Add -Zinstrument-mcount=fentry to -Zinstrument-mcount by pmur · Pull Request #158036 · rust-lang/rust

pmur · 2026-06-17T15:46:33Z

fentry is essentially a simpler version of mcount where the counting function is called before any other function prologue actions happen.

fentry is still used by the linux x86-64 kernel. It's unclear if or when patchable-function-entry will replace it. It is also used by clang for some x86-64 mingw toolchains.

This is only supported on some x86, x86-64, and s390x targets.

rustbot · 2026-06-17T15:46:37Z

These commits modify compiler targets.
(See the Target Tier Policy.)

rustbot · 2026-06-17T15:46:40Z

r? @jackh726

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fentry is essentially a simpler version of mcount where the counting function is called before any other function prologue actions happen. fentry is still used by the linux x86-64 kernel. It's unclear if or when patchable-function-entry will replace it. It is also used by clang for some x86-64 mingw toolchains. This is only supported on some x86, x86-64, and s390x targets.

pmur · 2026-06-22T19:46:22Z

@rustbot reroll

chenyukang · 2026-06-22T22:53:39Z

@rustbot reroll

folkertdev · 2026-06-23T09:02:20Z

What's the context/impetus for adding this? Is it for RfL?

pmur · 2026-06-23T13:56:28Z

RfL. It is still used by x86, and likely not going away anytime soon.

x86 is using patchable function entries exclusively for security mitigation techniques.

folkertdev · 2026-06-23T15:08:55Z

+/// The different settings that the `-Z Instrument-mcount` flag can have.
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
+pub enum InstrumentMcount {
+    /// `-Z instrument-mcount=no`
+    Disabled,
+    /// `-Z instrument-mcount=yes`
+    Mcount,
+    /// `-Z instrument-mcount=fentry`
+    Fentry,
+}


The whole -Z instrument-mcount is missing an entry in the unstable book, can you add one in src/doc/unstable-book/src/compiler-flags/?

We at least try to document things from first principles, rather than just deferring to clang/LLVM.

View changes since the review

Agreed. I have been working on that. Making a simple example of a Rust mcount function has been a little a bit trickier than I'd expected. I'll add it.

folkertdev · 2026-06-23T22:13:03Z

+// Define a custom mcount function. This may partially or fully override the glibc
+// implementation depending on linker options.
+#[unsafe(naked)]
+#[no_mangle]
+#[cfg(all(target_os = "linux", target_arch = "x86_64"))]
+pub extern "C" fn mcount() {
+    core::arch::naked_asm!(
+        // A simplified version based on the glibc x86-64 mcount wrapper.
+        // Save register arguments to the stack, and call the mcount above.
+        "push rax",
+        "push rcx",
+        "push rdx",
+        "push rsi",
+        "push rdi",
+        "push r8",
+        "push r9",
+        "mov rsi, 56[rsp]",
+        "mov rdi, 8[rbp]",
+        "call __count_fn",
+        "pop r9",
+        "pop r8",
+        "pop rdi",
+        "pop rsi",
+        "pop rdx",
+        "pop rcx",
+        "pop rax",
+        "ret",
+    )
+}


Just linking this, mcount could use extern "custom" #140829 if the ABI is truly custom.

View changes since the review

folkertdev · 2026-06-23T22:20:14Z

+#[unsafe(naked)]
+#[no_mangle]
+#[cfg(all(target_os = "linux", target_arch = "x86_64"))]
+pub extern "C" fn mcount() {


Suggested change

pub extern "C" fn mcount() {

unsafe extern "C" fn mcount() {

A bunch of things are pub here that really don't need to be. And if this function really has a custom ABI then it should be unsafe to call. Practically it seems fine though because all relevant registers are restored. It would help to write that down as a safety comment on the naked_asm! though.

View changes since the review

folkertdev · 2026-06-23T22:28:26Z

+
+In essence, this is implementing the function `fn mcount(caller: *const std::ffi::c_void, callee: *const std::ffi::c_void)`. The calling convention for mcount follows its own ABI, which isn't usually the standard ABI for the target, but is enforced by preexisting convention.
+
+A trivial example on x86_64-unknown-linux-gnu looks something like the following. The `#[instrument_fn]` attribute can be used to disable profiling to simplify writing counting functions, but implementors must be very careful when calling other functions (or closures which fail to inline) as they may also call mcount.


