Rollup merge of rust-lang#71283 - Amanieu:zprofile, r=davidtwco

Minor improvements to -Zprofile

- `-Zprofile` is broken with codegen units because GCOV assumes that each source file corresponds to one object file. This PR makes `-Zprofile` automatically set codegen units to 1 and gives an error if `-Ccodegen-units=X` is specified on the command line (with `X != 1`).
- The `profiler_builtins` crate is not suitable for `no_std` applications since it contains C code that depends on libc. In such cases a custom implementation of the LLVM gcov API (`llvm_gcov_init`, `llvm_gcda_*`) is needed. This PR adds `-Zno-profiler-runtime` flag which inhibits automatic injection of the `profiler_builtins` crate.

cc @whitequark who implemented the original `-Zprofile` support

Author: Dylan-DPC

src/librustc_metadata/creader.rs

@@ -686,7 +686,9 @@ impl<'a> CrateLoader<'a> {
     }
 
     fn inject_profiler_runtime(&mut self) {
-        if self.sess.opts.debugging_opts.profile || self.sess.opts.cg.profile_generate.enabled() {
+        if (self.sess.opts.debugging_opts.profile || self.sess.opts.cg.profile_generate.enabled())
+            && !self.sess.opts.debugging_opts.no_profiler_runtime
+        {
             info!("loading profiler");
 
             let name = Symbol::intern("profiler_builtins");

src/librustc_session/config.rs

@@ -1655,7 +1655,7 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
     let output_types = parse_output_types(&debugging_opts, matches, error_format);
 
     let mut cg = build_codegen_options(matches, error_format);
-    let (disable_thinlto, codegen_units) = should_override_cgus_and_disable_thinlto(
+    let (disable_thinlto, mut codegen_units) = should_override_cgus_and_disable_thinlto(
         &output_types,
         matches,
         error_format,
@@ -1672,6 +1672,16 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
             "can't instrument with gcov profiling when compiling incrementally",
         );
     }
+    if debugging_opts.profile {
+        match codegen_units {
+            Some(1) => {}
+            None => codegen_units = Some(1),
+            Some(_) => early_error(
+                error_format,
+                "can't instrument with gcov profiling with multiple codegen units",
+            ),
+        }
+    }
 
     if cg.profile_generate.enabled() && cg.profile_use.is_some() {
         early_error(

src/librustc_session/options.rs

@@ -890,6 +890,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "extra arguments to prepend to the linker invocation (space separated)"),
     profile: bool = (false, parse_bool, [TRACKED],
                      "insert profiling code"),
+    no_profiler_runtime: bool = (false, parse_bool, [TRACKED],
+        "don't automatically inject the profiler_builtins crate"),
     relro_level: Option<RelroLevel> = (None, parse_relro_level, [TRACKED],
         "choose which RELRO level to use"),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],