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coverage: Move unused-function synthesis into its own submodule #115505

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coverage: Move unused-function synthesis into its own submodule #115505

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e7e6782
coverage: Move unused-function synthesis into its own submodule
Zalathar Sep 3, 2023
c035b65
coverage: Rename `add_unused_functions` to `synthesize_unused_functions`
Zalathar Sep 3, 2023
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71 changes: 2 additions & 69 deletions compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,15 +2,13 @@ use crate::common::CodegenCx;
use crate::coverageinfo;
use crate::coverageinfo::ffi::CounterMappingRegion;
use crate::coverageinfo::map_data::FunctionCoverage;
use crate::coverageinfo::unused;
use crate::llvm;

use rustc_codegen_ssa::traits::ConstMethods;
use rustc_data_structures::fx::FxIndexSet;
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_span::Symbol;
Expand Down Expand Up @@ -43,7 +41,7 @@ pub fn finalize(cx: &CodegenCx<'_, '_>) {
// MIR `Coverage` code regions to the `function_coverage_map`, before calling
// `ctx.take_function_coverage_map()`.
if cx.codegen_unit.is_code_coverage_dead_code_cgu() {
add_unused_functions(cx);
unused::synthesize_unused_functions(cx);
}

let function_coverage_map = match cx.coverage_context() {
Expand Down Expand Up @@ -276,68 +274,3 @@ fn save_function_record(
is_used,
);
}

/// 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, but were not
/// codegenned. (Note that `-Clink-dead-code` can force some unused code to be codegenned, but
/// that flag is known to cause other errors, when combined with `-C instrument-coverage`; and
/// `-Clink-dead-code` will not generate code for unused generic functions.)
///
/// We can find the unused functions (including generic functions) by the set difference of all MIR
/// `DefId`s (`tcx` query `mir_keys`) minus the codegenned `DefId`s (`tcx` query
/// `codegened_and_inlined_items`).
///
/// These unused functions are then codegen'd in one of the CGUs which is marked as the
/// "code coverage dead code cgu" during the partitioning process. This prevents us from generating
/// code regions for the same function more than once which can lead to linker errors regarding
/// duplicate symbols.
fn add_unused_functions(cx: &CodegenCx<'_, '_>) {
assert!(cx.codegen_unit.is_code_coverage_dead_code_cgu());

let tcx = cx.tcx;

let ignore_unused_generics = tcx.sess.instrument_coverage_except_unused_generics();

let eligible_def_ids: Vec<DefId> = tcx
.mir_keys(())
.iter()
.filter_map(|local_def_id| {
let def_id = local_def_id.to_def_id();
let kind = tcx.def_kind(def_id);
// `mir_keys` will give us `DefId`s for all kinds of things, not
// just "functions", like consts, statics, etc. Filter those out.
// If `ignore_unused_generics` was specified, filter out any
// generic functions from consideration as well.
if !matches!(
kind,
DefKind::Fn | DefKind::AssocFn | DefKind::Closure | DefKind::Generator
) {
return None;
}
if ignore_unused_generics && tcx.generics_of(def_id).requires_monomorphization(tcx) {
return None;
}
Some(local_def_id.to_def_id())
})
.collect();

let codegenned_def_ids = tcx.codegened_and_inlined_items(());

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 `#[no_coverage]`, then skip generating a
// dead code stub for it.
if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NO_COVERAGE) {
debug!("skipping unused fn marked #[no_coverage]: {:?}", non_codegenned_def_id);
continue;
}

debug!("generating unused fn: {:?}", non_codegenned_def_id);
cx.define_unused_fn(non_codegenned_def_id);
}
}
114 changes: 3 additions & 111 deletions compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,5 @@
use crate::llvm;

use crate::abi::Abi;
use crate::builder::Builder;
use crate::common::CodegenCx;
use crate::coverageinfo::ffi::{CounterExpression, CounterMappingRegion};
Expand All @@ -12,25 +11,19 @@ use rustc_codegen_ssa::traits::{
StaticMethods,
};
use rustc_data_structures::fx::FxHashMap;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_llvm::RustString;
use rustc_middle::bug;
use rustc_middle::mir::coverage::{CounterId, CoverageKind};
use rustc_middle::mir::coverage::CoverageKind;
use rustc_middle::mir::Coverage;
use rustc_middle::ty;
use rustc_middle::ty::layout::{FnAbiOf, HasTyCtxt};
use rustc_middle::ty::GenericArgs;
use rustc_middle::ty::layout::HasTyCtxt;
use rustc_middle::ty::Instance;
use rustc_middle::ty::Ty;

use std::cell::RefCell;

pub(crate) mod ffi;
pub(crate) mod map_data;
pub mod mapgen;

const UNUSED_FUNCTION_COUNTER_ID: CounterId = CounterId::START;
mod unused;

const VAR_ALIGN_BYTES: usize = 8;

Expand Down Expand Up @@ -76,28 +69,6 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
bug!("Could not get the `coverage_context`");
}
}

/// Functions with MIR-based coverage are normally codegenned _only_ if
/// called. LLVM coverage tools typically expect every function to be
/// defined (even if unused), with at least one call to LLVM intrinsic
/// `instrprof.increment`.
///
/// Codegen a small function that will never be called, with one counter
/// that will never be incremented.
///
/// For used/called functions, the coverageinfo was already added to the
/// `function_coverage_map` (keyed by function `Instance`) during codegen.
/// But in this case, since the unused function was _not_ previously
/// codegenned, collect the coverage `CodeRegion`s from the MIR and add
/// them. The first `CodeRegion` is used to add a single counter, with the
/// same counter ID used in the injected `instrprof.increment` intrinsic
/// call. Since the function is never called, all other `CodeRegion`s can be
/// added as `unreachable_region`s.
fn define_unused_fn(&self, def_id: DefId) {
let instance = declare_unused_fn(self, def_id);
codegen_unused_fn_and_counter(self, instance);
add_unused_function_coverage(self, instance, def_id);
}
}

impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
Expand Down Expand Up @@ -165,85 +136,6 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
}
}

fn declare_unused_fn<'tcx>(cx: &CodegenCx<'_, 'tcx>, def_id: DefId) -> Instance<'tcx> {
let tcx = cx.tcx;

let instance = Instance::new(
def_id,
GenericArgs::for_item(tcx, def_id, |param, _| {
if let ty::GenericParamDefKind::Lifetime = param.kind {
tcx.lifetimes.re_erased.into()
} else {
tcx.mk_param_from_def(param)
}
}),
);

let llfn = cx.declare_fn(
tcx.symbol_name(instance).name,
cx.fn_abi_of_fn_ptr(
ty::Binder::dummy(tcx.mk_fn_sig(
[Ty::new_unit(tcx)],
Ty::new_unit(tcx),
false,
hir::Unsafety::Unsafe,
Abi::Rust,
)),
ty::List::empty(),
),
None,
);

llvm::set_linkage(llfn, llvm::Linkage::PrivateLinkage);
llvm::set_visibility(llfn, llvm::Visibility::Default);

assert!(cx.instances.borrow_mut().insert(instance, llfn).is_none());

instance
}

fn codegen_unused_fn_and_counter<'tcx>(cx: &CodegenCx<'_, 'tcx>, instance: Instance<'tcx>) {
let llfn = cx.get_fn(instance);
let llbb = Builder::append_block(cx, llfn, "unused_function");
let mut bx = Builder::build(cx, llbb);
let fn_name = bx.get_pgo_func_name_var(instance);
let hash = bx.const_u64(0);
let num_counters = bx.const_u32(1);
let index = bx.const_u32(u32::from(UNUSED_FUNCTION_COUNTER_ID));
debug!(
"codegen intrinsic instrprof.increment(fn_name={:?}, hash={:?}, num_counters={:?},
index={:?}) for unused function: {:?}",
fn_name, hash, num_counters, index, instance
);
bx.instrprof_increment(fn_name, hash, num_counters, index);
bx.ret_void();
}

fn add_unused_function_coverage<'tcx>(
cx: &CodegenCx<'_, 'tcx>,
instance: Instance<'tcx>,
def_id: DefId,
) {
let tcx = cx.tcx;

let mut function_coverage = FunctionCoverage::unused(tcx, instance);
for (index, &code_region) in tcx.covered_code_regions(def_id).iter().enumerate() {
if index == 0 {
// Insert at least one real counter so the LLVM CoverageMappingReader will find expected
// definitions.
function_coverage.add_counter(UNUSED_FUNCTION_COUNTER_ID, code_region.clone());
} else {
function_coverage.add_unreachable_region(code_region.clone());
}
}

if let Some(coverage_context) = cx.coverage_context() {
coverage_context.function_coverage_map.borrow_mut().insert(instance, function_coverage);
} else {
bug!("Could not get the `coverage_context`");
}
}

/// Calls llvm::createPGOFuncNameVar() with the given function instance's
/// mangled function name. The LLVM API returns an llvm::GlobalVariable
/// containing the function name, with the specific variable name and linkage
Expand Down
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