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| <main> |
| <h2 id="debugging-llvm"><a class="header" href="#debugging-llvm">Debugging LLVM</a></h2> |
| <blockquote> |
| <p>NOTE: If you are looking for info about code generation, please see <a href="./codegen.html">this |
| chapter</a> instead.</p> |
| </blockquote> |
| <p>This section is about debugging compiler bugs in code generation (e.g. why the |
| compiler generated some piece of code or crashed in LLVM). LLVM is a big |
| project on its own that probably needs to have its own debugging document (not |
| that I could find one). But here are some tips that are important in a rustc |
| context:</p> |
| <h3 id="minimize-the-example"><a class="header" href="#minimize-the-example">Minimize the example</a></h3> |
| <p>As a general rule, compilers generate lots of information from analyzing code. |
| Thus, a useful first step is usually to find a minimal example. One way to do |
| this is to</p> |
| <ol> |
| <li> |
| <p>create a new crate that reproduces the issue (e.g. adding whatever crate is |
| at fault as a dependency, and using it from there)</p> |
| </li> |
| <li> |
| <p>minimize the crate by removing external dependencies; that is, moving |
| everything relevant to the new crate</p> |
| </li> |
| <li> |
| <p>further minimize the issue by making the code shorter (there are tools that |
| help with this like <code>creduce</code>)</p> |
| </li> |
| </ol> |
| <p>For more discussion on methodology for steps 2 and 3 above, there is an |
| <a href="https://blog.pnkfx.org/blog/2019/11/18/rust-bug-minimization-patterns/">epic blog post</a> from pnkfelix specifically about Rust program minimization.</p> |
| <h3 id="enable-llvm-internal-checks"><a class="header" href="#enable-llvm-internal-checks">Enable LLVM internal checks</a></h3> |
| <p>The official compilers (including nightlies) have LLVM assertions disabled, |
| which means that LLVM assertion failures can show up as compiler crashes (not |
| ICEs but “real” crashes) and other sorts of weird behavior. If you are |
| encountering these, it is a good idea to try using a compiler with LLVM |
| assertions enabled - either an “alt” nightly or a compiler you build yourself |
| by setting <code>[llvm] assertions=true</code> in your bootstrap.toml - and see whether |
| anything turns up.</p> |
| <p>The rustc build process builds the LLVM tools into |
| <code>./build/<host-triple>/llvm/bin</code>. They can be called directly. |
| These tools include:</p> |
| <ul> |
| <li><a href="https://llvm.org/docs/CommandGuide/llc.html"><code>llc</code></a>, which compiles bitcode (<code>.bc</code> files) to executable code; this can be used to |
| replicate LLVM backend bugs.</li> |
| <li><a href="https://llvm.org/docs/CommandGuide/opt.html"><code>opt</code></a>, a bitcode transformer that runs LLVM optimization passes.</li> |
| <li><a href="https://llvm.org/docs/Bugpoint.html"><code>bugpoint</code></a>, which reduces large test cases to small, useful ones.</li> |
| <li>and many others, some of which are referenced in the text below.</li> |
| </ul> |
| <p>By default, the Rust build system does not check for changes to the LLVM source code or |
| its build configuration settings. So, if you need to rebuild the LLVM that is linked |
| into <code>rustc</code>, first delete the file <code>.llvm-stamp</code>, which should be located |
| in <code>build/<host-triple>/llvm/</code>.</p> |
| <p>The default rustc compilation pipeline has multiple codegen units, which is |
| hard to replicate manually and means that LLVM is called multiple times in |
| parallel. If you can get away with it (i.e. if it doesn’t make your bug |
| disappear), passing <code>-C codegen-units=1</code> to rustc will make debugging easier.</p> |
| <h3 id="get-your-hands-on-raw-llvm-input"><a class="header" href="#get-your-hands-on-raw-llvm-input">Get your hands on raw LLVM input</a></h3> |
