7 files changed, 57 insertions, 15 deletions
diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index b11821b7db0..4d6bc838130 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -1746,7 +1746,7 @@ fn linker_with_args<'a, B: ArchiveBuilder<'a>>(
     );
 
     // OBJECT-FILES-NO, AUDIT-ORDER
-    if sess.opts.cg.profile_generate.enabled() || sess.opts.debugging_opts.instrument_coverage {
+    if sess.opts.cg.profile_generate.enabled() || sess.instrument_coverage() {
         cmd.pgo_gen();
     }
 
diff --git a/compiler/rustc_codegen_ssa/src/back/symbol_export.rs b/compiler/rustc_codegen_ssa/src/back/symbol_export.rs
index 9a6f8cde1b2..abad8281d3a 100644
--- a/compiler/rustc_codegen_ssa/src/back/symbol_export.rs
+++ b/compiler/rustc_codegen_ssa/src/back/symbol_export.rs
@@ -188,9 +188,7 @@ fn exported_symbols_provider_local(
         }
     }
 
-    if tcx.sess.opts.debugging_opts.instrument_coverage
-        || tcx.sess.opts.cg.profile_generate.enabled()
-    {
+    if tcx.sess.instrument_coverage() || tcx.sess.opts.cg.profile_generate.enabled() {
         // These are weak symbols that point to the profile version and the
         // profile name, which need to be treated as exported so LTO doesn't nix
         // them.
diff --git a/compiler/rustc_codegen_ssa/src/back/write.rs b/compiler/rustc_codegen_ssa/src/back/write.rs
index 490b3d33112..7a8d8fb1304 100644
--- a/compiler/rustc_codegen_ssa/src/back/write.rs
+++ b/compiler/rustc_codegen_ssa/src/back/write.rs
@@ -176,7 +176,7 @@ impl ModuleConfig {
 
                     // The rustc option `-Zinstrument_coverage` injects intrinsic calls to
                     // `llvm.instrprof.increment()`, which requires the LLVM `instrprof` pass.
-                    if sess.opts.debugging_opts.instrument_coverage {
+                    if sess.instrument_coverage() {
                         passes.push("instrprof".to_owned());
                     }
                     passes
diff --git a/compiler/rustc_codegen_ssa/src/coverageinfo/map.rs b/compiler/rustc_codegen_ssa/src/coverageinfo/map.rs
index af6482fdbc2..7a17bced1c0 100644
--- a/compiler/rustc_codegen_ssa/src/coverageinfo/map.rs
+++ b/compiler/rustc_codegen_ssa/src/coverageinfo/map.rs
@@ -31,27 +31,44 @@ pub struct Expression {
 pub struct FunctionCoverage<'tcx> {
     instance: Instance<'tcx>,
     source_hash: u64,
+    is_used: bool,
     counters: IndexVec<CounterValueReference, Option<CodeRegion>>,
     expressions: IndexVec<InjectedExpressionIndex, Option<Expression>>,
     unreachable_regions: Vec<CodeRegion>,
 }
 
 impl<'tcx> FunctionCoverage<'tcx> {
+    /// Creates a new set of coverage data for a used (called) function.
     pub fn new(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> Self {
+        Self::create(tcx, instance, true)
+    }
+
+    /// Creates a new set of coverage data for an unused (never called) function.
+    pub fn unused(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> Self {
+        Self::create(tcx, instance, false)
+    }
+
+    fn create(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>, is_used: bool) -> Self {
         let coverageinfo = tcx.coverageinfo(instance.def_id());
         debug!(
-            "FunctionCoverage::new(instance={:?}) has coverageinfo={:?}",
-            instance, coverageinfo
+            "FunctionCoverage::new(instance={:?}) has coverageinfo={:?}. is_used={}",
+            instance, coverageinfo, is_used
         );
         Self {
             instance,
             source_hash: 0, // will be set with the first `add_counter()`
+            is_used,
             counters: IndexVec::from_elem_n(None, coverageinfo.num_counters as usize),
             expressions: IndexVec::from_elem_n(None, coverageinfo.num_expressions as usize),
             unreachable_regions: Vec::new(),
         }
     }
 
