Auto merge of #113339 - lqd:respect-filters, r=tmiasko

Filter out short-lived LLVM diagnostics before they reach the rustc handler

During profiling I saw remark passes being unconditionally enabled: for example `Machine Optimization Remark Emitter`.

The diagnostic remarks enabled by default are [from missed optimizations and opt analyses](#113339 (comment)). They are created by LLVM, passed to the diagnostic handler on the C++ side, emitted to rust, where they are unpacked, C++ strings are converted to rust, etc.

Then they are discarded in the vast majority of the time (i.e. unless some kind of `-Cremark` has enabled some of these passes' output to be printed).

These unneeded allocations are very short-lived, basically only lasting between the LLVM pass emitting them and the rust handler where they are discarded. So it doesn't hugely impact max-rss, and is only a slight reduction in instruction count (cachegrind reports a reduction between 0.3% and 0.5%) _on linux_. It's possible that targets without `jemalloc` or with a worse allocator, may optimize these less.

It is however significant in the aggregate, looking at the total number of allocated bytes:
- it's the biggest source of allocations according to dhat, on the benchmarks I've tried e.g. `syn` or `cargo`
- allocations on `syn` are reduced by 440MB, 17% (from 2440722647 bytes total, to 2030461328 bytes)
- allocations on `cargo` are reduced by 6.6GB, 19% (from 35371886402 bytes total, to 28723987743 bytes)

Some of these diagnostics objects [are allocated in LLVM](#113339 (comment)) *before* they're emitted to our diagnostic handler, where they'll be filtered out. So we could remove those in the future, but that will require changing a few LLVM call-sites upstream, so I left a FIXME.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -381,29 +381,22 @@ unsafe extern "C" fn diagnostic_handler(info: &DiagnosticInfo, user: *mut c_void
         }
 
         llvm::diagnostic::Optimization(opt) => {
-            let enabled = match cgcx.remark {
-                Passes::All => true,
-                Passes::Some(ref v) => v.iter().any(|s| *s == opt.pass_name),
-            };
-
-            if enabled {
-                diag_handler.emit_note(FromLlvmOptimizationDiag {
-                    filename: &opt.filename,
-                    line: opt.line,
-                    column: opt.column,
-                    pass_name: &opt.pass_name,
-                    kind: match opt.kind {
-                        OptimizationDiagnosticKind::OptimizationRemark => "success",
-                        OptimizationDiagnosticKind::OptimizationMissed
-                        | OptimizationDiagnosticKind::OptimizationFailure => "missed",
-                        OptimizationDiagnosticKind::OptimizationAnalysis
-                        | OptimizationDiagnosticKind::OptimizationAnalysisFPCommute
-                        | OptimizationDiagnosticKind::OptimizationAnalysisAliasing => "analysis",
-                        OptimizationDiagnosticKind::OptimizationRemarkOther => "other",
-                    },
-                    message: &opt.message,
-                });
-            }
+            diag_handler.emit_note(FromLlvmOptimizationDiag {
+                filename: &opt.filename,
+                line: opt.line,
+                column: opt.column,
+                pass_name: &opt.pass_name,
+                kind: match opt.kind {
+                    OptimizationDiagnosticKind::OptimizationRemark => "success",
+                    OptimizationDiagnosticKind::OptimizationMissed
+                    | OptimizationDiagnosticKind::OptimizationFailure => "missed",
+                    OptimizationDiagnosticKind::OptimizationAnalysis
+                    | OptimizationDiagnosticKind::OptimizationAnalysisFPCommute
+                    | OptimizationDiagnosticKind::OptimizationAnalysisAliasing => "analysis",
+                    OptimizationDiagnosticKind::OptimizationRemarkOther => "other",
+                },
+                message: &opt.message,
+            });
         }
         llvm::diagnostic::PGO(diagnostic_ref) | llvm::diagnostic::Linker(diagnostic_ref) => {
             let message = llvm::build_string(|s| {

compiler/rustc_codegen_llvm/src/llvm/diagnostic.rs

@@ -9,7 +9,7 @@ use libc::c_uint;
 use super::{DiagnosticInfo, SMDiagnostic};
 use rustc_span::InnerSpan;
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub enum OptimizationDiagnosticKind {
     OptimizationRemark,
     OptimizationMissed,

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -1892,12 +1892,19 @@ extern "C" void LLVMRustContextConfigureDiagnosticHandler(
           LlvmRemarkStreamer(std::move(LlvmRemarkStreamer)) {}
 
     virtual bool handleDiagnostics(const DiagnosticInfo &DI) override {
-      if (this->LlvmRemarkStreamer) {
-        if (auto *OptDiagBase = dyn_cast<DiagnosticInfoOptimizationBase>(&DI)) {
-          if (OptDiagBase->isEnabled()) {
+      // If this diagnostic is one of the optimization remark kinds, we can check if it's enabled
+      // before emitting it. This can avoid many short-lived allocations when unpacking the
+      // diagnostic and converting its various C++ strings into rust strings.
+      // FIXME: some diagnostic infos still allocate before we get here, and avoiding that would be
+      // good in the future. That will require changing a few call sites in LLVM.
+      if (auto *OptDiagBase = dyn_cast<DiagnosticInfoOptimizationBase>(&DI)) {
+        if (OptDiagBase->isEnabled()) {
+          if (this->LlvmRemarkStreamer) {
             this->LlvmRemarkStreamer->emit(*OptDiagBase);
             return true;
           }
+        } else {
+          return true;
         }
       }
       if (DiagnosticHandlerCallback) {