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| <h1 id="debugger-visualizers"><a class="header" href="#debugger-visualizers">Debugger Visualizers</a></h1> |
| <p>These are typically the last step before the debugger displays the information, but the results may |
| be piped through a debug adapter such as an IDE’s debugger API.</p> |
| <p>The term “Visualizer” is a bit of a misnomer. The real goal isn’t just to prettify the output, but |
| to provide an interface for the user to interact with that is as useful as possible. In many cases |
| this means reconstructing the original type as closely as possible to its Rust representation, but |
| not always.</p> |
| <p>The visualizer interface allows generating “synthetic children” - fields that don’t exist in the |
| debug info, but can be derived from invariants about the language and the type itself. A simple |
| example is allowing one to interact with the elements of a <code>Vec<T></code> instead of just it’s <code>*mut u8</code> |
| heap pointer, length, and capacity.</p> |
| <h2 id="rust-lldb-rust-gdb-and-rust-windbgcmd"><a class="header" href="#rust-lldb-rust-gdb-and-rust-windbgcmd"><code>rust-lldb</code>, <code>rust-gdb</code>, and <code>rust-windbg.cmd</code></a></h2> |
| <p>These support scripts are distributed with Rust toolchains. They locate the appropriate debugger and |
| the toolchain’s visualizer scripts, then launch the debugger with the appropriate arguments to load |
| the visualizer scripts before a debugee is launched/attached to.</p> |
| <h2 id="debugger_visualizer"><a class="header" href="#debugger_visualizer"><code>#![debugger_visualizer]</code></a></h2> |
| <p><a href="https://doc.rust-lang.org/reference/attributes/debugger.html#the-debugger_visualizer-attribute">This attribute</a> allows Rust library authors to include pretty printers for their |
| types within the library itself. These pretty printers are of the same format as typical |
| visualizers, but are embedded directly into the compiled binary. These scripts are loaded |
| automatically by the debugger, allowing a seamless experience for users. This attribute currently |
| works for GDB and natvis scripts.</p> |
| <p>GDB python scripts are embedded in the <code>.debug_gdb_scripts</code> section of the binary. More information |
| can be found <a href="https://sourceware.org/gdb/current/onlinedocs/gdb.html/dotdebug_005fgdb_005fscripts-section.html">here</a>. Rustc accomplishes this in <a href="https://github.com/rust-lang/rust/blob/main/compiler/rustc_codegen_llvm/src/debuginfo/gdb.rs"><code>rustc_codegen_llvm/src/debuginfo/gdb.rs</code></a></p> |
| <p>Natvis files can be embedded in the PDB debug info using the <a href="https://learn.microsoft.com/en-us/cpp/build/reference/natvis-add-natvis-to-pdb?view=msvc-170"><code>/NATVIS</code> linker option</a>, |
| and have the <a href="https://learn.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects?view=visualstudio#BKMK_natvis_location">highest priority</a> when a type is resolving which visualizer to use. The |
| files specified by the attribute are collected into |
| <a href="https://github.com/rust-lang/rust/blob/e0e204f3e97ad5f79524b9c259dc38df606ed82c/compiler/rustc_codegen_ssa/src/lib.rs#L212"><code>CrateInfo::natvis_debugger_visualizers</code></a> which are then added as linker arguments in |
| <a href="https://github.com/rust-lang/rust/blob/main/compiler/rustc_codegen_ssa/src/back/linker.rs#L1106"><code>rustc_codegen_ssa/src/back/linker.rs</code></a></p> |
| <p>LLDB is not currently supported, but there are a few methods that could potentially allow support in |
| the future. Officially, the intended method is via a <a href="https://lldb.llvm.org/resources/formatterbytecode.html">formatter bytecode</a>. This was |
| created to offer a comparable experience to GDB’s, but without the safety concerns associated with |
| embedding an entire python script. The opcodes are limited, but it works with <code>SBValue</code> and <code>SBType</code> |
| in roughly the same way as python visualizer scripts. Implementing this would require writing some |
| sort of DSL/mini compiler.</p> |
| <p>Alternatively, it might be possible to copy GDB’s strategy entirely: create a bespoke section in the |
