Auto merge of #113040 - Kobzol:llvm-remark-streamer, r=tmiasko

Add `-Zremark-dir` unstable flag to write LLVM optimization remarks to YAML

This PR adds an option for `rustc` to emit LLVM optimization remarks to a set of YAML files, which can then be digested by existing tools, like https://github.com/OfekShilon/optview2. When `-Cremark-dir` is passed, and remarks are enabled (`-Cremark=all`), the remarks will be now written to the specified directory, **instead** of being printed to standard error output.  The files are named based on the CGU from which they are being generated.

Currently, the remarks are written using the LLVM streaming machinery, directly in the diagnostics handler. It seemed easier than going back to Rust and then form there back to C++ to use the streamer from the diagnostics handler. But there are many ways to implement this, of course, so I'm open to suggestions :)

I included some comments with questions into the code. Also, I'm not sure how to test this.

r? `@tmiasko`

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -35,6 +35,7 @@ use rustc_span::symbol::sym;
 use rustc_span::{BytePos, FileName, InnerSpan, Pos, Span};
 use rustc_target::spec::{MergeFunctions, SanitizerSet};
 
+use crate::errors::ErrorCreatingRemarkDir;
 use std::any::Any;
 use std::borrow::Cow;
 use std::fs;
@@ -345,6 +346,9 @@ pub struct CodegenContext<B: WriteBackendMethods> {
     pub diag_emitter: SharedEmitter,
     /// LLVM optimizations for which we want to print remarks.
     pub remark: Passes,
+    /// Directory into which should the LLVM optimization remarks be written.
+    /// If `None`, they will be written to stderr.
+    pub remark_dir: Option<PathBuf>,
     /// Worker thread number
     pub worker: usize,
     /// The incremental compilation session directory, or None if we are not
@@ -1062,6 +1066,17 @@ fn start_executing_work<B: ExtraBackendMethods>(
             tcx.backend_optimization_level(())
         };
     let backend_features = tcx.global_backend_features(());
+
+    let remark_dir = if let Some(ref dir) = sess.opts.unstable_opts.remark_dir {
+        let result = fs::create_dir_all(dir).and_then(|_| dir.canonicalize());
+        match result {
+            Ok(dir) => Some(dir),
+            Err(error) => sess.emit_fatal(ErrorCreatingRemarkDir { error }),
+        }
+    } else {
+        None
+    };
+
     let cgcx = CodegenContext::<B> {
         crate_types: sess.crate_types().to_vec(),
         each_linked_rlib_for_lto,
@@ -1073,6 +1088,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
         prof: sess.prof.clone(),
         exported_symbols,
         remark: sess.opts.cg.remark.clone(),
+        remark_dir,
         worker: 0,
         incr_comp_session_dir: sess.incr_comp_session_dir_opt().map(|r| r.clone()),
         cgu_reuse_tracker: sess.cgu_reuse_tracker.clone(),

compiler/rustc_codegen_ssa/src/errors.rs

@@ -1023,3 +1023,9 @@ pub struct TargetFeatureSafeTrait {
     #[label(codegen_ssa_label_def)]
     pub def: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(codegen_ssa_error_creating_remark_dir)]
+pub struct ErrorCreatingRemarkDir {
+    pub error: std::io::Error,
+}