| <p>For rustc to generate LLVM IR, you need to pass the <code>--emit=llvm-ir</code> flag. If |
| you are building via cargo, use the <code>RUSTFLAGS</code> environment variable (e.g. |
| <code>RUSTFLAGS='--emit=llvm-ir'</code>). This causes rustc to spit out LLVM IR into the |
| target directory.</p> |
| <p><code>cargo llvm-ir [options] path</code> spits out the LLVM IR for a particular function |
| at <code>path</code>. (<code>cargo install cargo-asm</code> installs <code>cargo asm</code> and <code>cargo llvm-ir</code>). <code>--build-type=debug</code> emits code for debug builds. There are also |
| other useful options. Also, debug info in LLVM IR can clutter the output a lot: |
| <code>RUSTFLAGS="-C debuginfo=0"</code> is really useful.</p> |
| <p><code>RUSTFLAGS="-C save-temps"</code> outputs LLVM bitcode at |
| different stages during compilation, which is sometimes useful. The output LLVM |
| bitcode will be in <code>.bc</code> files in the compiler’s output directory, set via the |
| <code>--out-dir DIR</code> argument to <code>rustc</code>.</p> |
| <ul> |
| <li> |
| <p>If you are hitting an assertion failure or segmentation fault from the LLVM |
| backend when invoking <code>rustc</code> itself, it is a good idea to try passing each |
| of these <code>.bc</code> files to the <code>llc</code> command, and see if you get the same |
| failure. (LLVM developers often prefer a bug reduced to a <code>.bc</code> file over one |
| that uses a Rust crate for its minimized reproduction.)</p> |
| </li> |
| <li> |
| <p>To get human readable versions of the LLVM bitcode, one just needs to convert |
| the bitcode (<code>.bc</code>) files to <code>.ll</code> files using <code>llvm-dis</code>, which should be in |
| the target local compilation of rustc.</p> |
| </li> |
| </ul> |
| <p>Note that rustc emits different IR depending on whether <code>-O</code> is enabled, even |
| without LLVM’s optimizations, so if you want to play with the IR rustc emits, |
| you should:</p> |
| <pre><code class="language-bash">$ rustc +local my-file.rs --emit=llvm-ir -O -C no-prepopulate-passes \ |
| -C codegen-units=1 |
| $ OPT=./build/$TRIPLE/llvm/bin/opt |
| $ $OPT -S -O2 < my-file.ll > my |
| </code></pre> |
| <p>If you just want to get the LLVM IR during the LLVM pipeline, to e.g. see which |
| IR causes an optimization-time assertion to fail, or to see when LLVM performs |
| a particular optimization, you can pass the rustc flag <code>-C llvm-args=-print-after-all</code>, and possibly add <code>-C llvm-args='-filter-print-funcs=EXACT_FUNCTION_NAME</code> (e.g. <code>-C llvm-args='-filter-print-funcs=_ZN11collections3str21_$LT$impl$u20$str$GT$\ 7replace17hbe10ea2e7c809b0bE'</code>).</p> |
| <p>That produces a lot of output into standard error, so you’ll want to pipe that |
| to some file. Also, if you are using neither <code>-filter-print-funcs</code> nor <code>-C codegen-units=1</code>, then, because the multiple codegen units run in parallel, the |
| printouts will mix together and you won’t be able to read anything.</p> |
| <ul> |
| <li> |
| <p>One caveat to the aforementioned methodology: the <code>-print</code> family of options |
| to LLVM only prints the IR unit that the pass runs on (e.g., just a |
| function), and does not include any referenced declarations, globals, |
| metadata, etc. This means you cannot in general feed the output of <code>-print</code> |
| into <code>llc</code> to reproduce a given problem.</p> |
| </li> |
| <li> |
| <p>Within LLVM itself, calling <code>F.getParent()->dump()</code> at the beginning of |
| <code>SafeStackLegacyPass::runOnFunction</code> will dump the whole module, which |
| may provide better basis for reproduction. (However, you |
| should be able to get that same dump from the <code>.bc</code> files dumped by |
| <code>-C save-temps</code>.)</p> |
| </li> |
| </ul> |
