Adds a dev guide section on Rust Coverage

src/SUMMARY.md

@@ -147,6 +147,7 @@
     - [Backend Agnostic Codegen](./backend/backend-agnostic.md)
     - [Implicit Caller Location](./backend/implicit-caller-location.md)
 - [Profile-guided Optimization](./profile-guided-optimization.md)
+- [LLVM Source-Based Code Coverage](./llvm-coverage-instrumentation.md)
 - [Sanitizers Support](./sanitizers.md)
 - [Debugging Support in the Rust Compiler](./debugging-support-in-rustc.md)
 

src/llvm-coverage-instrumentation.md

@@ -0,0 +1,627 @@
+# LLVM Source-Based Code Coverage
+
+`rustc` supports detailed source-based code and test coverage analysis
+with a command line option (`-Z instrument-coverage`) that instruments Rust
+libraries and binaries with additional instructions and data, at compile time.
+
+The coverage instrumentation injects calls to the LLVM intrinsic instruction
+[`llvm.instrprof.increment`][llvm-instrprof-increment] 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 [Coverage Map] that encodes source metadata,
+mapping counter IDs--directly and indirectly--to the file locations (with
+start and end line and column).
+
+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 (`default.profraw` or
+a custom file set through environment variable `LLVM_PROFILE_FILE`).
+
+Developers use existing LLVM coverage analysis tools to decode `.profraw`
+files, with corresponding Coverage Maps (from matching binaries that produced
+them), and generate various reports for analysis, for example:
+
+<img alt="Screenshot of sample `llvm-cov show` result, for function add_quoted_string"
+ src="img/llvm-cov-show-01.png" class="center"/>
+<br/>
+
+Detailed instructions and examples are documented in the
+[Rust Unstable Book (under _source-based-code-coverage_)][unstable-book-sbcc].
+
+[llvm-instrprof-increment]: https://llvm.org/docs/LangRef.html#llvm-instrprof-increment-intrinsic
+[Coverage Map]: https://llvm.org/docs/CoverageMappingFormat.html
+[unstable-book-sbcc]: https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/source-based-code-coverage.html
+
+### Rust symbol mangling
+
+`-Z instrument-coverage` automatically enables Rust symbol mangling `v0` (as
+if the user specified `-Z symbol-mangling-version=v0` option when invoking
+`rustc`) to ensure consistent and reversible name mangling. This has two
+important benefits:
+
+1. LLVM coverage tools can analyze coverage over multiple runs, including some
+   changes to source code; so mangled names must be consistent across compilations.
+2. 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.
+
+## Components of LLVM Coverage Instrumentation in `rustc`
+
+### LLVM Runtime Dependency
+
+Coverage data is only generated by running the executable Rust program. `rustc`
+statically links coverage-instrumented binaries with LLVM runtime code
+([compiler-rt][compiler-rt-profile]) that implements program hooks
+(such as an `exit` hook) to write the counter values to the `.profraw` file.
+
+In the `rustc` source tree, `library/profiler_builtins` bundles the LLVM
+`compiler-rt` code into a Rust library crate. (When building `rustc`, the
+`profiler_builtins` library is only included when `profiler = true` is set
+in `rustc`'s `config.toml`.)
+
+When compiling with `-Z instrument-coverage`,
+[`CrateLoader::postprocess()`][crate-loader-postprocess] dynamically loads the
+`profiler_builtins` library by calling `inject_profiler_runtime()`.
+
+[compiler-rt-profile]: https://github.com/llvm/llvm-project/tree/master/compiler-rt/lib/profile
+[crate-loader-postprocess]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/creader/struct.CrateLoader.html#method.postprocess
+
+### MIR Pass: `InstrumentCoverage`
+
+Coverage instrumentation is performed on the MIR with a [MIR pass][mir-passes]
+called [`InstrumentCoverage`][mir-instrument-coverage]. This MIR pass analyzes
+the control flow graph (CFG)--represented by MIR `BasicBlock`s--to identify
+code branches, and injects additional [`Coverage`][coverage-statement]
+statements into the `BasicBlock`s.
