rust/tex/paper/miri.tex

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\begin{document}

\title{Miri: \\ \smaller{An interpreter for Rust's mid-level intermediate representation}}
% \subtitle{test}
\author{Scott Olson\footnote{\href{mailto:scott@solson.me}{scott@solson.me}} \\
  \smaller{Supervised by Christopher Dutchyn}}
\date{April 8th, 2016}
\maketitle

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\section{Abstract}

The increasing need for safe low-level code in contexts like operating systems and browsers is
driving the development of Rust\footnote{\url{https://www.rust-lang.org}}, a programming language
backed by Mozilla promising blazing speed without the segfaults. To make programming more
convenient, it's often desirable to be able to generate code or perform some computation at
compile-time. The former is mostly covered by Rust's existing macro feature, but the latter is
currently restricted to a limited form of constant evaluation capable of little beyond simple math.

When the existing constant evaluator was built, it would have been difficult to make it more
powerful than it is. However, a new intermediate representation was recently
added\footnote{\href{https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md}{The MIR RFC}}
to the Rust compiler between the abstract syntax tree and the back-end LLVM IR, called mid-level
intermediate representation, or MIR for short. As it turns out, writing an interpreter for MIR is a
surprisingly effective approach for supporting a large proportion of Rust's features in compile-time
execution.

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\section{Background}

The Rust compiler generates an instance of \rust{Mir} for each function [\autoref{fig:mir}]. Each
\rust{Mir} structure represents a control-flow graph for a given function, and contains a list of
``basic blocks'' which in turn contain a list of statements followed by a single terminator. Each
statement is of the form \rust{lvalue = rvalue}. An \rust{Lvalue} is used for referencing variables
and calculating addresses such as when dereferencing pointers, accessing fields, or indexing arrays.
An \rust{Rvalue} represents the core set of operations possible in MIR, including reading a value
from an lvalue, performing math operations, creating new pointers, structs, and arrays, and so on.
Finally, a terminator decides where control will flow next, optionally based on a boolean or some
other condition.

\begin{figure}[ht]
  \begin{minted}[autogobble]{rust}
    struct Mir {
        basic_blocks: Vec<BasicBlockData>,
        // ...
    }

    struct BasicBlockData {
        statements: Vec<Statement>,
        terminator: Terminator,
        // ...
    }

    struct Statement {
        lvalue: Lvalue,
        rvalue: Rvalue
    }

    enum Terminator {
        Goto { target: BasicBlock },
        If {
            cond: Operand,
            targets: [BasicBlock; 2]
        },
        // ...
    }
  \end{minted}
  \caption{MIR (simplified)}
  \label{fig:mir}
\end{figure}

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\section{First implementation}

\subsection{Basic operation}

Initially, I wrote a simple version of Miri\footnote{\url{https://github.com/tsion/miri}} that was
quite capable despite its flaws. The structure of the interpreter closely mirrors the structure of
MIR itself. It starts executing a function by iterating the statement list in the starting basic
block, matching over the lvalue to produce a pointer and matching over the rvalue to decide what to
write into that pointer. Evaluating the rvalue may involve reads (such as for the two sides of a
binary operation) or construction of new values. Upon reaching the terminator, a similar matching is
done and a new basic block is selected. Finally, Miri returns to the top of the main interpreter
loop and this entire process repeats, reading statements from the new block.

\subsection{Function calls}

To handle function call terminators\footnote{Calls occur only as terminators, never as rvalues.},
Miri is required to store some information in a virtual call stack so that it may pick up where it
left off when the callee returns. Each stack frame stores a reference to the \rust{Mir} for the
function being executed, its local variables, its return value location\footnote{Return value
pointers are passed in by callers.}, and the basic block where execution should resume. To
facilitate returning, there is a \rust{Return} terminator which causes Miri to pop a stack frame and
resume the previous function. The entire execution of a program completes when the first function
that Miri called returns, rendering the call stack empty.

It should be noted that Miri does not itself recurse when a function is called; it merely pushes a
virtual stack frame and jumps to the top of the interpreter loop. Consequently, Miri can interpret
deeply recursive programs without crashing. It could also set a stack depth limit and report an
error when a program exceeds it.

\subsection{Flaws}

% TODO(tsion): Incorporate this text from the slides.
% At first I wrote a naive version with a number of downsides:
%  * I represented values in a traditional dynamic language format,
% where every value was the same size.
%  * I didn’t work well for aggregates (structs, enums, arrays, etc.).
%  *I made unsafe programming tricks that make assumptions
% about low-level value layout essentially impossible

% TODO(tsion): Find a place for this text.
Making Miri work was primarily an implementation problem. Writing an interpreter which models values
of varying sizes, stack and heap allocation, unsafe memory operations, and more requires some
unconventional techniques compared to many interpreters. Miri's execution remains safe even while
simulating execution of unsafe code, which allows it to detect when unsafe code does something
invalid.

\blindtext

\section{Data layout}

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\section{Future work}

Other possible uses for Miri include:

\begin{itemize}
  \item A graphical or text-mode debugger that steps through MIR execution one statement at a time,
    for figuring out why some compile-time execution is raising an error or simply learning how Rust
    works at a low level.
  \item An read-eval-print-loop (REPL) for Rust may be easier to implement on top of Miri than the
    usual LLVM back-end.
  \item An extended version of Miri could be developed apart from the purpose of compile-time
    execution that is able to run foreign functions from C/C++ and generally have full access to the
    operating system. Such a version of Miri could be used to more quickly prototype changes to the
    Rust language that would otherwise require changes to the LLVM back-end.
  \item Miri might be useful for unit-testing the compiler by comparing the results of Miri's
    execution against the results of LLVM-compiled machine code's execution. This would help to
    guarantee that compile-time execution works the same as runtime execution.
\end{itemize}

\end{document}