Auto merge of #116046 - Zalathar:fn-cov-info, r=cjgillot

coverage: Move most per-function coverage info into `mir::Body`

Currently, all of the coverage information collected by the `InstrumentCoverage` pass is smuggled through MIR in the form of individual `StatementKind::Coverage` statements, which must then be reassembled by coverage codegen.

That's awkward for a number of reasons:
- While some of the coverage statements do care about their specific position in the MIR control-flow graph, many of them don't, and are just tacked onto the function's first BB as metadata carriers.
- MIR inlining can result in coverage statements being duplicated, so coverage codegen has to jump through hoops to avoid emitting duplicate mappings.
- MIR optimizations that would delete coverage statements need to carefully copy them into the function's first BB so as not to omit them from coverage reports.
- The order in which coverage codegen sees coverage statements is dependent on MIR optimizations/inlining, which can cause unnecessary churn in the emitted coverage mappings.
- We don't have a good way to annotate MIR-level functions with extra coverage info that doesn't belong in a statement.

---

This PR therefore takes most of the per-function coverage info and stores it in a field in `mir::Body` as `Option<Box<FunctionCoverageInfo>>`.

(This adds one pointer to the size of `mir::Body`, even when coverage is not enabled.)

Coverage statements still need to be injected into MIR in some cases, but only when they actually affect codegen (counters) or are needed to detect code that has been optimized away as unreachable (counters/expressions).

---

By the end of this PR, the information stored in `FunctionCoverageInfo` is:

- A hash of the function's source code (needed by LLVM's coverage map format)
- The number of coverage counters added by coverage instrumentation
- A table of coverage expressions, associating each expression ID with its operator (add or subtract) and its two operands
- The list of mappings, associating each covered code region with a counter/expression/zero value

---

~~This is built on top of #115301, so I'll rebase and roll a reviewer once that lands.~~
r? `@ghost`
`@rustbot` label +A-code-coverage

1 files changed, 13 insertions, 17 deletions

diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
index d4e77525698..ef3647efd88 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
@@ -10,9 +10,8 @@ use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
 use rustc_index::IndexVec;
 use rustc_middle::bug;
-use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::coverage::CodeRegion;
-use rustc_middle::ty::TyCtxt;
+use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::Symbol;
 
 /// Generates and exports the Coverage Map.
@@ -60,10 +59,8 @@ pub fn finalize(cx: &CodegenCx<'_, '_>) {
 
     // Encode coverage mappings and generate function records
     let mut function_data = Vec::new();
-    for (instance, mut function_coverage) in function_coverage_map {
+    for (instance, function_coverage) in function_coverage_map {
         debug!("Generate function coverage for {}, {:?}", cx.codegen_unit.name(), instance);
-        function_coverage.simplify_expressions();
-        let function_coverage = function_coverage;
 
         let mangled_function_name = tcx.symbol_name(instance).name;
         let source_hash = function_coverage.source_hash();
@@ -170,10 +167,11 @@ fn encode_mappings_for_function(
     let mut virtual_file_mapping = IndexVec::<u32, u32>::new();
     let mut mapping_regions = Vec::with_capacity(counter_regions.len());
 
-    // Sort the list of (counter, region) mapping pairs by region, so that they
-    // can be grouped by filename. Prepare file IDs for each filename, and
-    // prepare the mapping data so that we can pass it through FFI to LLVM.
-    counter_regions.sort_by_key(|(_counter, region)| *region);
+    // Sort and group the list of (counter, region) mapping pairs by filename.
+    // (Preserve any further ordering imposed by `FunctionCoverage`.)
+    // Prepare file IDs for each filename, and prepare the mapping data so that
+    // we can pass it through FFI to LLVM.
+    counter_regions.sort_by_key(|(_counter, region)| region.file_name);
     for counter_regions_for_file in
         counter_regions.group_by(|(_, a), (_, b)| a.file_name == b.file_name)
     {
@@ -331,16 +329,14 @@ fn add_unused_functions(cx: &CodegenCx<'_, '_>) {
     for non_codegenned_def_id in
         eligible_def_ids.into_iter().filter(|id| !codegenned_def_ids.contains(id))
     {
-        let codegen_fn_attrs = tcx.codegen_fn_attrs(non_codegenned_def_id);
-
-        // If a function is marked `#[coverage(off)]`, then skip generating a
-        // dead code stub for it.
-        if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NO_COVERAGE) {
-            debug!("skipping unused fn marked #[coverage(off)]: {:?}", non_codegenned_def_id);
+        // Skip any function that didn't have coverage data added to it by the
+        // coverage instrumentor.
+        let body = tcx.instance_mir(ty::InstanceDef::Item(non_codegenned_def_id));
+        let Some(function_coverage_info) = body.function_coverage_info.as_deref() else {
             continue;
-        }
+        };
 
         debug!("generating unused fn: {:?}", non_codegenned_def_id);
-        cx.define_unused_fn(non_codegenned_def_id);
+        cx.define_unused_fn(non_codegenned_def_id, function_coverage_info);
     }
 }