rust-lang · pmur · Feb 16, 2026
diff --git a/compiler/rustc_codegen_llvm/src/attributes.rs b/compiler/rustc_codegen_llvm/src/attributes.rs
@@ -7,7 +7,9 @@ use rustc_middle::middle::codegen_fn_attrs::{
     TargetFeature,
 };
 use rustc_middle::ty::{self, Instance, TyCtxt};
-use rustc_session::config::{BranchProtection, FunctionReturn, OptLevel, PAuthKey, PacRet};
+use rustc_session::config::{
+    BranchProtection, FunctionReturn, InstrumentMcount, OptLevel, PAuthKey, PacRet,
+};
 use rustc_span::sym;
 use rustc_symbol_mangling::mangle_internal_symbol;
 use rustc_target::spec::{Arch, FramePointer, SanitizerSet, StackProbeType, StackProtector};
@@ -177,7 +179,7 @@ pub(crate) fn frame_pointer(sess: &Session) -> FramePointer {
     let opts = &sess.opts;
     // "mcount" function relies on stack pointer.
     // See <https://sourceware.org/binutils/docs/gprof/Implementation.html>.
-    if opts.unstable_opts.instrument_mcount {
+    if opts.unstable_opts.instrument_mcount == InstrumentMcount::Mcount {
         fp.ratchet(FramePointer::Always);
     }
     fp.ratchet(opts.cg.force_frame_pointers);
@@ -214,7 +216,7 @@ fn instrument_function_attr<'ll>(
     instrument_fn: InstrumentFnAttr,
 ) -> SmallVec<[&'ll Attribute; 4]> {
     let mut attrs = SmallVec::new();
-    if sess.opts.unstable_opts.instrument_mcount {
+    if sess.opts.unstable_opts.instrument_mcount != InstrumentMcount::Disabled {
         // Similar to `clang -pg` behavior. Handled by the
         // `post-inline-ee-instrument` LLVM pass.
 
@@ -224,18 +226,26 @@ fn instrument_function_attr<'ll>(
         };
 
         if instrument_entry {
-            // The function name varies on platforms.
-            // See test/CodeGen/mcount.c in clang.
-            let mcount_name = match &sess.target.llvm_mcount_intrinsic {
-                Some(llvm_mcount_intrinsic) => llvm_mcount_intrinsic.as_ref(),
-                None => sess.target.mcount.as_ref(),
-            };
-
-            attrs.push(llvm::CreateAttrStringValue(
-                cx.llcx,
-                "instrument-function-entry-inlined",
-                mcount_name,
-            ));
+            match sess.opts.unstable_opts.instrument_mcount {
+                InstrumentMcount::Mcount => {
+                    // The function name varies on platforms.
+                    // See test/CodeGen/mcount.c in clang.
+                    let mcount_name = match &sess.target.llvm_mcount_intrinsic {
+                        Some(llvm_mcount_intrinsic) => llvm_mcount_intrinsic.as_ref(),
+                        None => sess.target.mcount.as_ref(),
+                    };
+
+                    attrs.push(llvm::CreateAttrStringValue(
+                        cx.llcx,
+                        "instrument-function-entry-inlined",
+                        mcount_name,
+                    ));
+                }
+                InstrumentMcount::Fentry => {
+                    attrs.push(llvm::CreateAttrStringValue(cx.llcx, "fentry-call", "true"));
+                }
+                InstrumentMcount::Disabled => {}
+            }
         }
     }
     if let Some(options) = &sess.opts.unstable_opts.instrument_xray {

diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -35,8 +35,8 @@ use rustc_middle::middle::debugger_visualizer::DebuggerVisualizerFile;
 use rustc_middle::middle::dependency_format::Linkage;
 use rustc_middle::middle::exported_symbols::SymbolExportKind;
 use rustc_session::config::{
-    self, CFGuard, CrateType, DebugInfo, LinkerFeaturesCli, OutFileName, OutputFilenames,
-    OutputType, PrintKind, SplitDwarfKind, Strip,
+    self, CFGuard, CrateType, DebugInfo, InstrumentMcount, LinkerFeaturesCli, OutFileName,
+    OutputFilenames, OutputType, PrintKind, SplitDwarfKind, Strip,
 };
 use rustc_session::lint::builtin::LINKER_MESSAGES;
 use rustc_session::output::{check_file_is_writeable, invalid_output_for_target, out_filename};
@@ -2882,7 +2882,7 @@ fn add_order_independent_options(
         cmd.pgo_gen();
     }
 