Can you elaborate a bit more on what this example should do? I get

> RUSTFLAG="-Z instrument-mcount=yes" cargo +nightly run main() called > rustc +nightly -vV rustc 1.98.0-nightly (4429659e4 2026-06-22) binary: rustc commit-hash: 4429659e4745016bd3f26a4a421843edc7fbc422 commit-date: 2026-06-22 host: x86_64-unknown-linux-gnu release: 1.98.0-nightly LLVM version: 22.1.7

So __count_fn is not actually called?

View changes since the review

If it compiles and links as I'd hope, it should print something like:

mcount: call from 0x55b6b374478a to 0x55b6b37447f9 main() called

It's frustratingly frail when Rust also links the profiling libraries using the default options. Let me investigate, and the example probably shouldn't link default libraries anyways.

I'll also add an fentry hook example for x86.

Oh, I see why the example did not work. A small typo: RUSTFLAG -> RUSTFLAGS.

And... wrapping the example in cargo shows I need a bit more cleanup.

ah, right. Can you include the expected output too?

folkertdev · 2026-06-25T19:34:00Z

Looks good!

@bors r+

rust-bors · 2026-06-25T19:34:04Z

📌 Commit 87514a3 has been approved by folkertdev

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 4. This pull request will be tested once the tree is reopened.

Rollup of 10 pull requests Successful merges: - #158410 (Update LLVM for Mach-O __LINKEDIT alignment fix.) - #157397 (cmse: clear padding when crossing the secure boundary) - #158036 (Add -Zinstrument-mcount=fentry to -Zinstrument-mcount) - #158330 (llvm: use intrinsics for f16, f32 minimum/maximum) - #158359 (fix(tests): allow either branch direction in ilog_known_base) - #158067 (LLVM 23: Adapt codegen test to moved assume) - #158261 (Move part of the target checking for `#[may_dangle]` to the parser) - #158358 (Fix invalid E0609 raw pointer deref suggestion inside macros) - #158392 (delegation: add tests for defaults and infers in generics) - #158394 (Generate synthetic generic args only for delegation's child segment)

Rollup merge of #158036 - pmur:murp/add-fentry, r=folkertdev Add -Zinstrument-mcount=fentry to -Zinstrument-mcount fentry is essentially a simpler version of mcount where the counting function is called before any other function prologue actions happen. fentry is still used by the linux x86-64 kernel. It's unclear if or when patchable-function-entry will replace it. It is also used by clang for some x86-64 mingw toolchains. This is only supported on some x86, x86-64, and s390x targets.

rustbot assigned jackh726 Jun 17, 2026

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pmur force-pushed the murp/add-fentry branch from 245af6c to 6ce0650 Compare June 17, 2026 18:10

rustbot assigned chenyukang and unassigned jackh726 Jun 22, 2026

rustbot assigned folkertdev and unassigned chenyukang Jun 22, 2026

folkertdev reviewed Jun 23, 2026

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pmur force-pushed the murp/add-fentry branch 2 times, most recently from 0921fd6 to f22ba59 Compare June 25, 2026 16:42

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folkertdev reviewed Jun 25, 2026

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Comment thread src/doc/unstable-book/src/compiler-flags/instrument-mcount.md Outdated

doc/unstable-book: document -Zinstrument-mcount

87514a3

pmur force-pushed the murp/add-fentry branch from f22ba59 to 87514a3 Compare June 25, 2026 17:55

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 25, 2026

jhpratt mentioned this pull request Jun 25, 2026

Rollup of 10 pull requests #158419

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rust-bors Bot merged commit a82172d into rust-lang:main Jun 26, 2026
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rustbot added this to the 1.98.0 milestone Jun 26, 2026


		In essence, this is implementing the function `fn mcount(caller: const std::ffi::c_void, callee: const std::ffi::c_void)`. The calling convention for mcount follows its own ABI, which isn't usually the standard ABI for the target, but is enforced by preexisting convention.

		A trivial example on x86_64-unknown-linux-gnu looks something like the following. The `#[instrument_fn]` attribute can be used to disable profiling to simplify writing counting functions, but implementors must be very careful when calling other functions (or closures which fail to inline) as they may also call mcount.

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