coverage: Obtain the __llvm_covfun section name outside a per-function loop

This section name is always constant for a given target, but obtaining it from
LLVM requires a few intermediate allocations. There's no need to do so
repeatedly from inside a per-function loop.

Author: Zalathar

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -100,9 +100,11 @@ pub fn finalize(cx: &CodegenCx<'_, '_>) {
     // Generate the LLVM IR representation of the coverage map and store it in a well-known global
     let cov_data_val = mapgen.generate_coverage_map(cx, version, filenames_size, filenames_val);
 
+    let covfun_section_name = coverageinfo::covfun_section_name(cx);
     for (mangled_function_name, source_hash, is_used, coverage_mapping_buffer) in function_data {
         save_function_record(
             cx,
+            &covfun_section_name,
             mangled_function_name,
             source_hash,
             filenames_ref,
@@ -228,6 +230,7 @@ impl CoverageMapGenerator {
 /// specific, well-known section and name.
 fn save_function_record(
     cx: &CodegenCx<'_, '_>,
+    covfun_section_name: &str,
     mangled_function_name: &str,
     source_hash: u64,
     filenames_ref: u64,
@@ -254,7 +257,13 @@ fn save_function_record(
         /*packed=*/ true,
     );
 
-    coverageinfo::save_func_record_to_mod(cx, func_name_hash, func_record_val, is_used);
+    coverageinfo::save_func_record_to_mod(
+        cx,
+        covfun_section_name,
+        func_name_hash,
+        func_record_val,
+        is_used,
+    );
 }
 
 /// When finalizing the coverage map, `FunctionCoverage` only has the `CodeRegion`s and counters for

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -408,6 +408,7 @@ pub(crate) fn save_cov_data_to_mod<'ll, 'tcx>(
 
 pub(crate) fn save_func_record_to_mod<'ll, 'tcx>(
     cx: &CodegenCx<'ll, 'tcx>,
+    covfun_section_name: &str,
     func_name_hash: u64,
     func_record_val: &'ll llvm::Value,
     is_used: bool,
@@ -423,20 +424,33 @@ pub(crate) fn save_func_record_to_mod<'ll, 'tcx>(
     let func_record_var_name =
         format!("__covrec_{:X}{}", func_name_hash, if is_used { "u" } else { "" });
     debug!("function record var name: {:?}", func_record_var_name);
-
-    let func_record_section_name = llvm::build_string(|s| unsafe {
-        llvm::LLVMRustCoverageWriteFuncSectionNameToString(cx.llmod, s);
-    })
-    .expect("Rust Coverage function record section name failed UTF-8 conversion");
-    debug!("function record section name: {:?}", func_record_section_name);
+    debug!("function record section name: {:?}", covfun_section_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, &func_record_section_name);
+    llvm::set_section(llglobal, covfun_section_name);
     llvm::set_alignment(llglobal, VAR_ALIGN_BYTES);
     llvm::set_comdat(cx.llmod, llglobal, &func_record_var_name);
     cx.add_used_global(llglobal);
 }
+
+/// Returns the section name string to pass through to the linker when embedding
+/// per-function coverage information in the object file, according to the target
+/// platform's object file format.
+///
+/// LLVM's coverage tools read coverage mapping details from this section when
+/// producing coverage reports.
+///
+/// Typical values are:
+/// - `__llvm_covfun` on Linux
+/// - `__LLVM_COV,__llvm_covfun` on macOS (includes `__LLVM_COV,` segment prefix)
+/// - `.lcovfun$M` on Windows (includes `$M` sorting suffix)
+pub(crate) fn covfun_section_name(cx: &CodegenCx<'_, '_>) -> String {
+    llvm::build_string(|s| unsafe {
+        llvm::LLVMRustCoverageWriteFuncSectionNameToString(cx.llmod, s);
+    })
+    .expect("Rust Coverage function record section name failed UTF-8 conversion")
+}