-    if sess.opts.unstable_opts.instrument_mcount {
+    if sess.opts.unstable_opts.instrument_mcount != InstrumentMcount::Disabled {
         cmd.enable_profiling();
     }
 

diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
@@ -13,11 +13,11 @@ use rustc_session::config::{
     AnnotateMoves, AutoDiff, BranchProtection, CFGuard, Cfg, CodegenRetagOptions, CoverageLevel,
     CoverageOptions, DebugInfo, DumpMonoStatsFormat, ErrorOutputType, ExternEntry, ExternLocation,
     Externs, FmtDebug, FunctionReturn, IncrementalStateAssertion, InliningThreshold, Input,
-    InstrumentCoverage, InstrumentXRay, LinkSelfContained, LinkerPluginLto, LocationDetail, LtoCli,
-    MirIncludeSpans, NextSolverConfig, Offload, Options, OutFileName, OutputType, OutputTypes,
-    PAuthKey, PacRet, Passes, PatchableFunctionEntry, Polonius, ProcMacroExecutionStrategy, Strip,
-    SwitchWithOptPath, SymbolManglingVersion, WasiExecModel, build_configuration,
-    build_session_options, rustc_optgroups,
+    InstrumentCoverage, InstrumentMcount, InstrumentXRay, LinkSelfContained, LinkerPluginLto,
+    LocationDetail, LtoCli, MirIncludeSpans, NextSolverConfig, Offload, Options, OutFileName,
+    OutputType, OutputTypes, PAuthKey, PacRet, Passes, PatchableFunctionEntry, Polonius,
+    ProcMacroExecutionStrategy, Strip, SwitchWithOptPath, SymbolManglingVersion, WasiExecModel,
+    build_configuration, build_session_options, rustc_optgroups,
 };
 use rustc_session::lint::Level;
 use rustc_session::search_paths::SearchPath;
@@ -834,7 +834,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(inline_mir, Some(true));
     tracked!(inline_mir_hint_threshold, Some(123));
     tracked!(inline_mir_threshold, Some(123));
-    tracked!(instrument_mcount, true);
+    tracked!(instrument_mcount, InstrumentMcount::Mcount);
     tracked!(instrument_xray, Some(InstrumentXRay::default()));
     tracked!(link_directives, false);
     tracked!(link_only, true);

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
@@ -255,6 +255,17 @@ pub enum AnnotateMoves {
     Enabled(Option<u64>),
 }
 
+/// 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,
+}
+
 /// Settings for `-Z instrument-xray` flag.
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
 pub struct InstrumentXRay {
@@ -3085,11 +3096,11 @@ pub(crate) mod dep_tracking {
     use super::{
         AnnotateMoves, AutoDiff, BranchProtection, CFGuard, CFProtection, CodegenRetagOptions,
         CoverageOptions, CrateType, DebugInfo, DebugInfoCompression, ErrorOutputType, FmtDebug,
-        FunctionReturn, InliningThreshold, InstrumentCoverage, InstrumentXRay, LinkerPluginLto,
-        LocationDetail, LtoCli, MirStripDebugInfo, NextSolverConfig, Offload, OptLevel,
-        OutFileName, OutputType, OutputTypes, PatchableFunctionEntry, Polonius, ResolveDocLinks,
-        SourceFileHashAlgorithm, SplitDwarfKind, SwitchWithOptPath, SymbolManglingVersion,
-        WasiExecModel,
+        FunctionReturn, InliningThreshold, InstrumentCoverage, InstrumentMcount, InstrumentXRay,
+        LinkerPluginLto, LocationDetail, LtoCli, MirStripDebugInfo, NextSolverConfig, Offload,
+        OptLevel, OutFileName, OutputType, OutputTypes, PatchableFunctionEntry, Polonius,
+        ResolveDocLinks, SourceFileHashAlgorithm, SplitDwarfKind, SwitchWithOptPath,
+        SymbolManglingVersion, WasiExecModel,
     };
     use crate::lint;
     use crate::utils::NativeLib;
@@ -3151,6 +3162,7 @@ pub(crate) mod dep_tracking {
         TlsModel,
         InstrumentCoverage,
         CoverageOptions,
+        InstrumentMcount,
         InstrumentXRay,
         CrateType,
         MergeFunctions,

diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
@@ -786,6 +786,8 @@ mod desc {
     pub(crate) const parse_coverage_options: &str = "`block` | `branch` | `condition`";
     pub(crate) const parse_codegen_retag_options: &str =
         "either no value or a comma-separated list of settings: `no-precise-im`, `no-precise-pin`";
+    pub(crate) const parse_instrument_mcount: &str =
+        "either a boolean (`yes`, `no`, `on`, `off`, etc), or `fentry` on supported targets.";
     pub(crate) const parse_instrument_xray: &str = "either a boolean (`yes`, `no`, `on`, `off`, etc), or a comma separated list of settings: `always` or `never` (mutually exclusive), `ignore-loops`, `instruction-threshold=N`, `skip-entry`, `skip-exit`";
     pub(crate) const parse_unpretty: &str = "`string` or `string=string`";
     pub(crate) const parse_treat_err_as_bug: &str = "either no value or a non-negative number";
@@ -1565,6 +1567,19 @@ pub mod parse {
         true
     }
 
+    pub(crate) fn parse_instrument_mcount(slot: &mut InstrumentMcount, v: Option<&str>) -> bool {
+        let mut use_mcount = false;
+        if parse_bool(&mut use_mcount, v) {
+            *slot = if use_mcount { InstrumentMcount::Mcount } else { InstrumentMcount::Disabled };
+            true
+        } else if let Some("fentry") = v {
+            *slot = InstrumentMcount::Fentry;
+            true
+        } else {
+            false
+        }
+    }
+
     pub(crate) fn parse_instrument_xray(
         slot: &mut Option<InstrumentXRay>,
         v: Option<&str>,
@@ -2433,7 +2448,7 @@ options! {
         "a default MIR inlining threshold (default: 50)"),
     input_stats: bool = (false, parse_bool, [UNTRACKED],
         "print some statistics about AST and HIR (default: no)"),
-    instrument_mcount: bool = (false, parse_bool, [TRACKED],
+    instrument_mcount: InstrumentMcount = (InstrumentMcount::Disabled, parse_instrument_mcount, [TRACKED],
         "insert function instrument code for mcount-based tracing (default: no)"),
     instrument_xray: Option<InstrumentXRay> = (None, parse_instrument_xray, [TRACKED],
         "insert function instrument code for XRay-based tracing (default: no)

diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
@@ -38,7 +38,7 @@ use crate::code_stats::CodeStats;
 pub use crate::code_stats::{DataTypeKind, FieldInfo, FieldKind, SizeKind, VariantInfo};
 use crate::config::{
     self, Cfg, CheckCfg, CoverageLevel, CoverageOptions, CrateType, DebugInfo, ErrorOutputType,
-    FunctionReturn, Input, InstrumentCoverage, OptLevel, OutFileName, OutputType,
+    FunctionReturn, Input, InstrumentCoverage, InstrumentMcount, OptLevel, OutFileName, OutputType,
     SwitchWithOptPath,
 };
 use crate::filesearch::FileSearch;
@@ -1340,6 +1340,12 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
         }
     }
 
+    if sess.opts.unstable_opts.instrument_mcount == InstrumentMcount::Fentry
+        && !sess.target.options.supports_fentry
+    {
+        sess.dcx().emit_err(errors::InstrumentationNotSupported { us: "fentry".to_string() });
+    }
+
     if sess.opts.unstable_opts.instrument_xray.is_some() && !sess.target.options.supports_xray {
         sess.dcx().emit_err(errors::InstrumentationNotSupported { us: "XRay".to_string() });
     }

diff --git a/compiler/rustc_target/src/spec/json.rs b/compiler/rustc_target/src/spec/json.rs
@@ -223,6 +223,7 @@ impl Target {
         forward!(supports_stack_protector);
         forward!(small_data_threshold_support);
         forward!(entry_name);
+        forward!(supports_fentry);
         forward!(supports_xray);
 
