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href=/2015/11/welcome>Welcome, and an algorithm</a></ul></div></nav></aside><main class="col col--7"><article><header><h1 class=title_f1Hy>Allocations in Rust: Compiler optimizations</h1><div class="container_mt6G margin-vert--md"><time datetime=2019-02-08T12:00:00.000Z>February 8, 2019</time> · <!-- -->4 min read</div></header><div id=__blog-post-container class=markdown><p>Up to this point, we've been discussing memory usage in the Rust language by focusing on simple
rules that are mostly right for small chunks of code. We've spent time showing how those rules work
themselves out in practice, and become familiar with reading the assembly code needed to see each
memory type (global, stack, heap) in action.</p>
<p>Throughout the series so far, we've put a handicap on the code. In the name of consistent and
understandable results, we've asked the compiler to pretty please leave the training wheels on. Now
is the time where we throw out all the rules and take off the kid gloves. As it turns out, both the
Rust compiler and the LLVM optimizers are incredibly sophisticated, and we'll step back and let them
do their job.</p>
<p>Similar to
<a href="https://www.youtube.com/watch?v=bSkpMdDe4g4" target=_blank rel="noopener noreferrer">"What Has My Compiler Done For Me Lately?"</a>, we're
focusing on interesting things the Rust language (and LLVM!) can do with memory management. We'll
still be looking at assembly code to understand what's going on, but it's important to mention
again: <strong>please use automated tools like <a href=https://crates.io/crates/alloc_counter target=_blank rel="noopener noreferrer">alloc-counter</a> to
double-check memory behavior if it's something you care about</strong>. It's far too easy to mis-read
assembly in large code sections, you should always verify behavior if you care about memory usage.</p>
<p>The guiding principal as we move forward is this: <em>optimizing compilers won't produce worse programs
than we started with.</em> There won't be any situations where stack allocations get moved to heap
allocations. There will, however, be an opera of optimization.</p>
<p><strong>Update 2019-02-10</strong>: When debugging a
<a href=https://gitlab.com/sio4/code/alloc-counter/issues/1 target=_blank rel="noopener noreferrer">related issue</a>, it was discovered that the
original code worked because LLVM optimized out the entire function, rather than just the allocation
segments. The code has been updated with proper use of
<a href=https://doc.rust-lang.org/std/ptr/fn.read_volatile.html target=_blank rel="noopener noreferrer"><code>read_volatile</code></a>, and a previous section
on vector capacity has been removed.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=the-case-of-the-disappearing-box>The Case of the Disappearing Box<a href=#the-case-of-the-disappearing-box class=hash-link aria-label="Direct link to The Case of the Disappearing Box" title="Direct link to The Case of the Disappearing Box"></a></h2>
<p>Our first optimization comes when LLVM can reason that the lifetime of an object is sufficiently
short that heap allocations aren't necessary. In these cases, LLVM will move the allocation to the
stack instead! The way this interacts with <code>#[inline]</code> attributes is a bit opaque, but the important
part is that LLVM can sometimes do better than the baseline Rust language:</p>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">std</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">alloc</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">GlobalAlloc</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Layout</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">System</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">std</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">sync</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">atomic</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">AtomicBool</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Ordering</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">pub</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">cmp</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">x</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u32</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// Turn on panicking if we allocate on the heap</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token constant" style="color:hsl(35, 99%, 36%)">DO_PANIC</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">store</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token boolean" style="color:hsl(35, 99%, 36%)">true</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Ordering</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">SeqCst</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// The compiler is able to see through the constant `Box`</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// and directly compare `x` to 24 - assembly line 73</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">let</span><span class="token plain"> y </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Box</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">new</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">24</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">let</span><span class="token plain"> equals </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> x </span><span class="token operator" style="color:hsl(221, 87%, 60%)">==</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">*</span><span class="token plain">y</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// This call to drop is eliminated</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token function" style="color:hsl(221, 87%, 60%)">drop</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">y</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// Need to mark the comparison result as volatile so that</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// LLVM doesn't strip out all the code. If `y` is marked</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// volatile instead, allocation will be forced.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">unsafe</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"> </span><span class="token namespace">std</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">ptr</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">read_volatile</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&</span><span class="token plain">equals</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// Turn off panicking, as there are some deallocations</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// when we exit main.