Add -Z instrument-mcount

[quark-zju]

This flag inserts mcount function call to the beginning of every function
after inline processing. So tracing tools like uftrace 1 (or ftrace for
Linux kernel modules) have a chance to examine function calls.

It is similar to the -pg flag provided by gcc or clang, but without
generating a __gmon_start__ function for executables. If a program
runs without being traced, no gmon.out will be written to disk.

Under the hood, it simply adds "instrument-function-entry-inlined"="mcount"
attribute to every function. The post-inline-ee-instrument LLVM pass does
the actual job.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @estebank (or someone else) soon.

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[estebank]

@bors r+

CC @rust-lang/compiler

[bors]

📌 Commit 31a5066 has been approved by estebank

[bors]

⌛ Testing commit 31a5066 with merge 6efaef6...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: estebank
Pushing 6efaef6 to master...

[nagisa]

Is it known that all the tools use specifically the mcount function name? A codegen test would’ve been nice as well.

[quark-zju]

Good question. clang -pg defaults to mcount but platforms can override it. The rule seems rather complicated. I think a complete solution would be platform-dependent. I'll add a comment, and try to add a test.

EDIT: uftrace also uses different names on different platforms.

[eddie9712]

Hi all, I have tried the "-Z instrument-mcount" with rustc compiler (rustc 1.60.0-nightly (bd3cb5256 2022-01-16)). However, I didn't get any mcount function inserted in my executable file. Is the "-Z instrument-mcount" still supported ?

[dwlsalmeida]

@eddie9712 same here!

[honggyukim]

Hi, @quark-zju already analyzed this problem before at namhyung/uftrace#1392 (comment). There must be some changes in LLVM 13. @tlambertz found a workaround at namhyung/uftrace#1392 (comment) as follows.

rustc +nightly -Zinstrument-mcount -C passes="ee-instrument post-inline-ee-instrument" main.rs

For now, we have to add -C passes="ee-instrument post-inline-ee-instrument" along with -Zinstrument-mcount until LLVM fixes this issue.