mirror of https://github.com/rust-lang/nomicon
Steve Klabnik
8 years ago
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# Aliasing
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First off, let's get some important caveats out of this way:
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* We will be using the broadest possible definition of aliasing for the sake
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of discussion. Rust's definition will probably be more restricted to factor
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in mutations and liveness.
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* We will be assuming a single-threaded, interrupt-free, execution. We will also
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be ignoring things like memory-mapped hardware. Rust assumes these things
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don't happen unless you tell it otherwise. For more details, see the
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[Concurrency Chapter](concurrency.html).
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With that said, here's our working definition: variables and pointers *alias*
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if they refer to overlapping regions of memory.
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# Why Aliasing Matters
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So why should we care about aliasing?
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Consider this simple function:
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```rust
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fn compute(input: &u32, output: &mut u32) {
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if *input > 10 {
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*output = 1;
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}
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if *input > 5 {
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*output *= 2;
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}
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}
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```
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We would *like* to be able to optimize it to the following function:
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```rust
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fn compute(input: &u32, output: &mut u32) {
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let cached_input = *input; // keep *input in a register
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if cached_input > 10 {
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*output = 2; // x > 10 implies x > 5, so double and exit immediately
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} else if cached_input > 5 {
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*output *= 2;
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}
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}
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```
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In Rust, this optimization should be sound. For almost any other language, it
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wouldn't be (barring global analysis). This is because the optimization relies
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on knowing that aliasing doesn't occur, which most languages are fairly liberal
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with. Specifically, we need to worry about function arguments that make `input`
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and `output` overlap, such as `compute(&x, &mut x)`.
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With that input, we could get this execution:
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```rust,ignore
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// input == output == 0xabad1dea
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// *input == *output == 20
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if *input > 10 { // true (*input == 20)
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*output = 1; // also overwrites *input, because they are the same
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}
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if *input > 5 { // false (*input == 1)
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*output *= 2;
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}
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// *input == *output == 1
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```
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Our optimized function would produce `*output == 2` for this input, so the
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correctness of our optimization relies on this input being impossible.
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In Rust we know this input should be impossible because `&mut` isn't allowed to be
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aliased. So we can safely reject its possibility and perform this optimization.
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In most other languages, this input would be entirely possible, and must be considered.
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This is why alias analysis is important: it lets the compiler perform useful
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optimizations! Some examples:
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* keeping values in registers by proving no pointers access the value's memory
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* eliminating reads by proving some memory hasn't been written to since last we read it
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* eliminating writes by proving some memory is never read before the next write to it
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* moving or reordering reads and writes by proving they don't depend on each other
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These optimizations also tend to prove the soundness of bigger optimizations
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such as loop vectorization, constant propagation, and dead code elimination.
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In the previous example, we used the fact that `&mut u32` can't be aliased to prove
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that writes to `*output` can't possibly affect `*input`. This let us cache `*input`
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in a register, eliminating a read.
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By caching this read, we knew that the the write in the `> 10` branch couldn't
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affect whether we take the `> 5` branch, allowing us to also eliminate a
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read-modify-write (doubling `*output`) when `*input > 10`.
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The key thing to remember about alias analysis is that writes are the primary
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hazard for optimizations. That is, the only thing that prevents us
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from moving a read to any other part of the program is the possibility of us
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re-ordering it with a write to the same location.
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For instance, we have no concern for aliasing in the following modified version
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of our function, because we've moved the only write to `*output` to the very
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end of our function. This allows us to freely reorder the reads of `*input` that
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occur before it:
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```rust
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fn compute(input: &u32, output: &mut u32) {
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let mut temp = *output;
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if *input > 10 {
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temp = 1;
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}
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if *input > 5 {
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temp *= 2;
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}
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*output = temp;
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}
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```
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We're still relying on alias analysis to assume that `temp` doesn't alias
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`input`, but the proof is much simpler: the value of a local variable can't be
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aliased by things that existed before it was declared. This is an assumption
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every language freely makes, and so this version of the function could be
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optimized the way we want in any language.
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This is why the definition of "alias" that Rust will use likely involves some
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notion of liveness and mutation: we don't actually care if aliasing occurs if
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there aren't any actual writes to memory happening.
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Of course, a full aliasing model for Rust must also take into consideration things like
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function calls (which may mutate things we don't see), raw pointers (which have
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no aliasing requirements on their own), and UnsafeCell (which lets the referent
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of an `&` be mutated).
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