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| <h1 id="interpreter"><a class="header" href="#interpreter">Interpreter</a></h1> |
| <p>The interpreter is a virtual machine for executing MIR without compiling to |
| machine code. It is usually invoked via <code>tcx.const_eval_*</code> functions. The |
| interpreter is shared between the compiler (for compile-time function |
| evaluation, CTFE) and the tool <a href="https://github.com/rust-lang/miri/">Miri</a>, which |
| uses the same virtual machine to detect Undefined Behavior in (unsafe) Rust |
| code.</p> |
| <p>If you start out with a constant:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>const FOO: usize = 1 << 12; |
| <span class="boring">}</span></code></pre> |
| <p>rustc doesn’t actually invoke anything until the constant is either used or |
| placed into metadata.</p> |
| <p>Once you have a use-site like:</p> |
| <pre><code class="language-rust ignore">type Foo = [u8; FOO - 42];</code></pre> |
| <p>The compiler needs to figure out the length of the array before being able to |
| create items that use the type (locals, constants, function arguments, …).</p> |
| <p>To obtain the (in this case empty) parameter environment, one can call |
| <code>let param_env = tcx.param_env(length_def_id);</code>. The <code>GlobalId</code> needed is</p> |
| <pre><code class="language-rust ignore">let gid = GlobalId { |
| promoted: None, |
| instance: Instance::mono(length_def_id), |
| };</code></pre> |
| <p>Invoking <code>tcx.const_eval(param_env.and(gid))</code> will now trigger the creation of |
| the MIR of the array length expression. The MIR will look something like this:</p> |
| <pre><code class="language-mir">Foo::{{constant}}#0: usize = { |
| let mut _0: usize; |
| let mut _1: (usize, bool); |
| |
| bb0: { |
| _1 = CheckedSub(const FOO, const 42usize); |
| assert(!move (_1.1: bool), "attempt to subtract with overflow") -> bb1; |
| } |
| |
| bb1: { |
| _0 = move (_1.0: usize); |
| return; |
| } |
| } |
| </code></pre> |
| <p>Before the evaluation, a virtual memory location (in this case essentially a |
| <code>vec![u8; 4]</code> or <code>vec![u8; 8]</code>) is created for storing the evaluation result.</p> |
| <p>At the start of the evaluation, <code>_0</code> and <code>_1</code> are |
| <code>Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Undef))</code>. This is quite |
| a mouthful: <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/operand/enum.Operand.html"><code>Operand</code></a> can represent either data stored somewhere in the |
| <a href="#memory">interpreter memory</a> (<code>Operand::Indirect</code>), or (as an optimization) |
| immediate data stored in-line. And <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/enum.Immediate.html"><code>Immediate</code></a> can either be a single |
| (potentially uninitialized) <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/enum.Scalar.html">scalar value</a> (integer or thin pointer), |
| or a pair of two of them. In our case, the single scalar value is <em>not</em> (yet) |
| initialized.</p> |
| <p>When the initialization of <code>_1</code> is invoked, the value of the <code>FOO</code> constant is |
| required, and triggers another call to <code>tcx.const_eval_*</code>, which will not be shown |
| here. If the evaluation of FOO is successful, <code>42</code> will be subtracted from its |
| value <code>4096</code> and the result stored in <code>_1</code> as |
| <code>Operand::Immediate(Immediate::ScalarPair(Scalar::Raw { data: 4054, .. }, Scalar::Raw { data: 0, .. })</code>. The first part of the pair is the computed value, |
| the second part is a bool that’s true if an overflow happened. A <code>Scalar::Raw</code> |
| also stores the size (in bytes) of this scalar value; we are eliding that here.</p> |
| <p>The next statement asserts that said boolean is <code>0</code>. In case the assertion |
| fails, its error message is used for reporting a compile-time error.</p> |
| <p>Since it does not fail, <code>Operand::Immediate(Immediate::Scalar(Scalar::Raw { data: 4054, .. }))</code> is stored in the virtual memory it was allocated before the |
| evaluation. <code>_0</code> always refers to that location directly.</p> |
| <p>After the evaluation is done, the return value is converted from <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/operand/enum.Operand.html"><code>Operand</code></a> to |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/consts/enum.ConstValue.html"><code>ConstValue</code></a> by <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/const_eval/eval_queries/fn.op_to_const.html"><code>op_to_const</code></a>: the former representation is geared towards |
| what is needed <em>during</em> const evaluation, while <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/consts/enum.ConstValue.html"><code>ConstValue</code></a> is shaped by the |
| needs of the remaining parts of the compiler that consume the results of const |
| evaluation. As part of this conversion, for types with scalar values, even if |
| the resulting <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/operand/enum.Operand.html"><code>Operand</code></a> is <code>Indirect</code>, it will return an immediate |