+
+A MIR `Coverage` statement is a virtual instruction that indicates a counter
+should be incremented when its adjacent statemeents are executed, to count
+a span of code ([`CodeRegion`][code-region]). It counts the number of times a
+branch is executed, and also specifies the exact location of that code span in
+the Rust source code.
+
+Note that many of these `Coverage` statements will *not* be converted into
+physical counters (or any other executable instructions) in the final binary.
+Some of them will be (see `CoverageKind::`[`Counter`][counter-coverage-kind]),
+but other counters can be computed on the fly, when generating a coverage
+report, by mapping a `CodeRegion` to a
+`CoverageKind`::[`Expression`][expression-coverage-kind].
+
+As an example:
+
+```rust
+fn some_func(flag: bool) {
+    // increment Counter(1)
+    ...
+    if flag {
+        // increment Counter(2)
+        ...
+    } else {
+        // count = Expression(1) = Counter(1) - Counter(2)
+        ...
+    }
+    // count = Expression(2) = Counter(1) + Zero
+    //     or, alternatively, Expression(2) = Counter(2) + Expression(1)
+    ...
+}
+```
+
+In this example, four contiguous code regions are counted while only
+incrementing two counters.
+
+CFG analysis is used to not only determine *where* the branches are, for
+conditional expressions like `if`, `else`, `match`, and `loop`, but also to
+determine where expressions can be used in place of physical counters.
+
+The advantages of optimizing coverage through expressions are more pronounced
+with loops. Loops generally include at least one conditional branch that
+determines when to break out of a loop (a `while` condition, or an `if` or
+`match` with a `break`). In MIR, this is typically lowered to a `SwitchInt`,
+with one branch to stay in the loop, and another branch to break out of the
+loop. The branch that breaks out will almost always execute less often,
+so `InstrumentCoverage` chooses to add a `Counter` to that branch, and an
+`Expression(continue) = Counter(loop) - Counter(break)` to the branch that
+continues.
+
+The `InstrumentCoverage` MIR pass is documented in
+[more detail below][instrument-coverage-pass-details].
+
+[mir-passes]: mir/passes.md
+[mir-instrument-coverage]: https://github.com/rust-lang/rust/tree/master/compiler/rustc_mir/src/transform/coverage
+[code-region]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/coverage/struct.CodeRegion.html
+[counter-coverage-kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/coverage/enum.CoverageKind.html#variant.Counter
+[expression-coverage-kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/coverage/enum.CoverageKind.html#variant.Expression
+[coverage-statement]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.StatementKind.html#variant.Coverage
+[instrument-coverage-pass-details]: #implementation-details-of-the-instrumentcoverage-mir-pass
+
+### Counter Injection and Coverage Map Pre-staging
+
+When the compiler enters [the Codegen phase][backend-lowering-mir], with a
+coverage-enabled MIR, [`codegen_statement()`][codegen-statement] converts each
+MIR `Statement` into some backend-specific action or instruction.
+`codegen_statement()` forwards `Coverage` statements to
+[`codegen_coverage()`][codegen-coverage]:
+
+```rust
+    pub fn codegen_statement(&mut self, mut bx: Bx, statement: &mir::Statement<'tcx>) -> Bx {
+        ...
+        match statement.kind {
+            ...
+            mir::StatementKind::Coverage(box ref coverage) => {
+                self.codegen_coverage(&mut bx, coverage.clone());
+                bx
+            }
+```
+
+
+`codegen_coverage()` handles each `CoverageKind` as follows:
+
+* For all `CoverageKind`s, Coverage data (counter ID, expression equation
+  and ID, and code regions) are passed to the backend's `Builder`, to
+  populate data structures that will be used to generate the crate's
+  "Coverage Map". (See the [`FunctionCoverage`][function-coverage] `struct`.)
+* For `CoverageKind::Counter`s, an instruction is injected in the backend
+  IR to increment the physical counter, by calling the `BuilderMethod`
+  [`instrprof_increment()`][instrprof-increment].
+
+```rust
+    pub fn codegen_coverage(&self, bx: &mut Bx, coverage: Coverage) {
+        let Coverage { kind, code_region } = coverage;
+        match kind {
+            CoverageKind::Counter { function_source_hash, id } => {
+                if let Some(code_region) = code_region {
+                    bx.add_coverage_counter(self.instance, id, code_region);
+                }
+                ...
+                bx.instrprof_increment(fn_name, hash, num_counters, index);
+            }
+            CoverageKind::Expression { id, lhs, op, rhs } => {
+                bx.add_coverage_counter_expression(self.instance, id, lhs, op, rhs, code_region);
+            }
+            CoverageKind::Unreachable => {
+                ...
+```
+_[code snippet trimmed for brevity]_
+
+> The function name `instrprof_increment()` is taken from the LLVM intrinsic
+call of the same name ([`llvm.instrprof.increment`][llvm-instrprof-increment]),
+and uses the same arguments and types; but note that, up to and through this
+stage (even though modeled after LLVM's implementation for code coverage
+instrumentation), the data and instructions are not strictly LLVM-specific.
+>
+> But since LLVM is the only Rust-supported backend with the tooling to
+process this form of coverage instrumentation, the backend for `Coverage`
+statements is only implemented for LLVM, at this time.
+
+[backend-lowering-mir]: backend/lowering-mir.md
+[codegen-statement]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#method.codegen_statement
+[codegen-coverage]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/struct.FunctionCx.html#method.codegen_coverage
+[function-coverage]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/coverageinfo/map/struct.FunctionCoverage.html
+[instrprof-increment]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/traits/trait.BuilderMethods.html#tymethod.instrprof_increment
+
+### Coverage Map Generation
+
+With the instructions to increment counters now implemented in LLVM IR,
+the last remaining step is to inject the LLVM IR variables that hold the
+static data for the coverage map.
+
+`rustc_codegen_llvm`'s [`compile_codegen_unit()`][compile-codegen-unit] calls
+[`coverageinfo_finalze()`][coverageinfo-finalize],
+which delegates its implementation to the
+[`rustc_codegen_llvm::coverageinfo::mapgen`][mapgen-finalize] module.
+
+For each function `Instance` (code-generated from MIR, including multiple
+instances of the same MIR for generic functions that have different type
+substitution combinations), `mapgen`'s `finalize()` method queries the
+`Instance`-associated `FunctionCoverage` for its `Counter`s, `Expression`s,
+and `CodeRegion`s; and calls LLVM codegen APIs to generate
+properly-configured variables in LLVM IR, according to very specific
+details of the [_LLVM Coverage Mapping Format_][coverage-mapping-format]
+(Version 4).[^llvm-and-covmap-versions]
+
+[^llvm-and-covmap-versions]: The Rust compiler currently supports
+_LLVM Coverage Mapping Format_ Version 4 (the most up-to-date version
+of the format, at the time of this writing) for improved compatibility
+with other LLVM-based compilers (like _Clang_), and to take advantage
+of some format optimizations. Version 4 was introduced in _LLVM 11_,
+which is currently the default LLVM version for Rust. Note that the
+Rust compiler optionally supports some earlier LLVM versions,
+prior to _LLVM 11_. If `rustc` is configured to use an incompatible
+version of LLVM, compiling with `-Z instrument-coverage` will
+generate an error message.
+
+```rust
+pub fn finalize<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>) {
+    let mut function_coverage_map = match cx.coverage_context() {
+        Some(ctx) => ctx.take_function_coverage_map(),
+        None => return,
+    };
+    ...
+    add_unreachable_coverage(tcx, &mut function_coverage_map);
+
+    let mut mapgen = CoverageMapGenerator::new();
+
+    for (instance, function_coverage) in function_coverage_map {
+        ...
+        let coverage_mapping_buffer = llvm::build_byte_buffer(|coverage_mapping_buffer| {
+            mapgen.write_coverage_mapping(expressions, counter_regions, coverage_mapping_buffer);
+        });
+```
+_[code snippet trimmed for brevity]_
+
+One noteable step, performed by `mapgen::finalize()` before processing the
+`Instance`s and their `FunctionCoverage`s, is the call to
+[`add_unreachable_functions()`][add-unreachable-coverage].
+
+When finalizing the coverage map, `FunctionCoverage` only has the `CodeRegion`s and counters for
+the functions that went through codegen; such as public functions and "used" functions
+(functions referenced by other "used" or public items). Any other functions (considered unused
+or "Unreachable") were still parsed and processed through the MIR stage.
+
+The set of unreachable functions is computed via the set difference of all MIR
+`DefId`s (`tcx` query `mir_keys`) minus the codegenned `DefId`s
+(`tcx` query `collect_and_partition_mono_items`). `add_unreachable_functions()`
+computes the set of unreachable functions, queries the `tcx` for the
+previously-computed `CodeRegions`, for each unreachabe MIR, and adds those code
+regions to one of the non-generic codegenned functions (non-generic avoids
+potentially injecting the unreachable coverage multiple times for multiple
+instantiations).
+
+[compile-codegen-unit]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/base/fn.compile_codegen_unit.html
+[coverageinfo-finalize]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/context/struct.CodegenCx.html#method.coverageinfo_finalize
+[mapgen-finalize]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/coverageinfo/mapgen/fn.finalize.html
+[coverage-mapping-format]: https://llvm.org/docs/CoverageMappingFormat.html
+[add-unreachable-coverage]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/coverageinfo/mapgen/fn.add_unreachable_coverage.html
+
+## Testing LLVM Coverage
+
+Coverage instrumentation in the MIR is validated by a `mir-opt` test:
+[`instrument-coverage`][mir-opt-test].
+
+More complete testing of end-to-end coverage instrumentation and reports are done
+in the `run-make-fulldeps` tests, with sample Rust programs (to be instrumented)
+in the [`coverage`][coverage-test-samples] directory, and the actual tests and expected
+results in [`coverage-reports`].
+
+In addition to testing the final result, two intermediate results are also validated
+to catch potential regression errors early: Minimum `CoverageSpan`s computed during
+the `InstrumentCoverage` MIR pass are saved in `mir_dump` [Spanview][spanview-debugging]
+files and compared to expected results in [`coverage-spanview`].
+
+Finally, the [`coverage-llvmir`] test compares compiles a simple Rust program with
+`-Z instrument-coverage` 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.
+
+Expected results for both the `mir-opt` tests and the `coverage*` tests under
+`run-make-fulldeps` can be refreshed by running:
+
+```shell
+$ ./x.py test src/test/<test-type> --blessed
+```
+
+[mir-opt-test]: https://github.com/rust-lang/rust/blob/master/src/test/mir-opt/instrument_coverage.rs
+[coverage-test-samples]: https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/coverage
+[`coverage-reports`]: https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/coverage-reports
+[`coverage-spanview`]: https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/coverage-spanview
+[spanview-debugging]: compiler-debugging.md#viewing-spanview-output
+[`coverage-llvmir`]: https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/coverage-llvmir
+
+## Implementation Details of the `InstrumentCoverage` MIR Pass 
+
+The bulk of the implementation of the `InstrumentCoverage` MIR pass is performed
+by the [`Instrumentor`][instrumentor]. For each MIR (each non-const, non-inlined
+function, generic, or closure), the `Instrumentor`'s constructor prepares a
+[`CoverageGraph`][coverage-graph] and then executes
+[`inject_counters()`][inject-counters].
+
+```rust
+        Instrumentor::new(&self.name(), tcx, mir_body).inject_counters();
+```
+
+The `CoverageGraph` is a coverage-specific simplification of the MIR control
+flow graph (CFG). Its nodes are [`BasicCoverageBlock`s][bcb], which
+encompass one or more sequentially-executed MIR `BasicBlock`s
+(with no internal branching), plus a `CoverageKind` counter (to
+be added, via coverage analysis), and an optional set of additional counters
+to count incoming edges (if there are more than one).
+
+The `Instrumentor`'s `inject_counters()` uses the `CoverageGraph` to
+compute the best places to inject coverage counters, as MIR `Statement`s,
+with the following steps:
+
+1. Depending on the debugging configurations in `rustc`'s, `config.toml`,
+   and `rustc` command line flags, various debugging features may be enabled
+   to enhance `debug!()` messages in logs, and to generate various "dump" files,
+   to help developers understand the MIR transformation process for coverage.