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token constant" style="color:hsl(35, 99%, 36%)">DO_PANIC</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">store</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token boolean" style="color:hsl(35, 99%, 36%)">false</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Ordering</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">SeqCst</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">main</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token function" style="color:hsl(221, 87%, 60%)">cmp</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">12</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[global_allocator]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">static</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">A</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">PanicAllocator</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">PanicAllocator</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">static</span><span class="token plain"> </span><span class="token constant" style="color:hsl(35, 99%, 36%)">DO_PANIC</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">AtomicBool</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">AtomicBool</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">new</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token boolean" style="color:hsl(35, 99%, 36%)">false</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">struct</span><span class="token plain"> </span><span class="token type-definition class-name" style="color:hsl(35, 99%, 36%)">PanicAllocator</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">unsafe</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">impl</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">GlobalAlloc</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">for</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">PanicAllocator</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">unsafe</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">alloc</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">self</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> layout</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Layout</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">-></span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">*</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">mut</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u8</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">        </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">if</span><span class="token plain"> </span><span class="token constant" style="color:hsl(35, 99%, 36%)">DO_PANIC</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">load</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Ordering</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">SeqCst</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">            </span><span class="token macro property" style="color:hsl(5, 74%, 59%)">panic!</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">"Unexpected allocation."</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">        </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">        </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">System</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">alloc</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">layout</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token 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punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> ptr</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">*</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">mut</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u8</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> layout</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Layout</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line 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plain">            </span><span class="token macro property" style="color:hsl(5, 74%, 59%)">panic!</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">"Unexpected deallocation."</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">        </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">        </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">System</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">dealloc</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">ptr</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> layout</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" 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<p>-- <a href=https://godbolt.org/z/BZ_Yp3 target=_blank rel="noopener noreferrer">Compiler Explorer</a></p>
<p>-- <a href="https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=4a765f753183d5b919f62c71d2109d5d" target=_blank rel="noopener noreferrer">Rust Playground</a></p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=dr-array-or-how-i-learned-to-love-the-optimizer>Dr. Array or: how I learned to love the optimizer<a href=#dr-array-or-how-i-learned-to-love-the-optimizer class=hash-link aria-label="Direct link to Dr. Array or: how I learned to love the optimizer" title="Direct link to Dr. Array or: how I learned to love the optimizer"></a></h2>
<p>Finally, this isn't so much about LLVM figuring out different memory behavior, but LLVM stripping
out code that doesn't do anything. Optimizations of this type have a lot of nuance to them; if
you're not careful, they can make your benchmarks look
<a href="https://www.youtube.com/watch?v=nXaxk27zwlk&feature=youtu.be&t=1199" target=_blank rel="noopener noreferrer">impossibly good</a>. In Rust, the
<code>black_box</code> function (implemented in both
<a href=https://doc.rust-lang.org/1.1.0/test/fn.black_box.html target=_blank rel="noopener noreferrer"><code>libtest</code></a> and
<a href=https://docs.rs/criterion/0.2.10/criterion/fn.black_box.html target=_blank rel="noopener noreferrer"><code>criterion</code></a>) will tell the compiler
to disable this kind of optimization. But if you let LLVM remove unnecessary code, you can end up
running programs that previously caused errors:</p>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[derive(Default)]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">struct</span><span class="token plain"> </span><span class="token type-definition class-name" style="color:hsl(35, 99%, 36%)">TwoFiftySix</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    _a</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u64</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">32</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[derive(Default)]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">struct</span><span class="token plain"> </span><span class="token type-definition class-name" style="color:hsl(35, 99%, 36%)">EightK</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    _a</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">TwoFiftySix</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">32</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[derive(Default)]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">struct</span><span class="token plain"> </span><span class="token type-definition class-name" style="color:hsl(35, 99%, 36%)">TwoFiftySixK</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    _a</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">EightK</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">32</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[derive(Default)]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">struct</span><span class="token plain"> </span><span class="token type-definition class-name" style="color:hsl(35, 99%, 36%)">EightM</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    _a</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">TwoFiftySixK</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">32</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">pub</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">main</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// Normally this blows up because we can't reserve size on stack</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// for the `EightM` struct. But because the compiler notices we</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// never do anything with `_x`, it optimizes out the stack storage</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// and the program completes successfully.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">let</span><span class="token plain"> _x </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">EightM</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">default</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
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