undef_mask.get(src.offset + Size::from_bytes(i)) == cur {
+                cur_len += 1;
+            } else {
+                ranges.push(cur_len);
+                cur_len = 1;
+                cur = !cur;
+            }
+        }
 
-        for i in 0..size.bytes() {
-            let defined = undef_mask.get(src.offset + Size::from_bytes(i));
+        // now fill in all the data
+        let dest_allocation = self.get_mut(dest.alloc_id)?;
+        // an optimization where we can just overwrite an entire range of definedness bits if
+        // they are going to be uniformly `1` or `0`.
+        if ranges.is_empty() {
+            dest_allocation.undef_mask.set_range_inbounds(
+                dest.offset,
+                dest.offset + size * repeat,
+                first,
+            );
+            return Ok(())
+        }
 
-            for j in 0..repeat {
-                dest_allocation.undef_mask.set(
-                    dest.offset + Size::from_bytes(i + (size.bytes() * j)),
-                    defined
+        // remember to fill in the trailing bits
+        ranges.push(cur_len);
+
+        for mut j in 0..repeat {
+            j *= size.bytes();
+            j += dest.offset.bytes();
+            let mut cur = first;
+            for range in &ranges {
+                let old_j = j;
+                j += range;
+                dest_allocation.undef_mask.set_range_inbounds(
+                    Size::from_bytes(old_j),
+                    Size::from_bytes(j),
+                    cur,
                 );
+                cur = !cur;
             }
         }
-
         Ok(())
     }
 }

src/test/run-pass-fulldeps/undef_mask.rs

@@ -0,0 +1,26 @@
+// ignore-cross-compile
+// ignore-stage1
+
+#![feature(rustc_private)]
+
+extern crate rustc;
+
+use rustc::mir::interpret::UndefMask;
+use rustc::ty::layout::Size;
+
+fn main() {
+    let mut mask = UndefMask::new(Size::from_bytes(500), false);
+    assert!(!mask.get(Size::from_bytes(499)));
+    mask.set(Size::from_bytes(499), true);
+    assert!(mask.get(Size::from_bytes(499)));
+    mask.set_range_inbounds(Size::from_bytes(100), Size::from_bytes(256), true);
+    for i in 0..100 {
+        assert!(!mask.get(Size::from_bytes(i)));
+    }
+    for i in 100..256 {
+        assert!(mask.get(Size::from_bytes(i)));
+    }
+    for i in 256..499 {
+        assert!(!mask.get(Size::from_bytes(i)));
+    }
+}