1 files changed, 5 insertions, 1 deletions
diff --git a/compiler/rustc_codegen_llvm/src/back/write.rs b/compiler/rustc_codegen_llvm/src/back/write.rs
index c45d637177e..085935b94df 100644
--- a/compiler/rustc_codegen_llvm/src/back/write.rs
+++ b/compiler/rustc_codegen_llvm/src/back/write.rs
@@ -549,7 +549,11 @@ pub(crate) unsafe fn optimize(
                     continue;
                 }
                 if pass_name == "insert-gcov-profiling" || pass_name == "instrprof" {
-                    // Instrumentation should be inserted before optimization.
+                    // Instrumentation must be inserted before optimization,
+                    // otherwise LLVM may optimize some functions away which
+                    // breaks llvm-cov.
+                    //
+                    // This mirrors what Clang does in lib/CodeGen/BackendUtil.cpp.
                     llvm::LLVMRustAddPass(mpm, find_pass(pass_name).unwrap());
                     continue;
                 }