coverage: Emit the filenames section before encoding per-function map…

…pings

Most coverage metadata is encoded into two sections in the final executable.
The `__llvm_covmap` section mostly just contains a list of filenames, while the
`__llvm_covfun` section contains encoded coverage maps for each instrumented
function.

The catch is that each per-function record also needs to contain a hash of the
filenames list that it refers to. Historically this was handled by assembling
most of the per-function data into a temporary list, then assembling the
filenames buffer, then using the filenames hash to emit the per-function data,
and then finally emitting the filenames table itself.

However, now that we build the filenames table up-front (via a separate
traversal of the per-function data), we can hash and emit that part first, and
then emit each of the per-function records immediately after building. This
removes the awkwardness of having to temporarily store nearly-complete
per-function records.

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -67,8 +67,20 @@ pub fn finalize(cx: &CodegenCx<'_, '_>) {
  function_coverage_entries.iter().flat_map(|(_, fn_cov)| fn_cov.all_file_names());
  let global_file_table = GlobalFileTable::new(tcx, all_file_names);
 
+ // Encode all filenames referenced by counters/expressions in this module
+ let filenames_buffer = global_file_table.filenames_buffer();
+ let filenames_size = filenames_buffer.len();
+ let filenames_val = cx.const_bytes(filenames_buffer);
+ let filenames_ref = coverageinfo::hash_bytes(filenames_buffer);
+
+ // Generate the LLVM IR representation of the coverage map and store it in a well-known global
+ let cov_data_val = generate_coverage_map(cx, version, filenames_size, filenames_val);
+ coverageinfo::save_cov_data_to_mod(cx, cov_data_val);
+
+ let mut unused_function_names = Vec::new();
+ let covfun_section_name = coverageinfo::covfun_section_name(cx);
+
  // Encode coverage mappings and generate function records
- let mut function_data = Vec::new();
  for (instance, function_coverage) in function_coverage_entries {
  debug!("Generate function coverage for {}, {:?}", cx.codegen_unit.name(), instance);
 
@@ -91,23 +103,6 @@ pub fn finalize(cx: &CodegenCx<'_, '_>) {
  }
  }
 
- function_data.push((mangled_function_name, source_hash, is_used, coverage_mapping_buffer));
- }
-
- // Encode all filenames referenced by counters/expressions in this module
- let filenames_buffer = global_file_table.filenames_buffer();
-
- let filenames_size = filenames_buffer.len();
- let filenames_val = cx.const_bytes(filenames_buffer);
- let filenames_ref = coverageinfo::hash_bytes(filenames_buffer);
-
- // Generate the LLVM IR representation of the coverage map and store it in a well-known global
- let cov_data_val = generate_coverage_map(cx, version, filenames_size, filenames_val);
-
- let mut unused_function_names = Vec::new();
-
- let covfun_section_name = coverageinfo::covfun_section_name(cx);
- for (mangled_function_name, source_hash, is_used, coverage_mapping_buffer) in function_data {
  if !is_used {
  unused_function_names.push(mangled_function_name);
  }
@@ -141,9 +136,6 @@ pub fn finalize(cx: &CodegenCx<'_, '_>) {
  llvm::set_linkage(array, llvm::Linkage::InternalLinkage);
  llvm::set_initializer(array, initializer);
  }
-
- // Save the coverage data value to LLVM IR
- coverageinfo::save_cov_data_to_mod(cx, cov_data_val);
 }
 
 /// Maps "global" (per-CGU) file ID numbers to their underlying filenames.