Implemented a basic memory arena in a sub-crate.

Cargo.lock

@@ -5,6 +5,7 @@ dependencies = [
  "crossbeam 0.2.10 (registry+https://github.com/rust-lang/crates.io-index)",
  "docopt 0.6.86 (registry+https://github.com/rust-lang/crates.io-index)",
  "lodepng 0.8.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "mem_arena 0.1.0",
  "nom 1.2.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "num_cpus 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "openexr 0.1.0 (git+https://github.com/cessen/openexr-rs?rev=612fc6c81c031970ffddcab15509236711613de8)",
@@ -85,6 +86,10 @@ dependencies = [
  "rgb 0.5.4 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "mem_arena"
+version = "0.1.0"
+
 [[package]]
 name = "memchr"
 version = "0.1.11"

Cargo.toml

@@ -1,3 +1,6 @@
+[workspace]
+members = ["mem_arena"]
+
 [package]
 name = "psychopath"
 version = "0.1.0"
@@ -10,6 +13,7 @@ simd_perf = ["simd"]
 debug = true
 
 [dependencies]
+# Crates.io dependencies
 time = "0.1"
 docopt = "0.6"
 rustc-serialize = "0.3"
@@ -18,6 +22,11 @@ scoped_threadpool = "0.1"
 crossbeam = "0.2"
 num_cpus = "1.0"
 lodepng = "0.8"
+simd = { version = "0.1.1", optional = true }
+
+# Github dependencies
 openexr = { git = "https://github.com/cessen/openexr-rs", rev = "612fc6c81c031970ffddcab15509236711613de8" }
 
-simd = { version = "0.1.1", optional = true }
+# Local crate dependencies
+[dependencies.mem_arena]
+path = "mem_arena"

mem_arena/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "mem_arena"
+version = "0.1.0"
+authors = ["Nathan Vegdahl <cessen@cessen.com>"]
+license = "MIT"
+
+[lib]
+name = "mem_arena"
+path = "src/lib.rs"

mem_arena/src/lib.rs

@@ -0,0 +1,152 @@
+use std::slice;
+use std::mem::{size_of, align_of};
+
+const DEFAULT_BLOCK_SIZE: usize = (1 << 20) * 32; // 32 MiB
+const DEFAULT_LARGE_ALLOCATION_THRESHOLD: usize = 1 << 20;
+
+fn alignment_offset(addr: usize, alignment: usize) -> usize {
+    (alignment - (addr % alignment)) % alignment
+}
+
+/// A growable memory arena for Copy types.
+///
+/// The arena works by allocating memory in blocks of a fixed size.  It doles
+/// out memory from the current block until an amount of memory is requested that
+/// doesn't fit in the remainder of the current block, and then allocates a new
+/// block.
+///
+/// Additionally, to minimize unused space in blocks, allocations above a specified
+/// size (the large allocation threshold) are given their own block.
+///
+/// The block size and large allocation threshold size are configurable.
+pub struct Arena {
+    blocks: Vec<Vec<u8>>,
+    block_size: usize,
+    large_alloc_threshold: usize,
+}
+
+impl Arena {
+    /// Create a new arena, with default block size and large allocation threshold
+    pub fn new() -> Arena {
+        Arena {
+            blocks: vec![Vec::with_capacity(DEFAULT_BLOCK_SIZE)],
+            block_size: DEFAULT_BLOCK_SIZE,
+            large_alloc_threshold: DEFAULT_LARGE_ALLOCATION_THRESHOLD,
+        }
+    }
+
+    pub fn new_with_settings(block_size: usize, large_alloc_threshold: usize) -> Arena {
+        assert!(large_alloc_threshold <= block_size);
+
+        Arena {
+            blocks: vec![Vec::with_capacity(block_size)],
+            block_size: block_size,
+            large_alloc_threshold: large_alloc_threshold,
+        }
+    }
+
+    /// Frees all memory currently allocated by the arena, resetting itself to start
+    /// fresh.
+    ///
+    /// CAUTION: this is unsafe because it does NOT ensure that all references to the data are
+    /// gone, so this can potentially lead to dangling references.
+    pub unsafe fn free_all_and_reset(&mut self) {
+        self.blocks.clear();
+        self.blocks.shrink_to_fit();
+        self.blocks.push(Vec::with_capacity(self.block_size));
+    }
+
+    /// Allocates memory for and initializes a type T, returning a mutable reference to it.
+    pub fn alloc<'a, T: Copy>(&'a mut self, value: T) -> &'a mut T {
+        let mut memory = unsafe { self.alloc_uninitialized() };
+        *memory = value;
+        memory
+    }
+
+    /// Allocates memory for a type `T`, returning a mutable reference to it.
+    ///
+    /// CAUTION: the memory returned is uninitialized.  Make sure to initalize before using!
+    pub unsafe fn alloc_uninitialized<'a, T: Copy>(&'a mut self) -> &'a mut T {
+        let memory = self.alloc_space(size_of::<T>(), align_of::<T>()) as *mut T;
+
+        memory.as_mut().unwrap()
+    }
+
+    /// Allocates memory for `len` values of type `T`, returning a mutable slice to it.
+    /// All elements are initialized to the given `value`.
+    pub fn alloc_array<'a, T: Copy>(&'a mut self, len: usize, value: T) -> &'a mut [T] {
+        let memory = unsafe { self.alloc_array_uninitialized(len) };
+
+        for v in memory.iter_mut() {
+            *v = value;
+        }
+
+        memory
+    }
+
+    /// Allocates memory for `len` values of type `T`, returning a mutable slice to it.
+    /// All elements are initialized to the given `value`.
+    pub unsafe fn alloc_array_uninitialized<'a, T: Copy>(&'a mut self, len: usize) -> &'a mut [T] {
+        let array_mem_size = {
+            let alignment_padding = alignment_offset(size_of::<T>(), align_of::<T>());
+            let aligned_type_size = size_of::<T>() + alignment_padding;
+            aligned_type_size * len
+        };
+
+        let memory = self.alloc_space(array_mem_size, align_of::<T>()) as *mut T;
+
+        slice::from_raw_parts_mut(memory, len)
+    }
+
+    /// Allocates space with a given size and alignment.
+    ///
+    /// CAUTION: this is unsafe because it returns uninitialized memory.
+    /// Make sure to initialize the memory after calling.
+    unsafe fn alloc_space<'a>(&'a mut self, size: usize, alignment: usize) -> *mut u8 {
+        assert!(size > 0);
+        assert!(alignment > 0);
+
+        // If the desired size is considered a "large allocation", give it its own memory block.
+        if size > self.large_alloc_threshold {
+            self.blocks.push(Vec::with_capacity(size + alignment - 1));
+            self.blocks.last_mut().unwrap().set_len(size + alignment - 1);
+
+            let start_index = alignment_offset(self.blocks.last().unwrap().as_ptr() as usize,
+                                               alignment);
+
+            let block_ptr = self.blocks.last_mut().unwrap().as_mut_ptr();
+            return block_ptr.offset(start_index as isize);
+        }
+        // If the desired size is not a "large allocation", try to fit it into the current
+        // block, and only create a new block if doesn't fit.
+        else {
+            let start_index = {
+                let block_addr = self.blocks.first().unwrap().as_ptr() as usize;
+                let block_filled = self.blocks.first().unwrap().len();
+                block_filled + alignment_offset(block_addr + block_filled, alignment)
+            };
+
+            // If it will fit in the current block, use the current block.
+            if (start_index + size) <= self.blocks.first().unwrap().capacity() {
+                self.blocks.first_mut().unwrap().set_len(start_index + size);
+
+                let block_ptr = self.blocks.first_mut().unwrap().as_mut_ptr();
+                return block_ptr.offset(start_index as isize);
+            }
+            // If it won't fit in the current block, create a new block and use that.
+            else {
+                self.blocks.push(Vec::with_capacity(self.block_size));
+                let block_count = self.blocks.len();
+                self.blocks.swap(0, block_count - 1);
+
+                let start_index = alignment_offset(self.blocks.first().unwrap().as_ptr() as usize,
+                                                   alignment);
+
+                self.blocks.first_mut().unwrap().set_len(start_index + size);
+
+                let block_ptr = self.blocks.first_mut().unwrap().as_mut_ptr();
+                return block_ptr.offset(start_index as isize);
+            }
+        }
+    }
+}