Pass -pg to linker when using -Zinstrument-mcount by pmur · Pull Request #152457 · rust-lang/rust

pmur · 2026-02-10T22:32:52Z

This selects a slightly different crt on gnu targets which enables the profiler within glibc.

This makes using gprof a little easier with Rust binaries. Otherwise, rustc must be passed -Clink-args=-pg to ensure the correct startup code is linked.

rustbot · 2026-02-10T22:32:57Z

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mati865 · 2026-02-11T01:19:08Z

The change itself is correct, but perhaps it deserves a test?

g(r)crt1.o enabled by -pg is a startup code for the c runtime. Is it right to call it the crt?

bjorn3 · 2026-02-11T09:20:09Z

Is this also correct for musl and other targets that use a gnu toolchain?

mati865 · 2026-02-11T14:19:20Z

Good question.
This flag is supported by GCC and Clang, so any target that uses them should accept it. If GCC/Clang does a wrong thing for a specific target, this is arguably not our fault, yet we still might want to add workarounds.

pmur · 2026-02-11T15:12:10Z

g(r)crt1.o enabled by -pg is a startup code for the c runtime. Is it right to call it the crt?

That's maybe a detail about the glibc specific implementation which can be redacted? non-gnu targets seem to have a little more varied behavior.

Is this also correct for musl and other targets that use a gnu toolchain?

musl, no. Linking should fail with unresolved mcount-ish symbols. Looking around at Clang, it does seem a few other platforms should respect this option. @mati865, I think you're right. If the target lacks support for this option, it will likely fail similar to musl. Removing the gnu check is likely helpful for other targets.

mati865 · 2026-02-12T12:53:03Z

Just to double check we don't regress here.
Musl (and likely other libc like uclibc) failed to before and still do after this PR, albeit with different errors, right?

FreeBSD seems to support it: https://man.freebsd.org/cgi/man.cgi?query=moncontrol&sektion=3&apropos=0&manpath=freebsd

Even mingw-w64 provides the necessary objects.

pmur · 2026-02-12T20:01:14Z

Just to double check we don't regress here.
Musl (and likely other libc like uclibc) failed to before and still do after this PR, albeit with different errors, right?

I verified musl will fail to compile in the same way with and without -pg. It fails to link due to the missing mcount symbol. I think uclibc will fail similarly, it appears to lack an mcount implementation too.

pmur · 2026-02-25T22:10:59Z

ping

davidtwco · 2026-03-10T13:06:30Z

@bors r+

rust-bors · 2026-03-10T13:06:39Z

📌 Commit a4a5208 has been approved by davidtwco

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.

Pass -pg to linker when using -Zinstrument-mcount This selects a slightly different crt on gnu targets which enables the profiler within glibc. This makes using gprof a little easier with Rust binaries. Otherwise, rustc must be passed `-Clink-args=-pg` to ensure the correct startup code is linked.

…uwer Rollup of 8 pull requests Successful merges: - #149130 (Implement coercions between `&pin (mut|const) T` and `&(mut) T` when `T: Unpin`) - #152457 (Pass -pg to linker when using -Zinstrument-mcount) - #153143 (Allow `./x test` to run tests without doc tests and without benchmarks) - #153653 (scalable vector: type renames and simple checks) - #152302 (fix: don't suggest replacing `env!("CARGO_BIN_NAME")` with itself) - #153641 (Move `Spanned`.) - #153643 (Avoid projection-only suggestions for inherent assoc types) - #153657 (triagebot: remove myself from some mention groups)

Pass -pg to linker when using -Zinstrument-mcount This selects a slightly different crt on gnu targets which enables the profiler within glibc. This makes using gprof a little easier with Rust binaries. Otherwise, rustc must be passed `-Clink-args=-pg` to ensure the correct startup code is linked.

…uwer Rollup of 13 pull requests Successful merges: - #149130 (Implement coercions between `&pin (mut|const) T` and `&(mut) T` when `T: Unpin`) - #152457 (Pass -pg to linker when using -Zinstrument-mcount) - #153143 (Allow `./x test` to run tests without doc tests and without benchmarks) - #153471 (Refactor `ActiveJobGuard`) - #153595 (`QueryLatch` cleanups) - #153653 (scalable vector: type renames and simple checks) - #152302 (fix: don't suggest replacing `env!("CARGO_BIN_NAME")` with itself) - #153479 (Add rationale for intentional potential_query_instability allows) - #153600 (add test for proc-macros with custom panic payloads) - #153641 (Move `Spanned`.) - #153643 (Avoid projection-only suggestions for inherent assoc types) - #153657 (triagebot: remove myself from some mention groups) - #153659 (Mark an unreachable match arm as such)

Pass -pg to linker when using -Zinstrument-mcount This selects a slightly different crt on gnu targets which enables the profiler within glibc. This makes using gprof a little easier with Rust binaries. Otherwise, rustc must be passed `-Clink-args=-pg` to ensure the correct startup code is linked.

pmur · 2026-03-11T17:33:35Z

It seems only-gnu also applies to non-glibc targets.

only-gnu and only-linux compiletest directives should only test a glibc target?

mati865 · 2026-03-12T07:55:31Z

Mingw-w64 provides the symbols it failed on in libgmon, so it should have worked.

Some of the symbols that linker couldnt find: https://github.com/mingw-w64/mingw-w64/blob/59cd5ce805b12cdefced08ce567d3518c6053b40/mingw-w64-crt/profile/gmon.c#L88 (other symbols are in different files).