+   Most of the debugging features are implemented in the [`debug`][debug]
+   sub-module.
+2. [`generate_coverage_spans()`][generate-coverage-spans] computes the minimum set of distinct,
+   non-branching code regions, from the MIR. These `CoverageSpan`s
+   represent a span of code that must be counted.
+3. [`make_bcb_counters()`][make-bcb-counters] generates `CoverageKind::Counter`s and
+   `CoverageKind::Expression`s for each `CoverageSpan`, plus additional
+   `intermediate_expressions`[^intermediate-expressions], not associated with any `CodeRegion`, but
+   are required to compute a final `Expression` value for a `CodeRegion`.
+4. Inject the new counters into the MIR, as new `StatementKind::Coverage`
+   statements. This is done by three distinct functions:
+   - `inject_coverage_span_counters()`
+   - `inject_indirect_counters()`
+   - `inject_intermediate_expression()`, called for each intermediate expression
+     returned from `make_bcb_counters()`
+
+[^intermediate-expressions]: Intermediate expressions are sometimes required
+because `Expression`s are limited to binary additions or subtractions. For
+example, `A + (B - C)` might represent an `Expression` count computed from three
+other counters, `A`, `B`, and `C`, but computing that value requires an
+intermediate expression for `B - C`.
+
+[instrumentor]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/struct.Instrumentor.html
+[coverage-graph]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/graph/struct.CoverageGraph.html
+[inject-counters]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/struct.Instrumentor.html#method.inject_counters
+[bcb]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/graph/struct.BasicCoverageBlock.html
+[debug]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/debug
+[generate-coverage-spans]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/spans/struct.CoverageSpans.html#method.generate_coverage_spans
+[make-bcb-counters]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/counters/struct.BcbCounters.html#method.make_bcb_counters
+
+### The `CoverageGraph`
+
+The [`CoverageGraph`][coverage-graph] is derived from the MIR (`mir::Body`).
+
+```rust
+        let basic_coverage_blocks = CoverageGraph::from_mir(mir_body);
+```
+
+Like `mir::Body`, the `CoverageGraph` is also a
+[`DirectedGraph`][directed-graph]. Both graphs represent the function's
+fundamental control flow, with many of the same
+[`graph trait`][graph-traits]s, supporting `start_node()`, `num_nodes()`,
+`successors()`, `predecessors()`, and `is_dominated_by()`.
+
+For anyone that knows how to work with the [MIR, as a CFG][mir-dev-guide], the
+`CoverageGraph` will be familiar, and can be used in much the same way.
+The nodes of the `CoverageGraph` are `BasicCoverageBlock`s (BCBs), which
+index into an `IndexVec` of `BasicCoverageBlockData`. This is analagous
+to the MIR CFG of `BasicBlock`s that index `BasicBlockData`.
+
+Each `BasicCoverageBlockData` captures one or more MIR `BasicBlock`s,
+exclusively, and represents the maximal-length sequence of `BasicBlocks`
+without conditional branches.
+
+[`compute_basic_coverage_blocks()`][compute-basic-coverage-blocks] builds the
+`CoverageGraph` as a coverage-specific simplification of the MIR CFG. In
+contrast with the [`SimplifyCfg`][simplify-cfg] MIR pass, this step does
+not alter the MIR itself, because the `CoverageGraph` aggressively simplifies
+the CFG, and ignores nodes that are not relevant to coverage. For example:
+
+  * The BCB CFG ignores (excludes) branches considered not relevant
+    to the current coverage solution. It excludes unwind-related code[^78544]
+    that is injected by the Rust compiler but has no physical source
+    code to count, which allows a `Call`-terminated BasicBlock
+    to be merged with its successor, within a single BCB.
+  * A `Goto`-terminated `BasicBlock` can be merged with its successor
+    ***as long as*** it has the only incoming edge to the successor `BasicBlock`.
+  * Some BasicBlock terminators support Rust-specific concerns--like borrow-checking--that are
+    not relevant to coverage analysis. `FalseUnwind`, for example, can be treated the same as
+    a `Goto` (potentially merged with its successor into the same BCB).