| binary and embed a python script in it. LLDB will not load it automatically, but the python API does |
| allow one to access the <a href="https://lldb.llvm.org/python_api/lldb.SBSection.html#sbsection">raw sections of the debug info</a>. With this, it may be possible |
| to extract the python script from our bespoke section and then load it in during the startup of |
| Rust’s visualizer scripts.</p> |
| <h2 id="performance"><a class="header" href="#performance">Performance</a></h2> |
| <p>Before tackling the visualizers themselves, it’s important to note that these are part of a |
| performance-sensitive system. Please excuse the break in formality, but: if I have to spend |
| significant time debugging, I’m annoyed. If I have to <em>wait on my debugger</em>, I’m pissed.</p> |
| <p>Every millisecond spent in these visualizers is a millisecond longer for the user to see output. |
| This can be especially painful for large stackframes that contain many/large container types. |
| Debugger GUI’s such as VSCode will request the whole stack frame at once, and this can result in |
| delays of tens of seconds (or even minutes) before being able to interact with any variables in the |
| frame.</p> |
| <p>There is a tendancy to balk at the idea of optimizing Python code, but it really can have a |
| substantial impact. Remember, there is no compiler to help keep the code fast. Even simple |
| transformations are not done for you. It can be difficult to find Python performance tips through |
| all the noise of people suggesting you don’t bother optimizing Python, so here are some things to |
| keep in mind that are relevant to these scripts:</p> |
| <ul> |
| <li>Everything allocates, even <code>int</code></li> |
| <li>Use tuples when possible. <code>list</code> is effectively <code>Vec<Box<[Any]>></code>, whereas tuples are equivalent |
| to <code>Box<[Any]></code>. They have one less layer of indirection, don’t carry extra capacity and can’t |
| grow/shrink which can be advantageous in many cases. An additional benefit is that Python caches and |
| recycles the underlying allocations of all tuples up to size 20.</li> |
| <li>Regexes are slow and should be avoided when simple string manipulation will do</li> |
| <li>Strings are immutable, thus many string operations implictly copy the contents.</li> |
| <li>When concatenating large lists of strings, <code>"".join(iterable_of_strings)</code> is typically the fastest |
| way to do it.</li> |
| <li>f-strings are generally the fastest way to do small, simple string transformations such as |
| surrounding a string with parentheses.</li> |
| <li>The act of calling a function is somewhat slow (even if the function is completely empty). If the |
| code section is very hot, consider inlining the function manually.</li> |
| <li>Local variable access is significantly faster than global and built-in function access</li> |
| <li>Member/method access via the <code>.</code> operator is also slow, consider reassigning deeply nested values |
| to local variables to avoid this cost (e.g. <code>h = a.b.c.d.e.f.g.h</code>).</li> |
| <li>Accessing inherited methods and fields is about 2x slower than base-class methods and fields. |
| Avoid inheritance whenever possible.</li> |
| <li>Use <a href="https://wiki.python.org/moin/UsingSlots"><code>__slots__</code></a> wherever possible. <code>__slots__</code> is a way |
| to indicate to Python that your class’s fields won’t change and speeds up field access by a |
| noticable amount. This does require you to name your fields in advance and initialize them in |
| <code>__init__</code>, but it’s a small price to pay for the benefits.</li> |
| <li>Match statements/if..elif..else are not optimized in any way. The conditions are checked in order, |
| 1 by 1. If possible, use an alternative such as dictionary dispatch or a table of values</li> |
| <li>Compute lazily when possible</li> |
| <li>List comprehensions are typically faster than loops, generator comprehensions are a bit slower |
| than list comprehensions, but use less memory. You can think of comprehensions as equivalent to |
| Rust’s <code>iter.map()</code>. List comprehensions effectively call <code>collect::<Vec<_>></code> at the end, whereas |
| generator comprehensions do not.</li> |
| </ul> |
| |
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