| <p>If you want just the IR for a specific function (say, you want to see why it |
| causes an assertion or doesn’t optimize correctly), you can use <code>llvm-extract</code>, |
| e.g.</p> |
| <pre><code class="language-bash">$ ./build/$TRIPLE/llvm/bin/llvm-extract \ |
| -func='_ZN11collections3str21_$LT$impl$u20$str$GT$7replace17hbe10ea2e7c809b0bE' \ |
| -S \ |
| < unextracted.ll \ |
| > extracted.ll |
| </code></pre> |
| <h3 id="investigate-llvm-optimization-passes"><a class="header" href="#investigate-llvm-optimization-passes">Investigate LLVM optimization passes</a></h3> |
| <p>If you are seeing incorrect behavior due to an optimization pass, a very handy |
| LLVM option is <code>-opt-bisect-limit</code>, which takes an integer denoting the index |
| value of the highest pass to run. Index values for taken passes are stable |
| from run to run; by coupling this with software that automates bisecting the |
| search space based on the resulting program, an errant pass can be quickly |
| determined. When an <code>-opt-bisect-limit</code> is specified, all runs are displayed |
| to standard error, along with their index and output indicating if the |
| pass was run or skipped. Setting the limit to an index of -1 (e.g., |
| <code>RUSTFLAGS="-C llvm-args=-opt-bisect-limit=-1"</code>) will show all passes and |
| their corresponding index values.</p> |
| <p>If you want to play with the optimization pipeline, you can use the <a href="https://llvm.org/docs/CommandGuide/opt.html"><code>opt</code></a> tool |
| from <code>./build/<host-triple>/llvm/bin/</code> with the LLVM IR emitted by rustc.</p> |
| <p>When investigating the implementation of LLVM itself, you should be |
| aware of its <a href="https://llvm.org/docs/ProgrammersManual.html#the-llvm-debug-macro-and-debug-option">internal debug infrastructure</a>. |
| This is provided in LLVM Debug builds, which you enable for rustc |
| LLVM builds by changing this setting in the bootstrap.toml:</p> |
| <pre><code>[llvm] |
| # Indicates whether the LLVM assertions are enabled or not |
| assertions = true |
| |
| # Indicates whether the LLVM build is a Release or Debug build |
| optimize = false |
| </code></pre> |
| <p>The quick summary is:</p> |
| <ul> |
| <li>Setting <code>assertions=true</code> enables coarse-grain debug messaging. |
| <ul> |
| <li>beyond that, setting <code>optimize=false</code> enables fine-grain debug messaging.</li> |
| </ul> |
| </li> |
| <li><code>LLVM_DEBUG(dbgs() << msg)</code> in LLVM is like <code>debug!(msg)</code> in <code>rustc</code>.</li> |
| <li>The <code>-debug</code> option turns on all messaging; it is like setting the |
| environment variable <code>RUSTC_LOG=debug</code> in <code>rustc</code>.</li> |
| <li>The <code>-debug-only=<pass1>,<pass2></code> variant is more selective; it is like |
| setting the environment variable <code>RUSTC_LOG=path1,path2</code> in <code>rustc</code>.</li> |
| </ul> |
| <h3 id="getting-help-and-asking-questions"><a class="header" href="#getting-help-and-asking-questions">Getting help and asking questions</a></h3> |
| <p>If you have some questions, head over to the <a href="https://rust-lang.zulipchat.com/">rust-lang Zulip</a> and |
| specifically the <code>#t-compiler/wg-llvm</code> channel.</p> |
| <h3 id="compiler-options-to-know-and-love"><a class="header" href="#compiler-options-to-know-and-love">Compiler options to know and love</a></h3> |
| <p>The <code>-C help</code> and <code>-Z help</code> compiler switches will list out a variety |
| of interesting options you may find useful. Here are a few of the most |
| common that pertain to LLVM development (some of them are employed in the |
| tutorial above):</p> |
| <ul> |
| <li>The <code>--emit llvm-ir</code> option emits a <code><filename>.ll</code> file with LLVM IR in textual format |
| <ul> |
| <li>The <code>--emit llvm-bc</code> option emits in bytecode format (<code><filename>.bc</code>)</li> |
| </ul> |
| </li> |
| <li>Passing <code>-C llvm-args=<foo></code> allows passing pretty much all the |
| options that tools like llc and opt would accept; |
| e.g. <code>-C llvm-args=-print-before-all</code> to print IR before every LLVM |
| pass.</li> |
| <li>The <code>-C no-prepopulate-passes</code> will avoid pre-populate the LLVM pass |
| manager with a list of passes. This will allow you to view the LLVM |
| IR that rustc generates, not the LLVM IR after optimizations.</li> |
| <li>The <code>-C passes=val</code> option allows you to supply a space separated list of extra LLVM passes to run</li> |
| <li>The <code>-C save-temps</code> option saves all temporary output files during compilation</li> |
| <li>The <code>-Z print-llvm-passes</code> option will print out LLVM optimization passes being run</li> |
| <li>The <code>-Z time-llvm-passes</code> option measures the time of each LLVM pass</li> |
| <li>The <code>-Z verify-llvm-ir</code> option will verify the LLVM IR for correctness</li> |
| <li>The <code>-Z no-parallel-backend</code> will disable parallel compilation of distinct compilation units</li> |
| <li>The <code>-Z llvm-time-trace</code> option will output a Chrome profiler compatible JSON file |
| which contains details and timings for LLVM passes.</li> |
| <li>The <code>-C llvm-args=-opt-bisect-limit=<index></code> option allows for bisecting LLVM |
| optimizations.</li> |
| </ul> |
| <h3 id="filing-llvm-bug-reports"><a class="header" href="#filing-llvm-bug-reports">Filing LLVM bug reports</a></h3> |
| <p>When filing an LLVM bug report, you will probably want some sort of minimal |
| working example that demonstrates the problem. The Godbolt compiler explorer is |
| really helpful for this.</p> |
| <ol> |
| <li> |
| <p>Once you have some LLVM IR for the problematic code (see above), you can |
| create a minimal working example with Godbolt. Go to |
| <a href="https://llvm.godbolt.org">llvm.godbolt.org</a>.</p> |
| </li> |
| <li> |
| <p>Choose <code>LLVM-IR</code> as programming language.</p> |
| </li> |
| <li> |
| <p>Use <code>llc</code> to compile the IR to a particular target as is:</p> |
| <ul> |
| <li>There are some useful flags: <code>-mattr</code> enables target features, <code>-march=</code> |
| selects the target, <code>-mcpu=</code> selects the CPU, etc.</li> |
| <li>Commands like <code>llc -march=help</code> output all architectures available, which |
| is useful because sometimes the Rust arch names and the LLVM names do not |
| match.</li> |
| <li>If you have compiled rustc yourself somewhere, in the target directory |
| you have binaries for <code>llc</code>, <code>opt</code>, etc.</li> |
| </ul> |
| </li> |
| <li> |
| <p>If you want to optimize the LLVM-IR, you can use <code>opt</code> to see how the LLVM |
| optimizations transform it.</p> |
| </li> |
| <li> |
| <p>Once you have a godbolt link demonstrating the issue, it is pretty easy to |
| fill in an LLVM bug. Just visit their <a href="https://github.com/llvm/llvm-project/issues">github issues page</a>.</p> |
| </li> |
| </ol> |
| <h3 id="porting-bug-fixes-from-llvm"><a class="header" href="#porting-bug-fixes-from-llvm">Porting bug fixes from LLVM</a></h3> |
| <p>Once you’ve identified the bug as an LLVM bug, you will sometimes |
| find that it has already been reported and fixed in LLVM, but we haven’t |
| gotten the fix yet (or perhaps you are familiar enough with LLVM to fix it yourself).</p> |
| <p>In that case, we can sometimes opt to port the fix for the bug |
| directly to our own LLVM fork, so that rustc can use it more easily. |
| Our fork of LLVM is maintained in <a href="https://github.com/rust-lang/llvm-project/">rust-lang/llvm-project</a>. Once |
| you’ve landed the fix there, you’ll also need to land a PR modifying |
| our submodule commits – ask around on Zulip for help.</p> |
| |
| </main> |
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