| <code>ConstValue::Scalar(computed_value)</code> (instead of the usual <code>ConstValue::ByRef</code>). |
| This makes using the result much more efficient and also more convenient, as no |
| further queries need to be executed in order to get at something as simple as a |
| <code>usize</code>.</p> |
| <p>Future evaluations of the same constants will not actually invoke |
| the interpreter, but just use the cached result.</p> |
| <h2 id="datastructures"><a class="header" href="#datastructures">Datastructures</a></h2> |
| <p>The interpreter’s outside-facing datastructures can be found in |
| <a href="https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_middle/src/mir/interpret">rustc_middle/src/mir/interpret</a>. |
| This is mainly the error enum and the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/consts/enum.ConstValue.html"><code>ConstValue</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/enum.Scalar.html"><code>Scalar</code></a> types. A |
| <code>ConstValue</code> can be either <code>Scalar</code> (a single <code>Scalar</code>, i.e., integer or thin |
| pointer), <code>Slice</code> (to represent byte slices and strings, as needed for pattern |
| matching) or <code>ByRef</code>, which is used for anything else and refers to a virtual |
| allocation. These allocations can be accessed via the methods on |
| <code>tcx.interpret_interner</code>. A <code>Scalar</code> is either some <code>Raw</code> integer or a pointer; |
| see <a href="#memory">the next section</a> for more on that.</p> |
| <p>If you are expecting a numeric result, you can use <code>eval_usize</code> (panics on |
| anything that can’t be represented as a <code>u64</code>) or <code>try_eval_usize</code> which results |
| in an <code>Option<u64></code> yielding the <code>Scalar</code> if possible.</p> |
| <h2 id="memory"><a class="header" href="#memory">Memory</a></h2> |
| <p>To support any kind of pointers, the interpreter needs to have a “virtual memory” that the |
| pointers can point to. This is implemented in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/interpret/struct.Memory.html"><code>Memory</code></a> type. In the |
| simplest model, every global variable, stack variable and every dynamic |
| allocation corresponds to an <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/struct.Allocation.html"><code>Allocation</code></a> in that memory. (Actually using an |
| allocation for every MIR stack variable would be very inefficient; that’s why we |
| have <code>Operand::Immediate</code> for stack variables that are both small and never have |
| their address taken. But that is purely an optimization.)</p> |
| <p>Such an <code>Allocation</code> is basically just a sequence of <code>u8</code> storing the value of |
| each byte in this allocation. (Plus some extra data, see below.) Every |
| <code>Allocation</code> has a globally unique <code>AllocId</code> assigned in <code>Memory</code>. With that, a |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/struct.Pointer.html"><code>Pointer</code></a> consists of a pair of an <code>AllocId</code> (indicating the allocation) and |
| an offset into the allocation (indicating which byte of the allocation the |
| pointer points to). It may seem odd that a <code>Pointer</code> is not just an integer |
| address, but remember that during const evaluation, we cannot know at which |
| actual integer address the allocation will end up – so we use <code>AllocId</code> as |
| symbolic base addresses, which means we need a separate offset. (As an aside, |
| it turns out that pointers at run-time are |
| <a href="https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#pointer-provenance">more than just integers, too</a>.)</p> |
| <p>These allocations exist so that references and raw pointers have something to |
| point to. There is no global linear heap in which things are allocated, but each |
| allocation (be it for a local variable, a static or a (future) heap allocation) |
| gets its own little memory with exactly the required size. So if you have a |
| pointer to an allocation for a local variable <code>a</code>, there is no possible (no |
| matter how unsafe) operation that you can do that would ever change said pointer |
| to a pointer to a different local variable <code>b</code>. |
| Pointer arithmetic on <code>a</code> will only ever change its offset; the <code>AllocId</code> stays the same.</p> |
| <p>This, however, causes a problem when we want to store a <code>Pointer</code> into an |
| <code>Allocation</code>: we cannot turn it into a sequence of <code>u8</code> of the right length! |
| <code>AllocId</code> and offset together are twice as big as a pointer “seems” to be. This |
| is what the <code>relocation</code> field of <code>Allocation</code> is for: the byte offset of the |
| <code>Pointer</code> gets stored as a bunch of <code>u8</code>, while its <code>AllocId</code> gets stored |
| out-of-band. The two are reassembled when the <code>Pointer</code> is read from memory. |