         // we're going to run `update_from_cli`, but that won't change the target's AbiMap
@@ -407,6 +408,7 @@ impl ToJson for Target {
         target_option_val!(small_data_threshold_support);
         target_option_val!(entry_name);
         target_option_val!(entry_abi);
+        target_option_val!(supports_fentry);
         target_option_val!(supports_xray);
 
         // Serializing `-Clink-self-contained` needs a dynamic key to support the
@@ -626,6 +628,7 @@ struct TargetSpecJson {
     supports_stack_protector: Option<bool>,
     small_data_threshold_support: Option<SmallDataThresholdSupport>,
     entry_name: Option<StaticCow<str>>,
+    supports_fentry: Option<bool>,
     supports_xray: Option<bool>,
     entry_abi: Option<ExternAbiWrapper>,
 }

diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
@@ -2688,6 +2688,9 @@ pub struct TargetOptions {
     /// Default value is `CanonAbi::C`
     pub entry_abi: CanonAbi,
 
+    /// Whether the target supports fentry instrumentation.
+    pub supports_fentry: bool,
+
     /// Whether the target supports XRay instrumentation.
     pub supports_xray: bool,
 
@@ -2929,6 +2932,7 @@ impl Default for TargetOptions {
             supports_stack_protector: true,
             entry_name: "main".into(),
             entry_abi: CanonAbi::C,
+            supports_fentry: false,
             supports_xray: false,
             default_address_space: rustc_abi::AddressSpace::ZERO,
             small_data_threshold_support: SmallDataThresholdSupport::DefaultForArch,

diff --git a/compiler/rustc_target/src/spec/targets/i686_unknown_linux_gnu.rs b/compiler/rustc_target/src/spec/targets/i686_unknown_linux_gnu.rs
@@ -22,6 +22,7 @@ pub(crate) fn target() -> Target {
     base.supported_sanitizers = SanitizerSet::ADDRESS;
     base.add_pre_link_args(LinkerFlavor::Gnu(Cc::Yes, Lld::No), &["-m32"]);
     base.stack_probes = StackProbeType::Inline;
+    base.supports_fentry = true;
 
     Target {
         llvm_target: "i686-unknown-linux-gnu".into(),

diff --git a/compiler/rustc_target/src/spec/targets/i686_unknown_linux_musl.rs b/compiler/rustc_target/src/spec/targets/i686_unknown_linux_musl.rs
@@ -12,6 +12,7 @@ pub(crate) fn target() -> Target {
     base.max_atomic_width = Some(64);
     base.add_pre_link_args(LinkerFlavor::Gnu(Cc::Yes, Lld::No), &["-m32", "-Wl,-melf_i386"]);
     base.stack_probes = StackProbeType::Inline;
+    base.supports_fentry = true;
     // FIXME(compiler-team#422): musl targets should be dynamically linked by default.
     base.crt_static_default = true;
 

diff --git a/compiler/rustc_target/src/spec/targets/s390x_unknown_linux_gnu.rs b/compiler/rustc_target/src/spec/targets/s390x_unknown_linux_gnu.rs
@@ -12,6 +12,7 @@ pub(crate) fn target() -> Target {
     base.stack_probes = StackProbeType::Inline;
     base.supported_sanitizers =
         SanitizerSet::ADDRESS | SanitizerSet::LEAK | SanitizerSet::MEMORY | SanitizerSet::THREAD;
+    base.supports_fentry = true;
 
     Target {
         llvm_target: "s390x-unknown-linux-gnu".into(),

diff --git a/compiler/rustc_target/src/spec/targets/s390x_unknown_linux_musl.rs b/compiler/rustc_target/src/spec/targets/s390x_unknown_linux_musl.rs
@@ -13,6 +13,7 @@ pub(crate) fn target() -> Target {
     base.stack_probes = StackProbeType::Inline;
     base.supported_sanitizers =
         SanitizerSet::ADDRESS | SanitizerSet::LEAK | SanitizerSet::MEMORY | SanitizerSet::THREAD;
+    base.supports_fentry = true;
 
