Use a BufWriter in emit_module

For the coercions rustc-perf benchmark without this writing the object
file takes 265ms while codegen only takes 40ms. The huge amount of time
to write the object file is likely because of syscall overhead.

Author: bjorn3

src/driver/aot.rs

@@ -2,6 +2,7 @@
 //! standalone executable.
 
 use std::fs::{self, File};
+use std::io::BufWriter;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 use std::thread::JoinHandle;
@@ -397,14 +398,19 @@ fn emit_module(
     }
 
     let tmp_file = output_filenames.temp_path(OutputType::Object, Some(&name));
-    let mut file = match File::create(&tmp_file) {
+    let file = match File::create(&tmp_file) {
         Ok(file) => file,
         Err(err) => return Err(format!("error creating object file: {}", err)),
     };
 
+    let mut file = BufWriter::new(file);
     if let Err(err) = object.write_stream(&mut file) {
         return Err(format!("error writing object file: {}", err));
     }
+    let file = match file.into_inner() {
+        Ok(file) => file,
+        Err(err) => return Err(format!("error writing object file: {}", err)),
+    };
 
     prof.artifact_size("object_file", &*name, file.metadata().unwrap().len());