| The other bit of extra data an <code>Allocation</code> needs is <code>undef_mask</code> for keeping |
| track of which of its bytes are initialized.</p> |
| <h3 id="global-memory-and-exotic-allocations"><a class="header" href="#global-memory-and-exotic-allocations">Global memory and exotic allocations</a></h3> |
| <p><code>Memory</code> exists only during evaluation; it gets destroyed when the |
| final value of the constant is computed. In case that constant contains any |
| pointers, those get “interned” and moved to a global “const eval memory” that is |
| part of <code>TyCtxt</code>. These allocations stay around for the remaining computation |
| and get serialized into the final output (so that dependent crates can use |
| them).</p> |
| <p>Moreover, to also support function pointers, the global memory in <code>TyCtxt</code> can |
| also contain “virtual allocations”: instead of an <code>Allocation</code>, these contain an |
| <code>Instance</code>. That allows a <code>Pointer</code> to point to either normal data or a |
| function, which is needed to be able to evaluate casts from function pointers to |
| raw pointers.</p> |
| <p>Finally, the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/enum.GlobalAlloc.html"><code>GlobalAlloc</code></a> type used in the global memory also contains a |
| variant <code>Static</code> that points to a particular <code>const</code> or <code>static</code> item. This is |
| needed to support circular statics, where we need to have a <code>Pointer</code> to a |
| <code>static</code> for which we cannot yet have an <code>Allocation</code> as we do not know the |
| bytes of its value.</p> |
| <h3 id="pointer-values-vs-pointer-types"><a class="header" href="#pointer-values-vs-pointer-types">Pointer values vs Pointer types</a></h3> |
| <p>One common cause of confusion in the interpreter is that being a pointer <em>value</em> and having |
| a pointer <em>type</em> are entirely independent properties. By “pointer value”, we |
| refer to a <code>Scalar::Ptr</code> containing a <code>Pointer</code> and thus pointing somewhere into |
| the interpreter’s virtual memory. This is in contrast to <code>Scalar::Raw</code>, which is just some |
| concrete integer.</p> |
| <p>However, a variable of pointer or reference <em>type</em>, such as <code>*const T</code> or <code>&T</code>, |
| does not have to have a pointer <em>value</em>: it could be obtained by casting or |
| transmuting an integer to a pointer. |
| And similarly, when casting or transmuting a reference to some |
| actual allocation to an integer, we end up with a pointer <em>value</em> |
| (<code>Scalar::Ptr</code>) at integer <em>type</em> (<code>usize</code>). This is a problem because we |
| cannot meaningfully perform integer operations such as division on pointer |
| values.</p> |
| <h2 id="interpretation"><a class="header" href="#interpretation">Interpretation</a></h2> |
| <p>Although the main entry point to constant evaluation is the <code>tcx.const_eval_*</code> |
| functions, there are additional functions in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_const_eval/index.html">rustc_const_eval/src/const_eval</a> |
| that allow accessing the fields of a <code>ConstValue</code> (<code>ByRef</code> or otherwise). You should |
| never have to access an <code>Allocation</code> directly except for translating it to the |
| compilation target (at the moment just LLVM).</p> |
| <p>The interpreter starts by creating a virtual stack frame for the current constant that is |
| being evaluated. There’s essentially no difference between a constant and a |
| function with no arguments, except that constants do not allow local (named) |
| variables at the time of writing this guide.</p> |
| <p>A stack frame is defined by the <code>Frame</code> type in |
| <a href="https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_const_eval/src/interpret/eval_context.rs">rustc_const_eval/src/interpret/eval_context.rs</a> |
| and contains all the local |
| variables memory (<code>None</code> at the start of evaluation). Each frame refers to the |
| evaluation of either the root constant or subsequent calls to <code>const fn</code>. The |
| evaluation of another constant simply calls <code>tcx.const_eval_*</code>, which produce an |
| entirely new and independent stack frame.</p> |
| <p>The frames are just a <code>Vec<Frame></code>, there’s no way to actually refer to a |
| <code>Frame</code>’s memory even if horrible shenanigans are done via unsafe code. The only |
| memory that can be referred to are <code>Allocation</code>s.</p> |
| <p>The interpreter now calls the <code>step</code> method (in |
| <a href="https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_const_eval/src/interpret/step.rs">rustc_const_eval/src/interpret/step.rs</a> |
| ) until it either returns an error or has no further statements to execute. Each |
| statement will now initialize or modify the locals or the virtual memory |
| referred to by a local. This might require evaluating other constants or |
| statics, which just recursively invokes <code>tcx.const_eval_*</code>.</p> |
| |
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