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Auto merge of rust-lang#134163 - Zalathar:covfun, r=<try>
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coverage: Rearrange the code for embedding per-function coverage metadata

This is a series of refactorings to the code that prepares and embeds per-function coverage metadata records (“covfun records”) in the `__llvm_covfun` linker section of the final binary. The `llvm-cov` tool reads this metadata from the binary when preparing a coverage report.

Beyond general cleanup, a big motivation behind these changes is to pave the way for re-landing an updated version of rust-lang#133418.

---

There should be no change in compiler output, as demonstrated by the absence of (meaningful) changes to coverage tests.

---

try-job: x86_64-gnu
try-job: x86_64-msvc
try-job: aarch64-apple
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bors committed Dec 11, 2024
2 parents 33c245b + ec7eb7e commit 4bb99ca
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Showing 5 changed files with 259 additions and 184 deletions.
28 changes: 28 additions & 0 deletions compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -152,6 +152,34 @@ impl CoverageSpan {
}
}

/// Holds tables of the various region types in one struct.
///
/// Don't pass this struct across FFI; pass the individual region tables as
/// pointer/length pairs instead.
///
/// Each field name has a `_regions` suffix for improved readability after
/// exhaustive destructing, which ensures that all region types are handled.
#[derive(Clone, Debug, Default)]
pub(crate) struct Regions {
pub(crate) code_regions: Vec<CodeRegion>,
pub(crate) branch_regions: Vec<BranchRegion>,
pub(crate) mcdc_branch_regions: Vec<MCDCBranchRegion>,
pub(crate) mcdc_decision_regions: Vec<MCDCDecisionRegion>,
}

impl Regions {
/// Returns true if none of this structure's tables contain any regions.
pub(crate) fn has_no_regions(&self) -> bool {
let Self { code_regions, branch_regions, mcdc_branch_regions, mcdc_decision_regions } =
self;

code_regions.is_empty()
&& branch_regions.is_empty()
&& mcdc_branch_regions.is_empty()
&& mcdc_decision_regions.is_empty()
}
}

/// Must match the layout of `LLVMRustCoverageCodeRegion`.
#[derive(Clone, Debug)]
#[repr(C)]
Expand Down
7 changes: 3 additions & 4 deletions compiler/rustc_codegen_llvm/src/coverageinfo/llvm_cov.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -62,11 +62,10 @@ pub(crate) fn write_filenames_to_buffer<'a>(
pub(crate) fn write_function_mappings_to_buffer(
virtual_file_mapping: &[u32],
expressions: &[ffi::CounterExpression],
code_regions: &[ffi::CodeRegion],
branch_regions: &[ffi::BranchRegion],
mcdc_branch_regions: &[ffi::MCDCBranchRegion],
mcdc_decision_regions: &[ffi::MCDCDecisionRegion],
regions: &ffi::Regions,
) -> Vec<u8> {
let ffi::Regions { code_regions, branch_regions, mcdc_branch_regions, mcdc_decision_regions } =
regions;
llvm::build_byte_buffer(|buffer| unsafe {
llvm::LLVMRustCoverageWriteFunctionMappingsToBuffer(
virtual_file_mapping.as_ptr(),
Expand Down
202 changes: 25 additions & 177 deletions compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,3 @@
use std::ffi::CString;
use std::iter;

use itertools::Itertools as _;
Expand All @@ -9,21 +8,22 @@ use rustc_codegen_ssa::traits::{
use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_index::IndexVec;
use rustc_middle::mir::coverage::MappingKind;
use rustc_middle::ty::{self, TyCtxt};
use rustc_middle::{bug, mir};
use rustc_session::RemapFileNameExt;
use rustc_session::config::RemapPathScopeComponents;
use rustc_span::def_id::DefIdSet;
use rustc_span::{Span, Symbol};
use rustc_target::spec::HasTargetSpec;
use tracing::debug;

use crate::common::CodegenCx;
use crate::coverageinfo::llvm_cov;
use crate::coverageinfo::map_data::FunctionCoverage;
use crate::coverageinfo::{ffi, llvm_cov};
use crate::coverageinfo::mapgen::covfun::prepare_covfun_record;
use crate::llvm;

mod covfun;

/// Generates and exports the coverage map, which is embedded in special
/// linker sections in the final binary.
///
Expand Down Expand Up @@ -80,47 +80,25 @@ pub(crate) fn finalize(cx: &CodegenCx<'_, '_>) {
let filenames_val = cx.const_bytes(&filenames_buffer);
let filenames_ref = llvm_cov::hash_bytes(&filenames_buffer);

