coverage: Tidy up creation of covfun records

Author: Zalathar

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -75,7 +75,10 @@ pub(crate) fn finalize(cx: &CodegenCx<'_, '_>) {
 
     // Encode all filenames referenced by coverage mappings in this CGU.
     let filenames_buffer = global_file_table.make_filenames_buffer(tcx);
-    let filenames_ref = llvm_cov::hash_bytes(&filenames_buffer);
+    // The `llvm-cov` tool uses this hash to associate each covfun record with
+    // its corresponding filenames table, since the final binary will typically
+    // contain multiple covmap records from different compilation units.
+    let filenames_hash = llvm_cov::hash_bytes(&filenames_buffer);
 
     let mut unused_function_names = Vec::new();
 
@@ -98,7 +101,7 @@ pub(crate) fn finalize(cx: &CodegenCx<'_, '_>) {
     for covfun in &covfun_records {
         unused_function_names.extend(covfun.mangled_function_name_if_unused());
 
-        covfun::generate_covfun_record(cx, filenames_ref, covfun)
+        covfun::generate_covfun_record(cx, filenames_hash, covfun)
     }
 
     // For unused functions, we need to take their mangled names and store them

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen/covfun.rs

@@ -136,7 +136,7 @@ fn fill_region_tables<'tcx>(
 /// as a global variable in the `__llvm_covfun` section.
 pub(crate) fn generate_covfun_record<'tcx>(
     cx: &CodegenCx<'_, 'tcx>,
-    filenames_ref: u64,
+    filenames_hash: u64,
     covfun: &CovfunRecord<'tcx>,
 ) {
     let &CovfunRecord {
@@ -155,46 +155,45 @@ pub(crate) fn generate_covfun_record<'tcx>(
         regions,
     );
 
-    // Concatenate the encoded coverage mappings
-    let coverage_mapping_size = coverage_mapping_buffer.len();
-    let coverage_mapping_val = cx.const_bytes(&coverage_mapping_buffer);
-
+    // A covfun record consists of four target-endian integers, followed by the
+    // encoded mapping data in bytes. Note that the length field is 32 bits.
+    // <https://llvm.org/docs/CoverageMappingFormat.html#llvm-ir-representation>
+    // See also `src/llvm-project/clang/lib/CodeGen/CoverageMappingGen.cpp` and
+    // `COVMAP_V3` in `src/llvm-project/llvm/include/llvm/ProfileData/InstrProfData.inc`.
     let func_name_hash = llvm_cov::hash_bytes(mangled_function_name.as_bytes());
-    let func_name_hash_val = cx.const_u64(func_name_hash);
-    let coverage_mapping_size_val = cx.const_u32(coverage_mapping_size as u32);
-    let source_hash_val = cx.const_u64(source_hash);
-    let filenames_ref_val = cx.const_u64(filenames_ref);
-    let func_record_val = cx.const_struct(
+    let covfun_record = cx.const_struct(
         &[
-            func_name_hash_val,
-            coverage_mapping_size_val,
-            source_hash_val,
-            filenames_ref_val,
-            coverage_mapping_val,
+            cx.const_u64(func_name_hash),
+            cx.const_u32(coverage_mapping_buffer.len() as u32),
+            cx.const_u64(source_hash),
+            cx.const_u64(filenames_hash),
+            cx.const_bytes(&coverage_mapping_buffer),
         ],
-        /*packed=*/ true,
+        // This struct needs to be packed, so that the 32-bit length field
+        // doesn't have unexpected padding.
+        true,
     );
 
     // Choose a variable name to hold this function's covfun data.
     // Functions that are used have a suffix ("u") to distinguish them from
     // unused copies of the same function (from different CGUs), so that if a
     // linker sees both it won't discard the used copy's data.
-    let func_record_var_name =
-        CString::new(format!("__covrec_{:X}{}", func_name_hash, if is_used { "u" } else { "" }))
-            .unwrap();
-    debug!("function record var name: {:?}", func_record_var_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, cx.covfun_section_name());
+    let u = if is_used { "u" } else { "" };
+    let covfun_var_name = CString::new(format!("__covrec_{func_name_hash:X}{u}")).unwrap();
+    debug!("function record var name: {covfun_var_name:?}");
+
+    let covfun_global = llvm::add_global(cx.llmod, cx.val_ty(covfun_record), &covfun_var_name);
+    llvm::set_initializer(covfun_global, covfun_record);
+    llvm::set_global_constant(covfun_global, true);
+    llvm::set_linkage(covfun_global, llvm::Linkage::LinkOnceODRLinkage);
+    llvm::set_visibility(covfun_global, llvm::Visibility::Hidden);
+    llvm::set_section(covfun_global, cx.covfun_section_name());
     // LLVM's coverage mapping format specifies 8-byte alignment for items in this section.
     // <https://llvm.org/docs/CoverageMappingFormat.html>
-    llvm::set_alignment(llglobal, Align::EIGHT);
+    llvm::set_alignment(covfun_global, Align::EIGHT);
     if cx.target_spec().supports_comdat() {
-        llvm::set_comdat(cx.llmod, llglobal, &func_record_var_name);
+        llvm::set_comdat(cx.llmod, covfun_global, &covfun_var_name);
     }
-    cx.add_used_global(llglobal);
+
+    cx.add_used_global(covfun_global);
 }