Enable AutoFDO.

This largely involves implementing the options debug-info-for-profiling
and profile-sample-use and forwarding them on to LLVM.

AutoFDO can be used on x86-64 Linux like this:
rustc -O -Cdebug-info-for-profiling main.rs -o main
perf record -b ./main
create_llvm_prof --binary=main --out=code.prof
rustc -O -Cprofile-sample-use=code.prof main.rs -o main2

Now `main2` will have feedback directed optimization applied to it.

The create_llvm_prof tool can be obtained from this github repository:
https://github.com/google/autofdo

Fixes #64892.

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -263,6 +263,10 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
         attributes::emit_uwtable(llfn, true);
     }
 
+    if cx.sess().opts.debugging_opts.profile_sample_use.is_some() {
+        llvm::AddFunctionAttrString(llfn, Function, cstr!("use-sample-profile"));
+    }
+
     // FIXME: none of these three functions interact with source level attributes.
     set_frame_pointer_type(cx, llfn);
     set_instrument_function(cx, llfn);

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -370,6 +370,13 @@ fn get_pgo_use_path(config: &ModuleConfig) -> Option<CString> {
         .map(|path_buf| CString::new(path_buf.to_string_lossy().as_bytes()).unwrap())
 }
 
+fn get_pgo_sample_use_path(config: &ModuleConfig) -> Option<CString> {
+    config
+        .pgo_sample_use
+        .as_ref()
+        .map(|path_buf| CString::new(path_buf.to_string_lossy().as_bytes()).unwrap())
+}
+
 pub(crate) fn should_use_new_llvm_pass_manager(config: &ModuleConfig) -> bool {
     // The new pass manager is enabled by default for LLVM >= 13.
     // This matches Clang, which also enables it since Clang 13.
@@ -389,6 +396,7 @@ pub(crate) unsafe fn optimize_with_new_llvm_pass_manager(
     let using_thin_buffers = opt_stage == llvm::OptStage::PreLinkThinLTO || config.bitcode_needed();
     let pgo_gen_path = get_pgo_gen_path(config);
     let pgo_use_path = get_pgo_use_path(config);
+    let pgo_sample_use_path = get_pgo_sample_use_path(config);
     let is_lto = opt_stage == llvm::OptStage::ThinLTO || opt_stage == llvm::OptStage::FatLTO;
     // Sanitizer instrumentation is only inserted during the pre-link optimization stage.
     let sanitizer_options = if !is_lto {
@@ -439,6 +447,8 @@ pub(crate) unsafe fn optimize_with_new_llvm_pass_manager(
         pgo_use_path.as_ref().map_or(std::ptr::null(), |s| s.as_ptr()),
         config.instrument_coverage,
         config.instrument_gcov,
+        pgo_sample_use_path.as_ref().map_or(std::ptr::null(), |s| s.as_ptr()),
+        config.debug_info_for_profiling,
         llvm_selfprofiler,
         selfprofile_before_pass_callback,
         selfprofile_after_pass_callback,
@@ -544,6 +554,9 @@ pub(crate) unsafe fn optimize(
             if config.instrument_coverage {
                 llvm::LLVMRustAddPass(mpm, find_pass("instrprof").unwrap());
             }
+            if config.debug_info_for_profiling {
+                llvm::LLVMRustAddPass(mpm, find_pass("add-discriminators").unwrap());
+            }
 
             add_sanitizer_passes(config, &mut extra_passes);
 
@@ -1001,6 +1014,7 @@ pub unsafe fn with_llvm_pmb(
     let inline_threshold = config.inline_threshold;
     let pgo_gen_path = get_pgo_gen_path(config);
     let pgo_use_path = get_pgo_use_path(config);
+    let pgo_sample_use_path = get_pgo_sample_use_path(config);
 
     llvm::LLVMRustConfigurePassManagerBuilder(
         builder,
@@ -1011,6 +1025,7 @@ pub unsafe fn with_llvm_pmb(
         prepare_for_thin_lto,
         pgo_gen_path.as_ref().map_or(ptr::null(), |s| s.as_ptr()),
         pgo_use_path.as_ref().map_or(ptr::null(), |s| s.as_ptr()),
+        pgo_sample_use_path.as_ref().map_or(ptr::null(), |s| s.as_ptr()),
     );
 
     llvm::LLVMPassManagerBuilderSetSizeLevel(builder, opt_size as u32);

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2176,6 +2176,7 @@ extern "C" {
         PrepareForThinLTO: bool,
         PGOGenPath: *const c_char,
         PGOUsePath: *const c_char,
+        PGOSampleUsePath: *const c_char,
     );
     pub fn LLVMRustAddLibraryInfo(
         PM: &PassManager<'a>,
@@ -2210,6 +2211,8 @@ extern "C" {
         PGOUsePath: *const c_char,
         InstrumentCoverage: bool,
         InstrumentGCOV: bool,
+        PGOSampleUsePath: *const c_char,
+        DebugInfoForProfiling: bool,
         llvm_selfprofiler: *mut c_void,
         begin_callback: SelfProfileBeforePassCallback,
         end_callback: SelfProfileAfterPassCallback,