Pass -pg to linker when using -Zinstrument-mcount

Clang and gcc use this option to control linking behavior too. Some
targets need to be linked against a special crt which enables
profiling at runtime.

This makes using gprof a little easier with Rust binaries. Otherwise,
rustc must be passed `-Clink-args=-pg` to ensure the correct startup
code is linked.

Author: pmur

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -2672,6 +2672,10 @@ fn add_order_independent_options(
         cmd.pgo_gen();
     }
 
+    if sess.opts.unstable_opts.instrument_mcount {
+        cmd.enable_profiling();
+    }
+
     if sess.opts.cg.control_flow_guard != CFGuard::Disabled {
         cmd.control_flow_guard();
     }

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -352,6 +352,7 @@ pub(crate) trait Linker {
     fn add_no_exec(&mut self) {}
     fn add_as_needed(&mut self) {}
     fn reset_per_library_state(&mut self) {}
+    fn enable_profiling(&mut self) {}
 }
 
 impl dyn Linker + '_ {
@@ -731,6 +732,12 @@ impl<'a> Linker for GccLinker<'a> {
         self.link_or_cc_args(&["-u", "__llvm_profile_runtime"]);
     }
 
+    fn enable_profiling(&mut self) {
+        // This flag is also used when linking to choose target specific
+        // libraries needed to enable profiling.
+        self.cc_arg("-pg");
+    }
+
     fn control_flow_guard(&mut self) {}
 
     fn ehcont_guard(&mut self) {}

tests/run-make/instrument-mcount-link-pg/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello World!");
+}

tests/run-make/instrument-mcount-link-pg/rmake.rs

@@ -0,0 +1,19 @@
+// When building a binary instrumented with mcount, verify the
+// binary is linked with the correct crt to enable profiling.
+//
+//@ only-gnu
+//@ ignore-cross-compile
+
+use run_make_support::{path, run, rustc};
+
+fn main() {
+    // Compile instrumentation enabled binary, and verify -pg is passed
+    let link_args =
+        rustc().input("main.rs").arg("-Zinstrument-mcount").print("link-args").run().stdout_utf8();
+    assert!(link_args.contains("\"-pg\""));
+
+    // Run it, and verify gmon.out is created
+    assert!(!path("gmon.out").exists());
+    run("main");
+    assert!(path("gmon.out").exists());
+}