+
+[^78544]: (Note, however, that Issue [#78544][rust-lang/rust#78544] considers
+providing future support for coverage of programs that intentionally
+`panic`, as an option, with some non-trivial cost.)
+
+The BCB CFG is critical to simplifying the coverage analysis by ensuring graph path-based
+queries (`is_dominated_by()`, `predecessors`, `successors`, etc.) have branch (control flow)
+significance.
+
+To visualize the `CoverageGraph`, you can generate a _graphviz_ `*.dot`
+file with the following `rustc` flags:[^graphviz-dark-mode]
+
+[^graphviz-dark-mode]: This image also applies `-Z graphviz-dark-mode`, to
+produce a Graphviz document with "dark mode" styling. If you use a dark mode or
+theme in your development environment, you will probably want to use this
+option so you can review the graphviz output without straining your vision.
+
+```shell
+$ rustc -Z instrument-coverage -Z dump-mir=InstrumentCoverage \
+    -Z dump-mir-graphviz some_rust_source.rs
+```
+
+The `-Z dump-mir` flag requests [MIR debugging
+output][mir-debugging] (generating `*.mir` files, by default).
+`-Z dump-mir-graphviz` additionally generates `*.dot` files.
+`-Z dump-mir=InstrumentCoverage` restricts these files to the
+`InstrumentCoverage` pass. All files are written to the `./mir_dump/`
+directory, by default.
+
+Files with names ending in `.-------.InstrumentCoverage.0.dot` contain the
+_graphviz_ representations of a `CoverageGraph` (one for each MIR, that is,
+for each function and closure):
+
+<img alt="cropped image of a sample CoverageGraph in graphviz format"
+ src="img/coverage-graphviz-01.png" style="border: 1px solid gray" class="center"/>
+<br/>
+
+This image shows each `BasicCoverageBlock` as a rectangular _node_, with
+directional edges (the arrows) leading from each node to its `successors()`.
+The nodes contain information in sections:
+
+1. The gray header has a label showing the BCB ID (or _index_ for looking up
+its `BasicCoverageBlockData`).
+2. The first content section shows the assigned `Counter` or `Expression` for
+each contiguous section of code. (There may be more than one `Expression`
+incremented by the same `Counter` for discontigous sections of code representing
+the same sequential actions.) Note the code is represented by the line and
+column ranges (for example: `52:28-52:33`, representing the original source
+line 52, for columns 28-33). These are followed by the MIR `Statement` or
+`Terminator` represented by that source range. (How these coverage regions
+are determined is discussed in the following section.)
+3. The final section(s) show the MIR `BasicBlock`s (by ID/index and its
+`TerminatorKind`) contained in this BCB. The last BCB is separated out because
+its `successors()` determine the edges leading out of the BCB, and into
+the `leading_bb()` (first `BasicBlock`) of each successor BCB.
+
+Note, to find the `BasicCoverageBlock` from a final BCB `Terminator`'s
+successor `BasicBlock`, there is an index and helper
+function--[`bcb_from_bb()`][bcb-from-bb]--to look up a `BasicCoverageBlock` from _any_
+contained `BasicBlock`.
+
+[directed-graph]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/graph/trait.DirectedGraph.html
+[graph-traits]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/graph/index.html#traits
+[mir-dev-guide]: mir/index.md
+[compute-basic-coverage-blocks]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/graph/struct.CoverageGraph.html#method.compute_basic_coverage_blocks
+[simplify-cfg]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/simplify/struct.SimplifyCfg.html
+[rust-lang/rust#78544]: https://github.com/rust-lang/rust/issues/78544
+[mir-debugging]: mir/debugging.md
+[bcb-from-bb]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/graph/struct.CoverageGraph.html#method.bcb_from_bb
+
+### `CoverageSpans`
+
+The `struct` [`CoverageSpans`][coverage-spans] builds and refines a final set of
+[`CoverageSpan`][coverage-span]s, each representing the largest contiguous `Span`
+of source within a single BCB. By definition--since each `Span` falls within a
+BCB, the `Span` is also non-branching; so if any code in that `Span` has executed,
+all code in the `Span` will have executed, the same number of times.
