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<ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="meet-safe-and-unsafe.html"><strong aria-hidden="true">1.</strong> Meet Safe and Unsafe</a></li><li><ol class="section"><li><a href="safe-unsafe-meaning.html"><strong aria-hidden="true">1.1.</strong> How Safe and Unsafe Interact</a></li><li><a href="what-unsafe-does.html"><strong aria-hidden="true">1.2.</strong> What Unsafe Can Do</a></li><li><a href="working-with-unsafe.html"><strong aria-hidden="true">1.3.</strong> Working with Unsafe</a></li></ol></li><li><a href="data.html"><strong aria-hidden="true">2.</strong> Data Layout</a></li><li><ol class="section"><li><a href="repr-rust.html"><strong aria-hidden="true">2.1.</strong> repr(Rust)</a></li><li><a href="exotic-sizes.html"><strong aria-hidden="true">2.2.</strong> Exotically Sized Types</a></li><li><a href="other-reprs.html"><strong aria-hidden="true">2.3.</strong> Other reprs</a></li></ol></li><li><a href="ownership.html"><strong aria-hidden="true">3.</strong> Ownership</a></li><li><ol class="section"><li><a href="references.html"><strong aria-hidden="true">3.1.</strong> References</a></li><li><a href="aliasing.html" class="active"><strong aria-hidden="true">3.2.</strong> Aliasing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.3.</strong> Lifetimes</a></li><li><a href="lifetime-mismatch.html"><strong aria-hidden="true">3.4.</strong> Limits of Lifetimes</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">3.5.</strong> Lifetime Elision</a></li><li><a href="unbounded-lifetimes.html"><strong aria-hidden="true">3.6.</strong> Unbounded Lifetimes</a></li><li><a href="hrtb.html"><strong aria-hidden="true">3.7.</strong> Higher-Rank Trait Bounds</a></li><li><a href="subtyping.html"><strong aria-hidden="true">3.8.</strong> Subtyping and Variance</a></li><li><a href="dropck.html"><strong aria-hidden="true">3.9.</strong> Drop Check</a></li><li><a href="phantom-data.html"><strong aria-hidden="true">3.10.</strong> PhantomData</a></li><li><a href="borrow-splitting.html"><strong aria-hidden="true">3.11.</strong> Splitting Borrows</a></li></ol></li><li><a href="conversions.html"><strong aria-hidden="true">4.</strong> Type Conversions</a></li><li><ol class="section"><li><a href="coercions.html"><strong aria-hidden="true">4.1.</strong> Coercions</a></li><li><a href="dot-operator.html"><strong aria-hidden="true">4.2.</strong> The Dot Operator</a></li><li><a href="casts.html"><strong aria-hidden="true">4.3.</strong> Casts</a></li><li><a href="transmutes.html"><strong aria-hidden="true">4.4.</strong> Transmutes</a></li></ol></li><li><a href="uninitialized.html"><strong aria-hidden="true">5.</strong> Uninitialized Memory</a></li><li><ol class="section"><li><a href="checked-uninit.html"><strong aria-hidden="true">5.1.</strong> Checked</a></li><li><a href="drop-flags.html"><strong aria-hidden="true">5.2.</strong> Drop Flags</a></li><li><a href="unchecked-uninit.html"><strong aria-hidden="true">5.3.</strong> Unchecked</a></li></ol></li><li><a href="obrm.html"><strong aria-hidden="true">6.</strong> Ownership Based Resource Management</a></li><li><ol class="section"><li><a href="constructors.html"><strong aria-hidden="true">6.1.</strong> Constructors</a></li><li><a href="destructors.html"><strong aria-hidden="true">6.2.</strong> Destructors</a></li><li><a href="leaking.html"><strong aria-hidden="true">6.3.</strong> Leaking</a></li></ol></li><li><a href="unwinding.html"><strong aria-hidden="true">7.</strong> Unwinding</a></li><li><ol class="section"><li><a href="exception-safety.html"><strong aria-hidden="true">7.1.</strong> Exception Safety</a></li><li><a href="poisoning.html"><strong aria-hidden="true">7.2.</strong> Poisoning</a></li></ol></li><li><a href="concurrency.html"><strong aria-hidden="true">8.</strong> Concurrency</a></li><li><ol class="section"><li><a href="races.html"><strong aria-hidden="true">8.1.</strong> Races</a></li><li><a href="send-and-sync.html"><strong aria-hidden="true">8.2.</strong> Send and Sync</a></li><li><a href="atomics.html"><strong aria-hidden="true">8.3.</strong> Atomics</a></li></ol></li><li><a href="vec.html"><strong aria-hidden="true">9.</strong> Implementing Vec</a></li><li><ol class="section"><li><a href="vec-layout.html"><strong aria-hidden="true">9.1.</strong> Layout</a></li><li><a href="vec-alloc.html"><strong aria-hidden="true">9.2.</strong> Allocating</a></li><li><a href="vec-push-pop.html"><strong aria-hidden="true">9.3.</strong> Push and Pop</a></li><li><a href="vec-dealloc.html"><strong aria-hidden="true">9.4.</strong> Deallocating</a></li><li><a href="vec-deref.html"><strong aria-hidden="true">9.5.</strong> Deref</a></li><li><a href="vec-insert-remove.html"><strong aria-hidden="true">9.6.</strong> Insert and Remove</a></li><li><a href="vec-into-iter.html"><strong aria-hidden="true">9.7.</strong> IntoIter</a></li><li><a href="vec-raw.html"><strong aria-hidden="true">9.8.</strong> RawVec</a></li><li><a href="vec-drain.html"><strong aria-hidden="true">9.9.</strong> Drain</a></li><li><a href="vec-zsts.html"><strong aria-hidden="true">9.10.</strong> Handling Zero-Sized Types</a></li><li><a href="vec-final.html"><strong aria-hidden="true">9.11.</strong> Final Code</a></li></ol></li><li><a href="arc-and-mutex.html"><strong aria-hidden="true">10.</strong> Implementing Arc and Mutex</a></li><li><a href="ffi.html"><strong aria-hidden="true">11.</strong> FFI</a></li></ol>
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<a class="header" href="aliasing.html#aliasing" id="aliasing"><h1>Aliasing</h1></a>
