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](https://github.com/rust-lang/rust/pull/113339#discussion_r1259480303). 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](https://github.com/rust-lang/rust/pull/113339#discussion_r1252387484) *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.

1 files changed, 10 insertions, 3 deletions

diff --git a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
index f509e2911f3..8ef39a6c866 100644
--- a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
+++ b/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) {