No coverage data on x86_64-unknown-none target

[YanWQ-monad]

Reproduce

src/main.rs:

#![no_std]
#![cfg_attr(not(test), no_main)]

use core::fmt::Write;

struct Writer;

impl Write for Writer {
    fn write_str(&mut self, s: &str) -> core::fmt::Result {
        let s = s.as_bytes();
        unsafe {
            core::arch::asm!(
                "syscall",
                in("rax") 1,
                in("rdi") 2,
                in("rsi") s.as_ptr(),
                in("rdx") s.len(),
            )
        }
        Ok(())
    }
}

#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
    loop {}
}

struct SizeCounter(usize);

impl minicov::CoverageWriter for SizeCounter {
    fn write(&mut self, data: &[u8]) -> Result<(), minicov::CoverageWriteError> {
        self.0 += data.len();
        Ok(())
    }
}

#[no_mangle]
pub fn _start(_: *const u8) {
    let mut buffer = SizeCounter(0);
    unsafe { minicov::capture_coverage(&mut buffer).unwrap() };
    writeln!(Writer, "Coverage data size: {:#x}", buffer.0).unwrap();

    loop {}
}

Cargo.toml:

[package]
name = "minicov-test"
edition = "2021"

[dependencies]
minicov = { version = "0.3", default-features = false }

Run:

$ RUSTFLAGS="-Cinstrument-coverage -Zno-profiler-runtime -Crelocation-model=static" cargo run --target x86_64-unknown-none --release
   Compiling minicov-test v0.0.0 (/minicov-test)
    Finished `release` profile [optimized] target(s) in 0.14s
     Running `target/x86_64-unknown-none/release/minicov-test`
Coverage data size: 0x80

And 0x80 is (seemingly) barely the size of the .profraw header, and does not contain any coverage data.

Possible Cause

On x86_64-unknown-none target, minicov is compiled with InstrProfilingPlatformOther.c because none of these is hit:

minicov/minicov/c/InstrProfilingPlatformLinux.c

Lines 9 to 11 in 31ed26c

	#if defined(__linux__) || defined(__FreeBSD__) || defined(__Fuchsia__) || \
	(defined(__sun__) && defined(__svr4__)) || defined(__NetBSD__) || \
	defined(_AIX)

Then, when capture_coverage gets the address of the counters, it gets NULL because CountersFirst is NULL, so it cannot collect the coverage counters.

In fact, the expected platform should be InstrProfilingPlatformLinux.c. I tried to remove the #if in InstrProfilingPlatformLinux.c (make it use Linux platform instead of Other), then minicov could collect the counters and dump the correct coverage data.

[Amanieu]

You need to execute static initializers to register counters at startup. This is normally done by libc but on -none targets you need to do this manually yourself.

[YanWQ-monad]

Thanks for your reply. May I ask what's the proper way to call the "static initializers"?

AFAIK, LLVM does not emit the runtime registration code, because the target is an ELF file:
https://github.com/llvm/llvm-project/blob/8caeb2e0c2fb8a5f1689c11775b81ceee76de958/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp#L1423

And I also look into what InstrProfilingLoweringPass does in my example: https://www.diffchecker.com/07GpyZjq/ . It seems that no initializers are generated in this pass?