Auto merge of #116922 - Zalathar:unused, r=cjgillot

coverage: Emit mappings for unused functions without generating stubs

For a while I've been annoyed by the fact that generating coverage maps for unused functions involves generating a stub function at the LLVM level.

As I suspected, generating that stub function isn't actually necessary, as long as we specifically tell LLVM about the symbol names of all the functions that have coverage mappings but weren't codegenned (due to being unused).

---

There is some helper code that gets moved around in the follow-up patches, so look at the first patch to see the most important functional changes.

---

`@rustbot` label +A-code-coverage

1 files changed, 2 insertions, 99 deletions

diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
index 204a73b788a..cf7c7e6be60 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
@@ -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};
@@ -12,17 +11,12 @@ 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, FunctionCoverageInfo};
+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;
 
@@ -30,8 +24,6 @@ pub(crate) mod ffi;
 pub(crate) mod map_data;
 pub mod mapgen;
 
-const UNUSED_FUNCTION_COUNTER_ID: CounterId = CounterId::START;
-
 const VAR_ALIGN_BYTES: usize = 8;
 
 /// A context object for maintaining all state needed by the coverageinfo module.
@@ -76,25 +68,6 @@ 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 function coverage info from MIR and add an
-    /// "unused" entry to the function coverage map.
-    fn define_unused_fn(&self, def_id: DefId, function_coverage_info: &'tcx FunctionCoverageInfo) {
-        let instance = declare_unused_fn(self, def_id);
-        codegen_unused_fn_and_counter(self, instance);
-        add_unused_function_coverage(self, instance, function_coverage_info);
-    }
 }
 
 impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
@@ -159,76 +132,6 @@ 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>,
-    function_coverage_info: &'tcx FunctionCoverageInfo,
-) {
-    // An unused function's mappings will automatically be rewritten to map to
-    // zero, because none of its counters/expressions are marked as seen.
-    let function_coverage = FunctionCoverage::unused(instance, function_coverage_info);
-
-    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