feat: harvest CPU more performance rustc/cargo build options

Author: GuillaumeLagrange

crates/codspeed/build.rs

@@ -7,6 +7,7 @@ fn main() {
 
     collect_rustc_info();
     collect_cargo_info();
+    collect_build_profile_info();
 
     let mut build = cc::Build::new();
     build
@@ -48,6 +49,41 @@ fn main() {
     }
 }
 
+/// Collect build profile and performance-relevant compiler options at build time.
+/// These env var names must be kept in sync with `src/instrument_hooks/mod.rs`.
+///
+/// Reference: <https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts>
+fn collect_build_profile_info() {
+    // PROFILE is set by Cargo to the name of the profile being built (e.g. "release", "bench", "debug").
+    if let Ok(profile) = std::env::var("PROFILE") {
+        println!("cargo:rustc-env=CODSPEED_BUILD_PROFILE={profile}");
+    }
+
+    // OPT_LEVEL is set by Cargo to the optimization level (0, 1, 2, 3, s, z).
+    if let Ok(opt_level) = std::env::var("OPT_LEVEL") {
+        println!("cargo:rustc-env=CODSPEED_BUILD_OPT_LEVEL={opt_level}");
+    }
+
+    // Extract performance-relevant codegen flags from rustflags.
+    //
+    // Cargo uses two mutually exclusive sources for rustflags (see
+    // <https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-reads>):
+    //   - RUSTFLAGS env var (space-separated, takes priority)
+    //   - CARGO_ENCODED_RUSTFLAGS env var (0x1f-separated, used when RUSTFLAGS is not set)
+    // We check both in priority order to accurately represent the flags used to build.
+    let rustflags: Vec<String> = if let Ok(raw) = std::env::var("RUSTFLAGS") {
+        raw.split_whitespace().map(String::from).collect()
+    } else if let Ok(encoded) = std::env::var("CARGO_ENCODED_RUSTFLAGS") {
+        encoded.split('\x1f').map(String::from).collect()
+    } else {
+        Vec::new()
+    };
+
+    if let Some(target_cpu) = extract_codegen_flag(&rustflags, "target-cpu") {
+        println!("cargo:rustc-env=CODSPEED_BUILD_TARGET_CPU={target_cpu}");
+    }
+}
+
 /// Collect rustc toolchain info at build time and expose as env vars.
 /// These env var names must be kept in sync with `src/instrument_hooks/mod.rs`.
 fn collect_rustc_info() {
@@ -98,3 +134,35 @@ fn collect_cargo_info() {
         println!("cargo:rustc-env=CODSPEED_CARGO_VERSION={rest}");
     }
 }
+
+/// Extract a codegen option value from a list of rustc flags.
+///
+/// Handles all three forms that rustc accepts:
+///   - `-Ckey=value` (no space)
+///   - `-C` `key=value` (space between -C and the option, split into two entries)
+///   - `-C` `key` `value` (space between key and value, split into three entries) — only for flags that accept it
+///
+/// Returns the value for the first match of `key`.
+fn extract_codegen_flag(flags: &[String], key: &str) -> Option<String> {
+    let mut iter = flags.iter();
+    while let Some(flag) = iter.next() {
+        // Form: -Ckey=value
+        if let Some(rest) = flag.strip_prefix("-C") {
+            if let Some(value) = rest.strip_prefix(key).and_then(|s| s.strip_prefix('=')) {
+                return Some(value.to_string());
+            }
+            // rest is not our key, skip
+            continue;
+        }
+
+        // Form: -C key=value  or  -C key value
+        if flag == "-C" {
+            if let Some(next) = iter.next() {
+                if let Some(value) = next.strip_prefix(key).and_then(|s| s.strip_prefix('=')) {
+                    return Some(value.to_string());
+                }
+            }
+        }
+    }
+    None
+}

crates/codspeed/src/instrument_hooks/mod.rs

@@ -59,6 +59,15 @@ mod linux_impl {
             if let Some(v) = option_env!("CODSPEED_CARGO_VERSION") {
                 let _ = self.set_environment(SECTION, "cargo", v);
             }
+            if let Some(v) = option_env!("CODSPEED_BUILD_PROFILE") {
+                let _ = self.set_environment(SECTION, "profile", v);
+            }
+            if let Some(v) = option_env!("CODSPEED_BUILD_OPT_LEVEL") {
+                let _ = self.set_environment(SECTION, "opt-level", v);
+            }
+            if let Some(v) = option_env!("CODSPEED_BUILD_TARGET_CPU") {
+                let _ = self.set_environment(SECTION, "target-cpu", v);
+            }
 
             if let Err(e) = self.write_environment() {
                 eprintln!("Warning: failed to write environment info: {e}");