Add -Zinstrument-fentry

The behavior is very similar to `-Zinstrument-mcount`, except this
option has no frame-pointer requirements, and is called before the
call frame is set up.

This only works on x86, x86-64, and s390x targets.

Author: pmur

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -204,14 +204,14 @@ fn instrument_function_attr<'ll>(
     instrument_fn: &Option<bool>,
 ) -> SmallVec<[&'ll Attribute; 4]> {
     let mut attrs = SmallVec::new();
-    if sess.opts.unstable_opts.instrument_mcount {
+    if sess.opts.unstable_opts.instrument_mcount || sess.opts.unstable_opts.instrument_fentry {
         // Similar to `clang -pg` behavior. Handled by the
         // `post-inline-ee-instrument` LLVM pass.
 
         // #[instrument_fn], the default is on.
         let instrument_entry = instrument_fn.unwrap_or_else(|| true);
 
-        if instrument_entry {
+        if instrument_entry && sess.opts.unstable_opts.instrument_mcount {
             // The function name varies on platforms.
             // See test/CodeGen/mcount.c in clang.
             let mcount_name = match &sess.target.llvm_mcount_intrinsic {
@@ -225,6 +225,10 @@ fn instrument_function_attr<'ll>(
                 mcount_name,
             ));
         }
+
+        if instrument_entry && sess.opts.unstable_opts.instrument_fentry {
+            attrs.push(llvm::CreateAttrStringValue(cx.llcx, "fentry-call", "true"));
+        }
     }
     if let Some(options) = &sess.opts.unstable_opts.instrument_xray {
         // XRay instrumentation is similar to __cyg_profile_func_{enter,exit}.

compiler/rustc_session/src/options.rs

@@ -2392,6 +2392,8 @@ 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_fentry: bool = (false, parse_bool, [TRACKED],
+        "insert function instrument code for fentry-based tracing (default: no)"),
     instrument_mcount: bool = (false, parse_bool, [TRACKED],
         "insert function instrument code for mcount-based tracing (default: no)"),
     instrument_xray: Option<InstrumentXRay> = (None, parse_instrument_xray, [TRACKED],

compiler/rustc_session/src/session.rs

@@ -1325,6 +1325,10 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
         }
     }
 
+    if sess.opts.unstable_opts.instrument_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() });
     }

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
@@ -628,6 +630,7 @@ 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>,
 }

compiler/rustc_target/src/spec/mod.rs

@@ -2702,6 +2702,9 @@ 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,
 
@@ -2944,6 +2947,7 @@ 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,

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(),

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;
 

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(),

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(),

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()
     };