This is then assembled to libgmon: https://github.com/mingw-w64/mingw-w64/blob/59cd5ce805b12cdefced08ce567d3518c6053b40/mingw-w64-crt/Makefile.am#L2049

pmur · 2026-03-12T14:18:52Z

Mingw-w64 provides the symbols it failed on in libgmon, so it should have worked.

Thanks for noting that. Investigating it, mingw expects the symbol to be named _mcount. That's a quick fix.

mati865 · 2026-03-12T15:29:04Z

Yeah. The other one is missing -lgmon because of Rust using -nodefaultlibs:

❯ x86_64-w64-mingw32-gcc ~/tmp/hello.c -pg -### 2>&1 | tr ' ' '\n' | rg gmon
"-lgmon"

❯ x86_64-w64-mingw32-gcc ~/tmp/hello.c -pg -### -nodefaultlibs 2>&1 | tr ' ' '\n' | rg gmon

So, I guess Rust should add -lgmon as well for all *-windows-gnu* targets.

pmur · 2026-03-12T17:09:03Z

So, I guess Rust should add -lgmon as well for all *-windows-gnu* targets.

Indeed, and that required readding a couple other libs too. I verified it links a simple gprof instrumented binary.

mati865 · 2026-03-12T18:02:50Z

compiler/rustc_codegen_ssa/src/back/linker.rs

        self.cc_arg("-pg");
+        // On windows-gnu targets, libgmon also needs to be linked, and this
+        // requires readding libraries to satisfy its dependencies.
+        if self.sess.target.os == Os::Windows {


You can check self.sess.target.is_like_windows here instead.

mati865 · 2026-03-12T18:05:50Z

Indeed, and that required readding a couple other libs too. I verified it links a simple gprof instrumented binary.

Right, I often forget how annoying ld.bfd is.

Thanks for noting that. Investigating it, mingw expects the symbol to be named _mcount. That's a quick fix.

Do you want to update base/windows_gnu_llvm.rs too (in the same commit as the other one)? If not, I can do it separately.

mingw exposes the `_mcount` symbol for profiling.

libgmon needs to be linked. This also requires readding a few other system libraries to satisfy its dependencies.

pmur · 2026-03-12T19:43:39Z

Do you want to update base/windows_gnu_llvm.rs too (in the same commit as the other one)? If not, I can do it separately.

I'll update the patch with both changes you've mentioned. Thanks.

pmur · 2026-03-24T19:55:21Z

Ping. This issue found in the rollup should be fixed.

davidtwco · 2026-03-26T14:22:30Z

@bors r+

rust-bors · 2026-03-26T14:22:34Z

📌 Commit 87c2552 has been approved by davidtwco

It is now in the queue for this repository.

Pass -pg to linker when using -Zinstrument-mcount This selects a slightly different crt on gnu targets which enables the profiler within glibc. This makes using gprof a little easier with Rust binaries. Otherwise, rustc must be passed `-Clink-args=-pg` to ensure the correct startup code is linked.

Rollup of 7 pull requests Successful merges: - #152457 (Pass -pg to linker when using -Zinstrument-mcount) - #154031 (Remove divergence check from check_expr_array) - #154418 (move many tests out of `ui/unsafe`) - #153662 (Suggest fully qualified path on method name collision) - #153675 (simd_add/sub/mul/neg: document overflow behavior) - #154110 (Change "error finalizing incremental compilation" text and emit it as a note, not a warning) - #154430 (Create GPU target notification group)

Pass -pg to linker when using -Zinstrument-mcount This selects a slightly different crt on gnu targets which enables the profiler within glibc. This makes using gprof a little easier with Rust binaries. Otherwise, rustc must be passed `-Clink-args=-pg` to ensure the correct startup code is linked.

Rollup of 7 pull requests Successful merges: - #152457 (Pass -pg to linker when using -Zinstrument-mcount) - #154031 (Remove divergence check from check_expr_array) - #154418 (move many tests out of `ui/unsafe`) - #154441 (bootstrap: force a CI LLVM stamp bump) - #153662 (Suggest fully qualified path on method name collision) - #153675 (simd_add/sub/mul/neg: document overflow behavior) - #154430 (Create GPU target notification group)

Rollup of 6 pull requests Successful merges: - #152457 (Pass -pg to linker when using -Zinstrument-mcount) - #154031 (Remove divergence check from check_expr_array) - #154418 (move many tests out of `ui/unsafe`) - #154441 (bootstrap: force a CI LLVM stamp bump) - #153675 (simd_add/sub/mul/neg: document overflow behavior) - #154430 (Create GPU target notification group)

rustbot assigned davidtwco Feb 10, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Feb 10, 2026

pmur force-pushed the murp/mcount-link-pg branch from 78a8cde to a4a5208 Compare February 11, 2026 17:46

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pmur force-pushed the murp/mcount-link-pg branch from f47f0f0 to 40eb8d1 Compare March 12, 2026 16:58

mati865 reviewed Mar 12, 2026

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pmur added 2 commits March 12, 2026 14:39

Fix mcount name for *-windows-gnu targets

cc2c5f9

mingw exposes the `_mcount` symbol for profiling.

Link extra libraries when using mcount on *-windows-gnu targets

87c2552

libgmon needs to be linked. This also requires readding a few other system libraries to satisfy its dependencies.

pmur force-pushed the murp/mcount-link-pg branch from 40eb8d1 to 87c2552 Compare March 12, 2026 19:53

mati865 approved these changes Mar 12, 2026

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rust-bors bot merged commit 433e106 into rust-lang:main Mar 27, 2026
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