// Generate the coverage map header, which contains the filenames used by
// this CGU's coverage mappings, and store it in a well-known global.
generate_covmap_record(cx, covmap_version, filenames_size, filenames_val);

let mut unused_function_names = Vec::new();

// Encode coverage mappings and generate function records
for (instance, function_coverage) in function_coverage_map {
debug!("Generate function coverage for {}, {:?}", cx.codegen_unit.name(), instance);

let mangled_function_name = tcx.symbol_name(instance).name;
let source_hash = function_coverage.source_hash();
let is_used = function_coverage.is_used();

let coverage_mapping_buffer =
encode_mappings_for_function(tcx, &global_file_table, &function_coverage);
let covfun_records = function_coverage_map
.into_iter()
.filter_map(|(instance, function_coverage)| {
prepare_covfun_record(tcx, &global_file_table, instance, &function_coverage)
})
.collect::<Vec<_>>();

if coverage_mapping_buffer.is_empty() {
if function_coverage.is_used() {
bug!(
"A used function should have had coverage mapping data but did not: {}",
mangled_function_name
);
} else {
debug!("unused function had no coverage mapping data: {}", mangled_function_name);
continue;
}
}
// If there are no covfun records for this CGU, don't generate a covmap record.
// This should prevent a repeat of <https://github.com/rust-lang/rust/issues/133606>.
if covfun_records.is_empty() {
return;
}

if !is_used {
unused_function_names.push(mangled_function_name);
}
for covfun in &covfun_records {
unused_function_names.extend(covfun.mangled_function_name_if_unused());

generate_covfun_record(
cx,
mangled_function_name,
source_hash,
filenames_ref,
coverage_mapping_buffer,
is_used,
);
covfun::generate_covfun_record(cx, filenames_ref, covfun)
}

// For unused functions, we need to take their mangled names and store them
Expand All @@ -141,6 +119,10 @@ pub(crate) fn finalize(cx: &CodegenCx<'_, '_>) {
llvm::set_linkage(array, llvm::Linkage::InternalLinkage);
llvm::set_initializer(array, initializer);
}

// Generate the coverage map header, which contains the filenames used by
// this CGU's coverage mappings, and store it in a well-known global.
generate_covmap_record(cx, covmap_version, filenames_size, filenames_val);
}

/// Maps "global" (per-CGU) file ID numbers to their underlying filenames.
Expand Down Expand Up @@ -208,7 +190,7 @@ rustc_index::newtype_index! {

/// Holds a mapping from "local" (per-function) file IDs to "global" (per-CGU)
/// file IDs.
#[derive(Default)]
#[derive(Debug, Default)]
struct VirtualFileMapping {
local_to_global: IndexVec<LocalFileId, GlobalFileId>,
global_to_local: FxIndexMap<GlobalFileId, LocalFileId>,
Expand All @@ -222,10 +204,8 @@ impl VirtualFileMapping {
.or_insert_with(|| self.local_to_global.push(global_file_id))
}

fn into_vec(self) -> Vec<u32> {
// This conversion should be optimized away to ~zero overhead.
// In any case, it's probably not hot enough to worry about.
self.local_to_global.into_iter().map(|global| global.as_u32()).collect()
fn to_vec(&self) -> Vec<u32> {
self.local_to_global.iter().map(|&global| GlobalFileId::as_u32(global)).collect()
}
}

Expand All @@ -236,83 +216,6 @@ fn span_file_name(tcx: TyCtxt<'_>, span: Span) -> Symbol {
Symbol::intern(&name)
}

/// Using the expressions and counter regions collected for a single function,
/// generate the variable-sized payload of its corresponding `__llvm_covfun`
/// entry. The payload is returned as a vector of bytes.
///
/// Newly-encountered filenames will be added to the global file table.
fn encode_mappings_for_function(
tcx: TyCtxt<'_>,
global_file_table: &GlobalFileTable,
function_coverage: &FunctionCoverage<'_>,
) -> Vec<u8> {
let counter_regions = function_coverage.counter_regions();
if counter_regions.is_empty() {
return Vec::new();
}

let expressions = function_coverage.counter_expressions().collect::<Vec<_>>();

let mut virtual_file_mapping = VirtualFileMapping::default();
let mut code_regions = vec![];
let mut branch_regions = vec![];
let mut mcdc_branch_regions = vec![];
let mut mcdc_decision_regions = vec![];

// Currently a function's mappings must all be in the same file as its body span.
let file_name = span_file_name(tcx, function_coverage.function_coverage_info.body_span);

// Look up the global file ID for that filename.
let global_file_id = global_file_table.global_file_id_for_file_name(file_name);

// Associate that global file ID with a local file ID for this function.
let local_file_id = virtual_file_mapping.local_id_for_global(global_file_id);
debug!(" file id: {local_file_id:?} => {global_file_id:?} = '{file_name:?}'");

