Unable to collect llvm statistics with `-C llvm-args=--stats`

[yugr]

I'd like Rust to dump standard LLVM compiler statistics via

$ cat test.rs
#[no_mangle]
pub fn foo(v: &[i32], n: usize) -> i32 {
    let mut ans = 0;
    for i in 0..n {
        ans += v[i];
    }
    ans
}
$ rustc --crate-type=rlib -O -C target-cpu=native -C llvm-args=--stats test.rs

but this does not work (output is empty).

This

$ rustc --crate-type=rlib -O -C target-cpu=native --emit=llvm-ir test.rs
$ ~/rust/build/x86_64-unknown-linux-gnu/ci-llvm/bin/opt test.ll --stats -O2 -o test.bc
===-------------------------------------------------------------------------===
                          ... Statistics Collected ...
===-------------------------------------------------------------------------===

  6 aa                - Number of NoAlias results
 12 basicaa           - Number of times a GEP is decomposed
  1 count-visits      - Max number of times we visited a function
  4 globalsmodref-aa  - Number of functions that do not access memory
...

works but of course is irrelevant because opt's pipeline is unrelated to rustc.

I tried both stable:

$ rustc --version
rustc 1.87.0 (17067e9ac 2025-05-09)

and nightly

$ rustc --version
rustc 1.89.0-nightly (6ccd44760 2025-06-08)

[dianqk]

This is the intended behavior; you must use a self-compiled rustc.

https://github.com/llvm/llvm-project/blob/c450dd7e8398ffa2c17ae80a21a965bc74fc94dd/llvm/lib/Support/Statistic.cpp#L245-L253

[workingjubilee]

Since this is working as intended because LLVM does not enable its version of self-profiling by default, I am closing this.

@yugr If you believe the overhead is negligible, then feel free to send a PR setting the relevant variables in our build configuration for LLVM and we can test the perf of it.

[yugr]

This is the intended behavior; you must use a self-compiled rustc.

https://github.com/llvm/llvm-project/blob/c450dd7e8398ffa2c17ae80a21a965bc74fc94dd/llvm/lib/Support/Statistic.cpp#L245-L253

The problem reproduces with self-compiled Rust as well with this bootstrap.toml:

# See bootstrap.example.toml for documentation of available options
#
profile = "compiler"  # Includes one of the default files in src/bootstrap/defaults
change-id = 138986

[llvm]
assertions=true

(and I verified that I have debug version LLVM because -Cllvm-args=-debug worked). Otherwise the opt invocation in my example wouldn't have worked.

Note that assertions=true should enable stats:

#if !defined(NDEBUG) || LLVM_FORCE_ENABLE_STATS
#define LLVM_ENABLE_STATS 1
#else
#define LLVM_ENABLE_STATS 0
#endif

(from include/llvm/ADT/Statistic.h).

Moreover I would expect rustc to emit a warning if stats are not supported, similar to clang:

$ clang -mllvm -stats -S -x c /dev/null
Statistics are disabled.  Build with asserts or with -DLLVM_FORCE_ENABLE_STATS

@workingjubilee @dianqk

[yugr]

Actually even adding

  llvm::EnableStatistics(true);

to LLVMRustSetLLVMOptions does not enable statistics print (not even the "Statistics are disabled" message).

[dianqk]

You'll likely need to set release-debuginfo to true.

[dianqk]

Moreover I would expect rustc to emit a warning if stats are not supported, similar to clang:

$ clang -mllvm -stats -S -x c /dev/null
Statistics are disabled.  Build with asserts or with -DLLVM_FORCE_ENABLE_STATS

It makes sense to me.