Instrumentation (-Zinstrument-mcount) broken due to missing LLVM-pass since LLVM13 update

[tlambertz]

Issue

I tried to compile a generic hello-world binary with the -Zinstrument-mcount compiler option.

I expected every function-entry to have a call to mcount().
Instead, there are no such calls inserted.

Triage

I have triaged the bug, and it first occurred in nightly-2021-08-22, the first version with LLVM13.

The LLVM-IR still contains the "instrument-function-entry-inlined"="mcount" attributes, but the corrosponding llvm-pass is not run anymore.

rustc +nightly-2021-08-21 -O -Zinstrument-mcount main.rs --emit asm -Z print-llvm-passes -Z no-parallel-llvm 2>&1 | grep mcount
    Instrument function entry/exit with calls to e.g. mcount() (pre inlining)
      Instrument function entry/exit with calls to e.g. mcount() (post inlining)
rustc +nightly-2021-08-22 -O -Zinstrument-mcount main.rs --emit asm -Z print-llvm-passes -Z no-parallel-llvm 2>&1 | grep mcount
[empty]

Verbose Versions:

❯ rustc +nightly-2021-08-21 --version --verbose
rustc 1.56.0-nightly (a0035916e 2021-08-20)
binary: rustc
commit-hash: a0035916e01d8e644ccd44554c57f0874cef8c8c
commit-date: 2021-08-20
host: x86_64-unknown-linux-gnu
release: 1.56.0-nightly
LLVM version: 12.0.1
❯ rustc +nightly-2021-08-22 --version --verbose
rustc 1.56.0-nightly (d3e2578c3 2021-08-21)
binary: rustc
commit-hash: d3e2578c31688619ddc0a10ddf8543bf4ebcba5b
commit-date: 2021-08-21
host: x86_64-unknown-linux-gnu
release: 1.56.0-nightly
LLVM version: 13.0.0

[tlambertz]

Manually adding the passes works with all nightly versions I have tested (including the current one):

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

For 2021-08-22 it also prints said passes. Sometime in between 09-01 and 10-01 the pass is not printed anymore, not sure why.

❯ rustc +nightly-2021-09-01 -Zinstrument-mcount -C passes="ee-instrument post-inline-ee-instrument" main.rs --emit asm -Z print-llvm-passes -Z no-parallel-llvm 2>&1 | grep mcount
      Instrument function entry/exit with calls to e.g. mcount() (pre inlining)
      Instrument function entry/exit with calls to e.g. mcount() (post inlining)
❯ rustc +nightly-2021-10-01 -Zinstrument-mcount -C passes="ee-instrument post-inline-ee-instrument" main.rs --emit asm -Z print-llvm-passes -Z no-parallel-llvm 2>&1 | grep mcount
[empty]

[tmiasko]

The responsibility for hooking in EntryExitInstrumenterPass moved from LLVM to Clang in https://reviews.llvm.org/D97608.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[mvtec-bergdolll]

Using RUSTFLAGS="-Z instrument-mcount -C passes=ee-instrument<post-inline>" I noticed that inlined functions may call mcount again, leading to skewed statistics, eg:

I'm not sure if clang generates different code, but I would have expected an inlined function to not contribute to the call count of its parent function.