// For each counter/region pair in this function+file, convert it to a
// form suitable for FFI.
for (mapping_kind, region) in counter_regions {
debug!("Adding counter {mapping_kind:?} to map for {region:?}");
let span = ffi::CoverageSpan::from_source_region(local_file_id, region);
match mapping_kind {
MappingKind::Code(term) => {
code_regions.push(ffi::CodeRegion { span, counter: ffi::Counter::from_term(term) });
}
MappingKind::Branch { true_term, false_term } => {
branch_regions.push(ffi::BranchRegion {
span,
true_counter: ffi::Counter::from_term(true_term),
false_counter: ffi::Counter::from_term(false_term),
});
}
MappingKind::MCDCBranch { true_term, false_term, mcdc_params } => {
mcdc_branch_regions.push(ffi::MCDCBranchRegion {
span,
true_counter: ffi::Counter::from_term(true_term),
false_counter: ffi::Counter::from_term(false_term),
mcdc_branch_params: ffi::mcdc::BranchParameters::from(mcdc_params),
});
}
MappingKind::MCDCDecision(mcdc_decision_params) => {
mcdc_decision_regions.push(ffi::MCDCDecisionRegion {
span,
mcdc_decision_params: ffi::mcdc::DecisionParameters::from(mcdc_decision_params),
});
}
}
}

// Encode the function's coverage mappings into a buffer.
llvm_cov::write_function_mappings_to_buffer(
&virtual_file_mapping.into_vec(),
&expressions,
&code_regions,
&branch_regions,
&mcdc_branch_regions,
&mcdc_decision_regions,
)
}

/// Generates the contents of the covmap record for this CGU, which mostly
/// consists of a header and a list of filenames. The record is then stored
/// as a global variable in the `__llvm_covmap` section.
Expand Down Expand Up @@ -350,61 +253,6 @@ fn generate_covmap_record<'ll>(
cx.add_used_global(llglobal);
}

/// Generates the contents of the covfun record for this function, which
/// contains the function's coverage mapping data. The record is then stored
/// as a global variable in the `__llvm_covfun` section.
fn generate_covfun_record(
cx: &CodegenCx<'_, '_>,
mangled_function_name: &str,
source_hash: u64,
filenames_ref: u64,
coverage_mapping_buffer: Vec<u8>,
is_used: bool,
) {
// Concatenate the encoded coverage mappings
let coverage_mapping_size = coverage_mapping_buffer.len();
let coverage_mapping_val = cx.const_bytes(&coverage_mapping_buffer);

let func_name_hash = llvm_cov::hash_bytes(mangled_function_name.as_bytes());
let func_name_hash_val = cx.const_u64(func_name_hash);
let coverage_mapping_size_val = cx.const_u32(coverage_mapping_size as u32);
let source_hash_val = cx.const_u64(source_hash);
let filenames_ref_val = cx.const_u64(filenames_ref);
let func_record_val = cx.const_struct(
&[
func_name_hash_val,
coverage_mapping_size_val,
source_hash_val,
filenames_ref_val,
coverage_mapping_val,
],
/*packed=*/ true,
);

// Choose a variable name to hold this function's covfun data.
// Functions that are used have a suffix ("u") to distinguish them from
// unused copies of the same function (from different CGUs), so that if a
// linker sees both it won't discard the used copy's data.
let func_record_var_name =
CString::new(format!("__covrec_{:X}{}", func_name_hash, if is_used { "u" } else { "" }))
.unwrap();
debug!("function record var name: {:?}", func_record_var_name);

let llglobal = llvm::add_global(cx.llmod, cx.val_ty(func_record_val), &func_record_var_name);
llvm::set_initializer(llglobal, func_record_val);
llvm::set_global_constant(llglobal, true);
llvm::set_linkage(llglobal, llvm::Linkage::LinkOnceODRLinkage);
llvm::set_visibility(llglobal, llvm::Visibility::Hidden);
llvm::set_section(llglobal, cx.covfun_section_name());
// LLVM's coverage mapping format specifies 8-byte alignment for items in this section.
// <https://llvm.org/docs/CoverageMappingFormat.html>
llvm::set_alignment(llglobal, Align::EIGHT);
if cx.target_spec().supports_comdat() {
llvm::set_comdat(cx.llmod, llglobal, &func_record_var_name);
}
cx.add_used_global(llglobal);
}

/// Each CGU will normally only emit coverage metadata for the functions that it actually generates.
/// But since we don't want unused functions to disappear from coverage reports, we also scan for
/// functions that were instrumented but are not participating in codegen.
Expand Down
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