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Merge pull request #721 from phil-opp/align-up-performance

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Provide multiple implementations of align_up and mention performance
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Philipp Oppermann
2020-01-28 10:39:20 +01:00
committed by GitHub
parent c0399b04db 16a6aca76a
commit 0c67111e68
1 changed files with 30 additions and 1 deletions
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blog/content/second-edition/posts/11-allocator-designs/index.md
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@@ -295,11 +295,14 @@ Compared to the previous prototype, the `alloc` implementation now respects alig
The `dealloc` function ignores the given pointer and `Layout` arguments. Instead, it just decreases the `allocations` counter. If the counter reaches `0` again, it means that all allocations were freed again. In this case, it resets the `next` address to the `heap_start` address to make the complete heap memory available again.
The `align_up` function is general enough that we can put it into the parent `allocator` module. It looks like this:
#### Address Alignment
The `align_up` function is general enough that we can put it into the parent `allocator` module. A basic implementation looks like this:
```rust
// in src/allocator.rs
/// Align the given address `addr` upwards to alignment `align`.
fn align_up(addr: usize, align: usize) -> usize {
let remainder = addr % align;
if remainder == 0 {
@@ -314,6 +317,32 @@ The function first computes the [remainder] of the division of `addr` by `align`
[remainder]: https://en.wikipedia.org/wiki/Euclidean_division
Note that this isn't the most efficient way to implement this function. A much faster implementation looks like this:
```rust
/// Align the given address `addr` upwards to alignment `align`.
///
/// Requires that `align` is a power of two.
fn align_up(addr: usize, align: usize) -> usize {
(addr + align - 1) & !(align - 1)
}
```
This method utilizes that the `GlobalAlloc` trait guarantees that `align` is always a power of two. This makes it possible to create a [bitmask] to align the address in a very efficient way. To understand how it works, let's go through it step by step starting on the right side:
[bitmask]: https://en.wikipedia.org/wiki/Mask_(computing)
- Since `align` is a power of two, its [binary representation] has only a single bit set (e.g. `0b000100000`). This means that `align - 1` has all the lower bits set (e.g. `0b00011111`).
- By creating the [bitwise `NOT`] through the `!` operator, we get a number that has all the bits set except for the bits lower than `align` (e.g. `0b…111111111100000`).
- By performing a [bitwise `AND`] on an address and `!(align - 1)`, we align the address _downwards_. This works by clearing all the bits that are lower than `align`.
- Since we want to align upwards instead of downwards, we increase the `addr` by `align - 1` before performing the bitwise `AND`. This way, already aligned addresses remain the same while non-aligned addresses are rounded to the next alignment boundary.
[binary representation]: https://en.wikipedia.org/wiki/Binary_number#Representation
[bitwise `NOT`]: https://en.wikipedia.org/wiki/Bitwise_operation#NOT
[bitwise `AND`]: https://en.wikipedia.org/wiki/Bitwise_operation#AND
Which variant you choose it up to you. Both compute the same result, only using different methods.
### Using It
To use the bump allocator instead of the `linked_list_allocator` crate, we need to update the `ALLOCATOR` static in `allocator.rs`: