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                         <h1 id="llvm-source-based-code-coverage"><a class="header" href="#llvm-source-based-code-coverage">LLVM source-based code coverage</a></h1>
 <ul>
 <li><a href="#recommended-bootstraptoml-settings">Recommended <code>bootstrap.toml</code> settings</a></li>
 <li><a href="#rust-symbol-mangling">Rust symbol mangling</a></li>
 <li><a href="#the-llvm-profiler-runtime">The LLVM profiler runtime</a></li>
 <li><a href="#testing-coverage-instrumentation">Testing coverage instrumentation</a></li>
 </ul>
 <p><code>rustc</code> supports detailed source-based code and test coverage analysis
 with a command line option (<code>-C instrument-coverage</code>) that instruments Rust
 libraries and binaries with additional instructions and data, at compile time.</p>
 <p>The coverage instrumentation injects calls to the LLVM intrinsic instruction
 <a href="https://llvm.org/docs/LangRef.html#llvm-instrprof-increment-intrinsic"><code>llvm.instrprof.increment</code></a> at code branches
 (based on a MIR-based control flow analysis), and LLVM converts these to
 instructions that increment static counters, when executed. The LLVM coverage
 instrumentation also requires a <a href="https://llvm.org/docs/CoverageMappingFormat.html">Coverage Map</a> that encodes source metadata,
 mapping counter IDs--directly and indirectly--to the file locations (with
 start and end line and column).</p>
 <p>Rust libraries, with or without coverage instrumentation, can be linked into
 instrumented binaries. When the program is executed and cleanly terminates,
 LLVM libraries write the final counter values to a file (<code>default.profraw</code> or
 a custom file set through environment variable <code>LLVM_PROFILE_FILE</code>).</p>
 <p>Developers use existing LLVM coverage analysis tools to decode <code>.profraw</code>
 files, with corresponding Coverage Maps (from matching binaries that produced
 them), and generate various reports for analysis, for example:</p>
 <p><img alt="Screenshot of sample `llvm-cov show` result, for function add_quoted_string"
  src="img/llvm-cov-show-01.png" class="center"/>
 <br/></p>
 <p>Detailed instructions and examples are documented in the
 <a href="https://doc.rust-lang.org/nightly/rustc/instrument-coverage.html">rustc book</a>.</p>
 <h2 id="recommended-bootstraptoml-settings"><a class="header" href="#recommended-bootstraptoml-settings">Recommended <code>bootstrap.toml</code> settings</a></h2>
 <p>When working on the coverage instrumentation code, it is usually necessary to
 <strong>enable the profiler runtime</strong> by setting <code>profiler = true</code> in <code>[build]</code>.
 This allows the compiler to produce instrumented binaries, and makes it possible
 to run the full coverage test suite.</p>
 <p>Enabling debug assertions in the compiler and in LLVM is recommended, but not
 mandatory.</p>
 <pre><code class="language-toml"># Similar to the "compiler" profile, but also enables debug assertions in LLVM.
 # These assertions can detect malformed coverage mappings in some cases.
 profile = "codegen"

 [build]
 # IMPORTANT: This tells the build system to build the LLVM profiler runtime.
 # Without it, the compiler can't produce coverage-instrumented binaries,
 # and many of the coverage tests will be skipped.
 profiler = true

 [rust]
 # Enable debug assertions in the compiler.
 debug-assertions = true
 </code></pre>
 <h2 id="rust-symbol-mangling"><a class="header" href="#rust-symbol-mangling">Rust symbol mangling</a></h2>
 <p><code>-C instrument-coverage</code> automatically enables Rust symbol mangling <code>v0</code> (as
 if the user specified <code>-C symbol-mangling-version=v0</code> option when invoking
 <code>rustc</code>) to ensure consistent and reversible name mangling. This has two
 important benefits:</p>
 <ol>
 <li>LLVM coverage tools can analyze coverage over multiple runs, including some
 changes to source code; so mangled names must be consistent across compilations.</li>
 <li>LLVM coverage reports can report coverage by function, and even separates
 out the coverage counts of each unique instantiation of a generic function,
 if invoked with multiple type substitution variations.</li>
 </ol>
 <h2 id="the-llvm-profiler-runtime"><a class="header" href="#the-llvm-profiler-runtime">The LLVM profiler runtime</a></h2>
 <p>Coverage data is only generated by running the executable Rust program. <code>rustc</code>
 statically links coverage-instrumented binaries with LLVM runtime code
 (<a href="https://github.com/llvm/llvm-project/tree/main/compiler-rt/lib/profile">compiler-rt</a>) that implements program hooks
 (such as an <code>exit</code> hook) to write the counter values to the <code>.profraw</code> file.</p>
 <p>In the <code>rustc</code> source tree,
 <code>library/profiler_builtins</code> bundles the LLVM <code>compiler-rt</code> code into a Rust library crate.
 Note that when building <code>rustc</code>,
 <code>profiler_builtins</code> is only included when <code>build.profiler = true</code> is set in <code>bootstrap.toml</code>.</p>
 <p>When compiling with <code>-C instrument-coverage</code>,
 <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/creader/struct.CrateLoader.html#method.postprocess"><code>CrateLoader::postprocess()</code></a> dynamically loads
 <code>profiler_builtins</code> by calling <code>inject_profiler_runtime()</code>.</p>
 <h2 id="testing-coverage-instrumentation"><a class="header" href="#testing-coverage-instrumentation">Testing coverage instrumentation</a></h2>
 <p><a href="./tests/compiletest.html#coverage-tests">(See also the compiletest documentation for the <code>tests/coverage</code>
 test suite.)</a></p>
 <p>Coverage instrumentation in the MIR is validated by a <code>mir-opt</code> test:
 <a href="https://github.com/rust-lang/rust/blob/master/tests/mir-opt/coverage/instrument_coverage.rs"><code>tests/mir-opt/coverage/instrument_coverage.rs</code></a>.</p>
 <p>Coverage instrumentation in LLVM IR is validated by the <a href="https://github.com/rust-lang/rust/tree/master/tests/coverage"><code>tests/coverage</code></a>
 test suite in <code>coverage-map</code> mode.
 These tests compile a test program to LLVM IR assembly, and then
 use the <a href="https://github.com/rust-lang/rust/tree/master/src/tools/coverage-dump"><code>src/tools/coverage-dump</code></a> tool to extract and pretty-print the
 coverage mappings that would be embedded in the final binary.</p>
 <p>End-to-end testing of coverage instrumentation and coverage reporting is
 performed by the <a href="https://github.com/rust-lang/rust/tree/master/tests/coverage"><code>tests/coverage</code></a> test suite in <code>coverage-run</code> mode,
 and by the <a href="https://github.com/rust-lang/rust/tree/master/tests/coverage-run-rustdoc"><code>tests/coverage-run-rustdoc</code></a> test suite.
 These tests compile and run a test program with coverage
 instrumentation, then use LLVM tools to convert the coverage data into a
 human-readable coverage report.</p>
 <blockquote>
 <p>Tests in <code>coverage-run</code> mode have an implicit <code>//@ needs-profiler-runtime</code>
 directive, so they will be skipped if the profiler runtime has not been
 <a href="#recommended-configtoml-settings">enabled in <code>bootstrap.toml</code></a>.</p>
 </blockquote>
 <p>Finally, the <a href="https://github.com/rust-lang/rust/blob/master/tests/mir-opt/coverage/instrument_coverage.rs"><code>tests/codegen/instrument-coverage/testprog.rs</code></a> test compiles a simple Rust program
 with <code>-C instrument-coverage</code> and compares the compiled program's LLVM IR to
 expected LLVM IR instructions and structured data for a coverage-enabled
 program, including various checks for Coverage Map-related metadata and the LLVM
 intrinsic calls to increment the runtime counters.</p>
 <p>Expected results for the <code>coverage</code>, <code>coverage-run-rustdoc</code>,
 and <code>mir-opt</code> tests can be refreshed by running:</p>
 <pre><code class="language-shell">./x test coverage --bless
 ./x test coverage-run-rustdoc --bless
 ./x test tests/mir-opt --bless
 </code></pre>

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	<h1 id="llvm-source-based-code-coverage"><a class="header" href="#llvm-source-based-code-coverage">LLVM source-based code coverage</a></h1>
	<ul>
	<li><a href="#recommended-bootstraptoml-settings">Recommended <code>bootstrap.toml</code> settings</a></li>
	<li><a href="#rust-symbol-mangling">Rust symbol mangling</a></li>
	<li><a href="#the-llvm-profiler-runtime">The LLVM profiler runtime</a></li>
	<li><a href="#testing-coverage-instrumentation">Testing coverage instrumentation</a></li>
	</ul>
	<p><code>rustc</code> supports detailed source-based code and test coverage analysis
	with a command line option (<code>-C instrument-coverage</code>) that instruments Rust
	libraries and binaries with additional instructions and data, at compile time.</p>
	<p>The coverage instrumentation injects calls to the LLVM intrinsic instruction
	<a href="https://llvm.org/docs/LangRef.html#llvm-instrprof-increment-intrinsic"><code>llvm.instrprof.increment</code></a> at code branches
	(based on a MIR-based control flow analysis), and LLVM converts these to
	instructions that increment static counters, when executed. The LLVM coverage
	instrumentation also requires a <a href="https://llvm.org/docs/CoverageMappingFormat.html">Coverage Map</a> that encodes source metadata,
	mapping counter IDs--directly and indirectly--to the file locations (with
	start and end line and column).</p>
	<p>Rust libraries, with or without coverage instrumentation, can be linked into
	instrumented binaries. When the program is executed and cleanly terminates,
	LLVM libraries write the final counter values to a file (<code>default.profraw</code> or
	a custom file set through environment variable <code>LLVM_PROFILE_FILE</code>).</p>
	<p>Developers use existing LLVM coverage analysis tools to decode <code>.profraw</code>
	files, with corresponding Coverage Maps (from matching binaries that produced
	them), and generate various reports for analysis, for example:</p>
	<p><img alt="Screenshot of sample `llvm-cov show` result, for function add_quoted_string"
	src="img/llvm-cov-show-01.png" class="center"/>
	<br/></p>
	<p>Detailed instructions and examples are documented in the
	<a href="https://doc.rust-lang.org/nightly/rustc/instrument-coverage.html">rustc book</a>.</p>
	<h2 id="recommended-bootstraptoml-settings"><a class="header" href="#recommended-bootstraptoml-settings">Recommended <code>bootstrap.toml</code> settings</a></h2>
	<p>When working on the coverage instrumentation code, it is usually necessary to
	<strong>enable the profiler runtime</strong> by setting <code>profiler = true</code> in <code>[build]</code>.
	This allows the compiler to produce instrumented binaries, and makes it possible
	to run the full coverage test suite.</p>
	<p>Enabling debug assertions in the compiler and in LLVM is recommended, but not
	mandatory.</p>
	<pre><code class="language-toml"># Similar to the "compiler" profile, but also enables debug assertions in LLVM.
	# These assertions can detect malformed coverage mappings in some cases.
	profile = "codegen"

	[build]
	# IMPORTANT: This tells the build system to build the LLVM profiler runtime.
	# Without it, the compiler can't produce coverage-instrumented binaries,
	# and many of the coverage tests will be skipped.
	profiler = true

	[rust]
	# Enable debug assertions in the compiler.
	debug-assertions = true
	</code></pre>
	<h2 id="rust-symbol-mangling"><a class="header" href="#rust-symbol-mangling">Rust symbol mangling</a></h2>
	<p><code>-C instrument-coverage</code> automatically enables Rust symbol mangling <code>v0</code> (as
	if the user specified <code>-C symbol-mangling-version=v0</code> option when invoking
	<code>rustc</code>) to ensure consistent and reversible name mangling. This has two
	important benefits:</p>
	<ol>
	<li>LLVM coverage tools can analyze coverage over multiple runs, including some
	changes to source code; so mangled names must be consistent across compilations.</li>
	<li>LLVM coverage reports can report coverage by function, and even separates
	out the coverage counts of each unique instantiation of a generic function,
	if invoked with multiple type substitution variations.</li>
	</ol>
	<h2 id="the-llvm-profiler-runtime"><a class="header" href="#the-llvm-profiler-runtime">The LLVM profiler runtime</a></h2>
	<p>Coverage data is only generated by running the executable Rust program. <code>rustc</code>
	statically links coverage-instrumented binaries with LLVM runtime code
	(<a href="https://github.com/llvm/llvm-project/tree/main/compiler-rt/lib/profile">compiler-rt</a>) that implements program hooks
	(such as an <code>exit</code> hook) to write the counter values to the <code>.profraw</code> file.</p>
	<p>In the <code>rustc</code> source tree,
	<code>library/profiler_builtins</code> bundles the LLVM <code>compiler-rt</code> code into a Rust library crate.
	Note that when building <code>rustc</code>,
	<code>profiler_builtins</code> is only included when <code>build.profiler = true</code> is set in <code>bootstrap.toml</code>.</p>
	<p>When compiling with <code>-C instrument-coverage</code>,
	<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/creader/struct.CrateLoader.html#method.postprocess"><code>CrateLoader::postprocess()</code></a> dynamically loads
	<code>profiler_builtins</code> by calling <code>inject_profiler_runtime()</code>.</p>
	<h2 id="testing-coverage-instrumentation"><a class="header" href="#testing-coverage-instrumentation">Testing coverage instrumentation</a></h2>
	<p><a href="./tests/compiletest.html#coverage-tests">(See also the compiletest documentation for the <code>tests/coverage</code>
	test suite.)</a></p>
	<p>Coverage instrumentation in the MIR is validated by a <code>mir-opt</code> test:
	<a href="https://github.com/rust-lang/rust/blob/master/tests/mir-opt/coverage/instrument_coverage.rs"><code>tests/mir-opt/coverage/instrument_coverage.rs</code></a>.</p>
	<p>Coverage instrumentation in LLVM IR is validated by the <a href="https://github.com/rust-lang/rust/tree/master/tests/coverage"><code>tests/coverage</code></a>
	test suite in <code>coverage-map</code> mode.
	These tests compile a test program to LLVM IR assembly, and then
	use the <a href="https://github.com/rust-lang/rust/tree/master/src/tools/coverage-dump"><code>src/tools/coverage-dump</code></a> tool to extract and pretty-print the
	coverage mappings that would be embedded in the final binary.</p>
	<p>End-to-end testing of coverage instrumentation and coverage reporting is
	performed by the <a href="https://github.com/rust-lang/rust/tree/master/tests/coverage"><code>tests/coverage</code></a> test suite in <code>coverage-run</code> mode,
	and by the <a href="https://github.com/rust-lang/rust/tree/master/tests/coverage-run-rustdoc"><code>tests/coverage-run-rustdoc</code></a> test suite.
	These tests compile and run a test program with coverage
	instrumentation, then use LLVM tools to convert the coverage data into a
	human-readable coverage report.</p>
	<blockquote>
	<p>Tests in <code>coverage-run</code> mode have an implicit <code>//@ needs-profiler-runtime</code>
	directive, so they will be skipped if the profiler runtime has not been
	<a href="#recommended-configtoml-settings">enabled in <code>bootstrap.toml</code></a>.</p>
	</blockquote>
	<p>Finally, the <a href="https://github.com/rust-lang/rust/blob/master/tests/mir-opt/coverage/instrument_coverage.rs"><code>tests/codegen/instrument-coverage/testprog.rs</code></a> test compiles a simple Rust program
	with <code>-C instrument-coverage</code> and compares the compiled program's LLVM IR to
	expected LLVM IR instructions and structured data for a coverage-enabled
	program, including various checks for Coverage Map-related metadata and the LLVM
	intrinsic calls to increment the runtime counters.</p>
	<p>Expected results for the <code>coverage</code>, <code>coverage-run-rustdoc</code>,
	and <code>mir-opt</code> tests can be refreshed by running:</p>
	<pre><code class="language-shell">./x test coverage --bless
	./x test coverage-run-rustdoc --bless
	./x test tests/mir-opt --bless
	</code></pre>

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