<p>First off, let's get some important caveats out of this way:</p>
<ul>
<li>
<p>We will be using the broadest possible definition of aliasing for the sake
of discussion. Rust's definition will probably be more restricted to factor
in mutations and liveness.</p>
</li>
<li>
<p>We will be assuming a single-threaded, interrupt-free, execution. We will also
be ignoring things like memory-mapped hardware. Rust assumes these things
don't happen unless you tell it otherwise. For more details, see the
<a href="concurrency.html">Concurrency Chapter</a>.</p>
</li>
</ul>
<p>With that said, here's our working definition: variables and pointers <em>alias</em>
if they refer to overlapping regions of memory.</p>
<a class="header" href="aliasing.html#why-aliasing-matters" id="why-aliasing-matters"><h1>Why Aliasing Matters</h1></a>
<p>So why should we care about aliasing?</p>
<p>Consider this simple function:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn compute(input: &u32, output: &mut u32) {
if *input > 10 {
*output = 1;
}
if *input > 5 {
*output *= 2;
}
}
#}</code></pre></pre>
<p>We would <em>like</em> to be able to optimize it to the following function:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn compute(input: &u32, output: &mut u32) {
let cached_input = *input; // keep *input in a register
if cached_input > 10 {
*output = 2; // x > 10 implies x > 5, so double and exit immediately
} else if cached_input > 5 {
*output *= 2;
}
}
#}</code></pre></pre>
<p>In Rust, this optimization should be sound. For almost any other language, it
wouldn't be (barring global analysis). This is because the optimization relies
on knowing that aliasing doesn't occur, which most languages are fairly liberal
with. Specifically, we need to worry about function arguments that make <code>input</code>
and <code>output</code> overlap, such as <code>compute(&x, &mut x)</code>.</p>
<p>With that input, we could get this execution:</p>
<pre><code class="language-rust ignore"> // input == output == 0xabad1dea
// *input == *output == 20
if *input > 10 { // true (*input == 20)
*output = 1; // also overwrites *input, because they are the same
}
if *input > 5 { // false (*input == 1)
*output *= 2;
}
// *input == *output == 1
</code></pre>
<p>Our optimized function would produce <code>*output == 2</code> for this input, so the
correctness of our optimization relies on this input being impossible.</p>
<p>In Rust we know this input should be impossible because <code>&mut</code> isn't allowed to be
aliased. So we can safely reject its possibility and perform this optimization.
In most other languages, this input would be entirely possible, and must be considered.</p>
<p>This is why alias analysis is important: it lets the compiler perform useful
optimizations! Some examples:</p>
<ul>
<li>keeping values in registers by proving no pointers access the value's memory</li>
<li>eliminating reads by proving some memory hasn't been written to since last we read it</li>
<li>eliminating writes by proving some memory is never read before the next write to it</li>
<li>moving or reordering reads and writes by proving they don't depend on each other</li>
</ul>
<p>These optimizations also tend to prove the soundness of bigger optimizations
such as loop vectorization, constant propagation, and dead code elimination.</p>
<p>In the previous example, we used the fact that <code>&mut u32</code> can't be aliased to prove
that writes to <code>*output</code> can't possibly affect <code>*input</code>. This let us cache <code>*input</code>
in a register, eliminating a read.</p>
<p>By caching this read, we knew that the the write in the <code>> 10</code> branch couldn't
affect whether we take the <code>> 5</code> branch, allowing us to also eliminate a
read-modify-write (doubling <code>*output</code>) when <code>*input > 10</code>.</p>
<p>The key thing to remember about alias analysis is that writes are the primary
hazard for optimizations. That is, the only thing that prevents us
from moving a read to any other part of the program is the possibility of us
re-ordering it with a write to the same location.</p>
<p>For instance, we have no concern for aliasing in the following modified version
of our function, because we've moved the only write to <code>*output</code> to the very
end of our function. This allows us to freely reorder the reads of <code>*input</code> that
occur before it:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn compute(input: &u32, output: &mut u32) {
let mut temp = *output;
if *input > 10 {
temp = 1;
}
if *input > 5 {
temp *= 2;
}
*output = temp;
}
#}</code></pre></pre>
<p>We're still relying on alias analysis to assume that <code>temp</code> doesn't alias
<code>input</code>, but the proof is much simpler: the value of a local variable can't be
aliased by things that existed before it was declared. This is an assumption
every language freely makes, and so this version of the function could be
optimized the way we want in any language.</p>
<p>This is why the definition of "alias" that Rust will use likely involves some
notion of liveness and mutation: we don't actually care if aliasing occurs if
there aren't any actual writes to memory happening.</p>
<p>Of course, a full aliasing model for Rust must also take into consideration things like
function calls (which may mutate things we don't see), raw pointers (which have
no aliasing requirements on their own), and UnsafeCell (which lets the referent
of an <code>&</code> be mutated).</p>
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