+    /// Returns true for a used (called) function, and false for an unused function.
+    pub fn is_used(&self) -> bool {
+        self.is_used
+    }
+
     /// Sets the function source hash value. If called multiple times for the same function, all
     /// calls should have the same hash value.
     pub fn set_function_source_hash(&mut self, source_hash: u64) {
@@ -128,8 +145,8 @@ impl<'tcx> FunctionCoverage<'tcx> {
         &'a self,
     ) -> (Vec<CounterExpression>, impl Iterator<Item = (Counter, &'a CodeRegion)>) {
         assert!(
-            self.source_hash != 0,
-            "No counters provided the source_hash for function: {:?}",
+            self.source_hash != 0 || !self.is_used,
+            "No counters provided the source_hash for used function: {:?}",
             self.instance
         );
 
diff --git a/compiler/rustc_codegen_ssa/src/mir/coverageinfo.rs b/compiler/rustc_codegen_ssa/src/mir/coverageinfo.rs
index 5ab1baafb57..e2f75b2e337 100644
--- a/compiler/rustc_codegen_ssa/src/mir/coverageinfo.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/coverageinfo.rs
@@ -33,7 +33,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
                     let coverageinfo = bx.tcx().coverageinfo(instance.def_id());
 
-                    let fn_name = bx.create_pgo_func_name_var(instance);
+                    let fn_name = bx.get_pgo_func_name_var(instance);
                     let hash = bx.const_u64(function_source_hash);
                     let num_counters = bx.const_u32(coverageinfo.num_counters);
                     let index = bx.const_u32(u32::from(id));
diff --git a/compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs b/compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs
index 95bddfb4b41..c7157528396 100644
--- a/compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs
@@ -1,14 +1,41 @@
 use super::BackendTypes;
+use rustc_hir::def_id::DefId;
 use rustc_middle::mir::coverage::*;
 use rustc_middle::ty::Instance;
 
-pub trait CoverageInfoMethods: BackendTypes {
+pub trait CoverageInfoMethods<'tcx>: BackendTypes {
     fn coverageinfo_finalize(&self);
-}
 
-pub trait CoverageInfoBuilderMethods<'tcx>: BackendTypes {
+    /// 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);
+
+    /// For LLVM codegen, returns a function-specific `Value` for a global
+    /// string, to hold the function name passed to LLVM intrinsic
+    /// `instrprof.increment()`. The `Value` is only created once per instance.
+    /// Multiple invocations with the same instance return the same `Value`.
+    fn get_pgo_func_name_var(&self, instance: Instance<'tcx>) -> Self::Value;
+
+    /// Creates a new PGO function name variable. This should only be called
+    /// to fill in the unused function names array.
     fn create_pgo_func_name_var(&self, instance: Instance<'tcx>) -> Self::Value;
+}
 
+pub trait CoverageInfoBuilderMethods<'tcx>: BackendTypes {
     /// Returns true if the function source hash was added to the coverage map (even if it had
     /// already been added, for this instance). Returns false *only* if `-Z instrument-coverage` is
     /// not enabled (a coverage map is not being generated).
diff --git a/compiler/rustc_codegen_ssa/src/traits/mod.rs b/compiler/rustc_codegen_ssa/src/traits/mod.rs
index 8ada6c10479..be2e0ea230f 100644
--- a/compiler/rustc_codegen_ssa/src/traits/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/mod.rs
@@ -58,7 +58,7 @@ pub trait CodegenMethods<'tcx>:
     + MiscMethods<'tcx>
     + ConstMethods<'tcx>
     + StaticMethods
-    + CoverageInfoMethods
+    + CoverageInfoMethods<'tcx>
     + DebugInfoMethods<'tcx>
     + AsmMethods
     + PreDefineMethods<'tcx>
@@ -74,7 +74,7 @@ impl<'tcx, T> CodegenMethods<'tcx> for T where
         + MiscMethods<'tcx>
         + ConstMethods<'tcx>
         + StaticMethods
-        + CoverageInfoMethods
+        + CoverageInfoMethods<'tcx>
         + DebugInfoMethods<'tcx>
         + AsmMethods
         + PreDefineMethods<'tcx>