     Target {
         llvm_target: "s390x-unknown-linux-musl".into(),

diff --git a/compiler/rustc_target/src/spec/targets/s390x_unknown_none_softfloat.rs b/compiler/rustc_target/src/spec/targets/s390x_unknown_none_softfloat.rs
@@ -20,6 +20,7 @@ pub(crate) fn target() -> Target {
         rustc_abi: Some(RustcAbi::Softfloat),
         stack_probes: StackProbeType::Inline,
         supported_sanitizers: SanitizerSet::KERNELADDRESS,
+        supports_fentry: true,
         ..Default::default()
     };
 

diff --git a/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_gnu.rs b/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_gnu.rs
@@ -19,6 +19,7 @@ pub(crate) fn target() -> Target {
         | SanitizerSet::SAFESTACK
         | SanitizerSet::THREAD
         | SanitizerSet::REALTIME;
+    base.supports_fentry = true;
     base.supports_xray = true;
 
     Target {

diff --git a/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_musl.rs b/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_musl.rs
@@ -15,6 +15,7 @@ pub(crate) fn target() -> Target {
         | SanitizerSet::LEAK
         | SanitizerSet::MEMORY
         | SanitizerSet::THREAD;
+    base.supports_fentry = true;
     base.supports_xray = true;
     // FIXME(compiler-team#422): musl targets should be dynamically linked by default.
     base.crt_static_default = true;

diff --git a/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_none.rs b/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_none.rs
@@ -10,6 +10,7 @@ pub(crate) fn target() -> Target {
     base.linker_flavor = LinkerFlavor::Gnu(Cc::No, Lld::Yes);
     base.linker = Some("rust-lld".into());
     base.panic_strategy = PanicStrategy::Abort;
+    base.supports_fentry = true;
 
     Target {
         llvm_target: "x86_64-unknown-linux-none".into(),

diff --git a/tests/assembly-llvm/fentry.rs b/tests/assembly-llvm/fentry.rs
@@ -0,0 +1,25 @@
+//@ assembly-output: emit-asm
+//@ compile-flags: -Zinstrument-mcount=fentry
+//@ add-minicore
+
+//@ revisions: X86 S390X
+//@[X86] compile-flags: --target=x86_64-unknown-linux-gnu -Cllvm-args=-x86-asm-syntax=intel
+//@[X86] needs-llvm-components: x86
+//@[S390X] compile-flags: --target=s390x-unknown-linux-gnu
+//@[S390X] needs-llvm-components: systemz
+
+#![crate_type = "lib"]
+#![feature(no_core)]
+#![no_core]
+
+extern crate minicore;
+
+// CHECK-LABEL: mcount_func:
+#[no_mangle]
+pub fn mcount_func(a: isize, b: isize) -> isize {
+    // X86: call __fentry__
+    // S390X: brasl %r0, __fentry__@PLT
+    a + b
+    // X86: ret
+    // S390X: br %r14
+}
diff --git a/tests/codegen-llvm/instrument-fentry.rs b/tests/codegen-llvm/instrument-fentry.rs
@@ -0,0 +1,25 @@
+//@ add-minicore
+//@ compile-flags: -Z instrument-mcount=fentry -Copt-level=0
+//
+//@ revisions: x86_64-linux
+//@[x86_64-linux] compile-flags: --target=x86_64-unknown-linux-gnu
+//@[x86_64-linux] needs-llvm-components: x86
+//
+//@ revisions: x86-linux
+//@[x86-linux] compile-flags: --target=i686-unknown-linux-gnu
+//@[x86-linux] needs-llvm-components: x86
+//
+//@ revisions: s390x-linux
+//@[s390x-linux] compile-flags: --target=s390x-unknown-linux-gnu
+//@[s390x-linux] needs-llvm-components: systemz
+
+#![feature(no_core)]
+#![no_std]
+#![no_core]
+#![crate_type = "lib"]
+
+extern crate minicore;
+use minicore::*;
+
+// CHECK: attributes #{{.*}} "fentry-call"="true"
+pub fn foo() {}