+
+[`CoverageSpans::generate_coverage_spans()`][generate-coverage-spans] constructs
+an initial set of `CoverageSpan`s from the `Span`s associated with each MIR
+`Statement` and `Terminator`.
+
+The final stage of `generate_coverage_spans()` is handled by
+[`to_refined_spans()`][to-refined-spans], which iterates through the `CoverageSpan`s,
+merges and de-duplicates them, and returns an optimial, minimal set of `CoverageSpan`s
+that can be used to assign coverage `Counter`s or `Expression`s, one-for-one.
+
+An visual, interactive representation of the final `CoverageSpan`s can be
+generated with the following `rustc` flags:
+
+```shell
+$ rustc -Zinstrument-coverage -Zdump-mir=InstrumentCoverage \
+    -Zdump-mir-spanview some_rust_source.rs
+```
+
+These flags request Spanview output for the `InstrumentCoverage` pass, and the
+resulting files (one for each MIR, that is, for each function or closure) can be
+found in the `mir_dump` directory (by default), with the extension:
+`.-------.InstrumentCoverage.0.html`.
+
+<img alt="cropped image of a sample Spanview in a browser"
+ src="img/coverage-spanview-01.png" style="border: 1px solid gray" class="center"/>
+<br/>
+
+The image above shows one such example. The orange and blue backgrounds
+highlight alternating `CoverageSpan`s from the refined set. Hovering over a
+line expands the output on that line to show the MIR `BasicBlock` IDs covered
+by each `CoverageSpan`. While hovering, the `CoverageSpan` under the pointer
+also has a _tooltip_ block of text, showing even more detail, including the
+MIR `Statement`s and `Terminator`s contributing to the `CoverageSpan`, and
+their individual `Span`s (which should be encapsulated within the code region
+of the refined `CoverageSpan`)
+
+[coverage-spans]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/spans/struct.CoverageSpans.html
+[coverage-span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/spans/struct.CoverageSpan.html
+[to-refined-spans]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/spans/struct.CoverageSpans.html#method.to_refined_spans
+
+### `make_bcb_counters()`
+
+[`make_bcb_counters()`][make-bcb-counters] traverses the `CoverageGraph` and adds a
+`Counter` or `Expression` to every BCB. It uses _Control Flow Analysis_
+to determine where an `Expression` can be used in place of a `Counter`.
+`Expressions` have no runtime overhead, so if a viable expression (adding or
+subtracting two other counters or expressions) can compute the same result as
+an embedded counter, an `Expression` is preferred.
+
+[`TraverseCoverageGraphWithLoops`][traverse-coverage-graph-with-loops]
+provides a traversal order that ensures all `BasicCoverageBlock` nodes in a
+loop are visited before visiting any node outside that loop. The traversal
+state includes a `context_stack`, with the current loop's context information
+(if in a loop), as well as context for nested loops.
+
+Within loops, nodes with multiple outgoing edges (generally speaking, these
+are BCBs terminated in a `SwitchInt`) can be optimized when at least one
+branch exits the loop and at least one branch stays within the loop. (For an
+`if` or `while`, there are only two branches, but a `match` may have more.)
+
+A branch that does not exit the loop should be counted by `Expression`, if
+possible. Note that some situations require assigning counters to BCBs before
+they are visited by traversal, so the `counter_kind` (`CoverageKind` for
+a `Counter` or `Expression`) may have already been assigned, in which case
+one of the other branches should get the `Expression`.
+
+For a node with more than two branches (such as for more than two
+`match` patterns), only one branch can be optimized by `Expression`. All
+others require a `Counter` (unless its BCB `counter_kind` was previously
+assigned).
