Rollup merge of #113723 - khei4:khei4/llvm-stats, r=oli-obk,nikic

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

This resurrects PR https://github.com/rust-lang/rust/pull/104000, which has sat idle for a while. And I want to see the effect of stack-move optimizations on LLVM (like https://reviews.llvm.org/D153453) :).

I have applied the changes requested by `@oli-obk` and `@nagisa`  https://github.com/rust-lang/rust/pull/104000#discussion_r1014625377 and https://github.com/rust-lang/rust/pull/104000#discussion_r1014642482 in the latest commits.

r? `@oli-obk`

-----

LLVM has a neat [statistics](https://llvm.org/docs/ProgrammersManual.html#the-statistic-class-stats-option) 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.

-----
(Edit: fix broken link
(Edit2: fix segmentation fault and use malloc

If `rustc` is built with
```toml
[llvm]
assertions = true
```
Then you can see like
```
rustc +stage1 -Z print-codegen-stats -C opt-level=3  tmp.rs
===-------------------------------------------------------------------------===
                          ... Statistics Collected ...
===-------------------------------------------------------------------------===
         3 aa                           - Number of MayAlias results
       193 aa                           - Number of MustAlias results
       531 aa                           - Number of NoAlias results
...
```

And the current default build emits only
```
$ rustc +stage1 -Z print-codegen-stats -C opt-level=3  tmp.rs
===-------------------------------------------------------------------------===
                          ... Statistics Collected ...
===-------------------------------------------------------------------------===
$
```
This might be better to emit the message to tell assertion flag necessity, but now I can't find how to do that...

compiler/rustc_codegen_llvm/src/lib.rs

@@ -46,6 +46,7 @@ use rustc_span::symbol::Symbol;
 
 use std::any::Any;
 use std::ffi::CStr;
+use std::io::Write;
 
 mod back {
     pub mod archive;
@@ -178,7 +179,28 @@ impl WriteBackendMethods for LlvmCodegenBackend {
     type ThinBuffer = back::lto::ThinBuffer;
     fn print_pass_timings(&self) {
         unsafe {
-            llvm::LLVMRustPrintPassTimings();
+            let mut size = 0;
+            let cstr = llvm::LLVMRustPrintPassTimings(&mut size as *mut usize);
+            if cstr.is_null() {
+                println!("failed to get pass timings");
+            } else {
+                let timings = std::slice::from_raw_parts(cstr as *const u8, size);
+                std::io::stdout().write_all(timings).unwrap();
+                libc::free(cstr as *mut _);
+            }
+        }
+    }
+    fn print_statistics(&self) {
+        unsafe {
+            let mut size = 0;
+            let cstr = llvm::LLVMRustPrintStatistics(&mut size as *mut usize);
+            if cstr.is_null() {
+                println!("failed to get pass stats");
+            } else {
+                let stats = std::slice::from_raw_parts(cstr as *const u8, size);
+                std::io::stdout().write_all(stats).unwrap();
+                libc::free(cstr as *mut _);
+            }
         }
     }
     fn run_link(

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1868,7 +1868,10 @@ extern "C" {
     pub fn LLVMRustGetLastError() -> *const c_char;
 
     /// Print the pass timings since static dtors aren't picking them up.
-    pub fn LLVMRustPrintPassTimings();
+    pub fn LLVMRustPrintPassTimings(size: *const size_t) -> *const c_char;
+
+    /// Print the statistics since static dtors aren't picking them up.
+    pub fn LLVMRustPrintStatistics(size: *const size_t) -> *const c_char;
 
     pub fn LLVMStructCreateNamed(C: &Context, Name: *const c_char) -> &Type;
 

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);
         }