rustc_llvm: Add a `-Z print-llvm-stats` option to expose LLVM statistics.

LLVM has a neat [statistics] feature that tracks how often optimizations kick
in. It's very handy for optimization work. Since we expose the LLVM pass
timings, I thought it made sense to expose the LLVM statistics too.

[statistics]: https://llvm.org/docs/ProgrammersManual.html#the-statistic-class-stats-option

compiler/rustc_codegen_gcc/src/lib.rs

@@ -239,6 +239,10 @@ impl WriteBackendMethods for GccCodegenBackend {
         unimplemented!();
     }
 
+    fn print_statistics(&self) {
+        unimplemented!()
+    }
+
     unsafe fn optimize(_cgcx: &CodegenContext<Self>, _diag_handler: &Handler, module: &ModuleCodegen<Self::Module>, config: &ModuleConfig) -> Result<(), FatalError> {
         module.module_llvm.context.set_optimization_level(to_gcc_opt_level(config.opt_level));
         Ok(())

compiler/rustc_codegen_llvm/src/lib.rs

@@ -181,6 +181,11 @@ impl WriteBackendMethods for LlvmCodegenBackend {
             llvm::LLVMRustPrintPassTimings();
         }
     }
+    fn print_statistics(&self) {
+        unsafe {
+            llvm::LLVMRustPrintStatistics();
+        }
+    }
     fn run_link(
         cgcx: &CodegenContext<Self>,
         diag_handler: &Handler,

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1870,6 +1870,9 @@ extern "C" {
     /// Print the pass timings since static dtors aren't picking them up.
     pub fn LLVMRustPrintPassTimings();
 
+    /// Print the statistics since static dtors aren't picking them up.
+    pub fn LLVMRustPrintStatistics();
+
     pub fn LLVMStructCreateNamed(C: &Context, Name: *const c_char) -> &Type;
 
     pub fn LLVMStructSetBody<'a>(

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -110,6 +110,10 @@ unsafe fn configure_llvm(sess: &Session) {
         // Use non-zero `import-instr-limit` multiplier for cold callsites.
         add("-import-cold-multiplier=0.1", false);
 
+        if sess.print_llvm_stats() {
+            add("-stats", false);
+        }
+
         for arg in sess_args {
             add(&(*arg), true);
         }

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -1945,6 +1945,10 @@ impl<B: ExtraBackendMethods> OngoingCodegen<B> {
             self.backend.print_pass_timings()
         }
 
+        if sess.print_llvm_stats() {
+            self.backend.print_statistics()
+        }
+
         (
             CodegenResults {
                 metadata: self.metadata,

compiler/rustc_codegen_ssa/src/traits/write.rs

@@ -35,6 +35,7 @@ pub trait WriteBackendMethods: 'static + Sized + Clone {
         cached_modules: Vec<(SerializedModule<Self::ModuleBuffer>, WorkProduct)>,
     ) -> Result<(Vec<LtoModuleCodegen<Self>>, Vec<WorkProduct>), FatalError>;
     fn print_pass_timings(&self);
+    fn print_statistics(&self);
     unsafe fn optimize(
         cgcx: &CodegenContext<Self>,
         diag_handler: &Handler,

compiler/rustc_interface/src/tests.rs

@@ -715,6 +715,7 @@ fn test_unstable_options_tracking_hash() {
     // `pre_link_arg` is omitted because it just forwards to `pre_link_args`.
     untracked!(pre_link_args, vec![String::from("abc"), String::from("def")]);
     untracked!(print_llvm_passes, true);
+    untracked!(print_llvm_stats, true);
     untracked!(print_mono_items, Some(String::from("abc")));
     untracked!(print_type_sizes, true);
     untracked!(proc_macro_backtrace, true);

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -1,4 +1,5 @@
 #include "LLVMWrapper.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DiagnosticHandler.h"
 #include "llvm/IR/DiagnosticInfo.h"
@@ -116,6 +117,11 @@ extern "C" void LLVMRustPrintPassTimings() {
   TimerGroup::printAll(OS);
 }
 
+extern "C" void LLVMRustPrintStatistics() {
+  raw_fd_ostream OS(2, false); // stderr.
+  llvm::PrintStatistics(OS);
+}
+
 extern "C" LLVMValueRef LLVMRustGetNamedValue(LLVMModuleRef M, const char *Name,
                                               size_t NameLen) {
   return wrap(unwrap(M)->getNamedValue(StringRef(Name, NameLen)));

compiler/rustc_session/src/options.rs

@@ -1672,6 +1672,9 @@ options! {
         "make rustc print the total optimization fuel used by a crate"),
     print_llvm_passes: bool = (false, parse_bool, [UNTRACKED],
         "print the LLVM optimization passes being run (default: no)"),
+    #[rustc_lint_opt_deny_field_access("use `Session::print_llvm_stats` instead of this field")]
+    print_llvm_stats: bool = (true, parse_bool, [UNTRACKED],
+        "print LLVM statistics (default: no)"),
     print_mono_items: Option<String> = (None, parse_opt_string, [UNTRACKED],
         "print the result of the monomorphization collection pass"),
     print_type_sizes: bool = (false, parse_bool, [UNTRACKED],

compiler/rustc_session/src/session.rs

@@ -1057,6 +1057,10 @@ impl Session {
         self.opts.unstable_opts.verbose
     }
 
+    pub fn print_llvm_stats(&self) -> bool {
+        self.opts.unstable_opts.print_llvm_stats
+    }
+
     pub fn verify_llvm_ir(&self) -> bool {
         self.opts.unstable_opts.verify_llvm_ir || option_env!("RUSTC_VERIFY_LLVM_IR").is_some()
     }