+
+A branch expression is derived from the equation:
+
+```text
+Counter(branching_node) = SUM(Counter(branches))
+```
+
+It's important to
+be aware that the `branches` in this equation are the outgoing _edges_
+from the `branching_node`, but a `branch`'s target node may have other
+incoming edges. Given the following graph, for example, the count for
+`B` is the sum of its two incoming edges:
+
+<img alt="Example graph with multiple incoming edges to a branch node"
+ src="img/coverage-branch-counting-01.png" class="center" style="width: 25%">
+<br/>
+
+In this situation, BCB node `B` may require an edge counter for its
+"edge from A", and that edge might be computed from an `Expression`,
+`Counter(A) - Counter(C)`. But an expression for the BCB _node_  `B`
+would be the sum of all incoming edges:
+
+```text
+Expression((Counter(A) - Counter(C)) + SUM(Counter(remaining_edges)))
+```
+
+Note that this is only one possible configuration. The actual choice
+of `Counter` vs. `Expression` also depends on the order of counter
+assignments, and whether a BCB or incoming edge counter already has
+its `Counter` or `Expression`.
+
+[bcb-counters]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/counters/struct.BcbCounters.html
+[traverse-coverage-graph-with-loops]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/graph/struct.TraverseCoverageGraphWithLoops.html
+
+### Injecting counters into a MIR `BasicBlock`
+
+With the refined `CoverageSpan`s, and after all `Counter`s and `Expression`s are
+created, the final step is to inject the `StatementKind::Coverage` statements
+into the MIR. There are three distinct sources, handled by the following
+functions:
+
+- [`inject_coverage_span_counters()`][inject-coverage-span-counters] injects the
+  counter from each `CoverageSpan`'s BCB.
+- [`inject_indirect_counters()`][inject-indirect-counters] injects counters
+  for any BCB not assigned to a `CoverageSpan`, and for all edge counters.
+  These counters don't have `CoverageSpan`s.
+- [`inject_intermediate_expression()`][inject-intermediate-expression] injects
+  the intermediate expressions returned from `make_bcb_counters()`. These
+  counters aren't associated with any BCB, edge, or `CoverageSpan`.
+
+These three functions inject the `Coverage` statements into the MIR.
+`Counter`s and `Expression`s with `CoverageSpan`s add `Coverage` statements
+to a corresponding `BasicBlock`, with a `CodeRegion` computed from the
+refined `Span` and current `SourceMap`.
+
+All other `Coverage` statements have a `CodeRegion` of `None`, but they
+still must be injected because they contribute to other `Expression`s.
+
+Finally, edge's with a `CoverageKind::Counter` require a new `BasicBlock`,
+so the counter is only incremented when traversing the branch edge.
+
+[inject-coverage-span-counters]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/struct.Instrumentor.html#method.inject_coverage_span_counters
+[inject-indirect-counters]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/struct.Instrumentor.html#method.inject_indirect_counters
+[inject-intermediate-expression]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/fn.inject_intermediate_expression.html
+
+### Additional Debugging Support
+
+See the
+[crate documentation for `rustc_mir::transform::coverage::debug`][coverage-debugging]
+for a detailed description of the debug output, logging, and configuration options
+available to developers working on the `InstrumentCoverage` pass.
+
+[coverage-debugging]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/coverage/debug/index.html

src/profile-guided-optimization.md

@@ -29,7 +29,8 @@ LLVM actually [supports multiple forms][clang-pgo] of PGO:
 - GCOV-based profiling, where code coverage infrastructure is used to collect
   profiling information.
 - Front-end based instrumentation, where the compiler front-end (e.g. Clang)
-  inserts instrumentation intrinsics into the LLVM IR it generates.
+  inserts instrumentation intrinsics into the LLVM IR it generates (but see the
+  [^note-instrument-coverage]"Note").
 - IR-level instrumentation, where LLVM inserts the instrumentation intrinsics
   itself during optimization passes.
 
@@ -44,6 +45,11 @@ optimized. Instrumentation-based PGO has two components: a compile-time
 component and run-time component, and one needs to understand the overall
 workflow to see how they interact.
 
+[^note-instrument-coverage]: Note: `rustc` now supports front-end-based coverage
+instrumentation, via the experimental option
+[`-Z instrument-coverage`](../llvm-coverage-instrumentation), but using these
+coverage results for PGO has not been attempted at this time.
+
 ### Overall Workflow
 
 Generating a PGO-optimized program involves the following four steps: