std_detect: Support run-time detection on OpenBSD using elf_aux_info

Author: brad0

library/std_detect/README.md

@@ -66,10 +66,12 @@ crate from working on applications in which `std` is not available.
 
 * FreeBSD:
   * `arm32`, `powerpc64`: `std_detect` supports these on FreeBSD by querying ELF
-    auxiliary vectors using `sysctl`.
+    auxiliary vectors using `elf_aux_info`.
   * `arm64`: run-time feature detection is implemented by directly querying `mrs`.
 
 * OpenBSD:
+   * `powerpc64`: `std_detect` supports these on OpenBSD by querying ELF auxiliary
+     vectors using `elf_aux_info`.
   * `arm64`: run-time feature detection is implemented by querying `sysctl`.
 
 * Windows:

library/std_detect/src/detect/mod.rs

@@ -61,11 +61,12 @@ cfg_select! {
         #[path = "os/freebsd/mod.rs"]
         mod os;
     }
-    all(target_os = "openbsd", target_arch = "aarch64", feature = "libc") => {
+    all(target_os = "openbsd", feature = "libc") => {
         #[allow(dead_code)] // we don't use code that calls the mrs instruction.
+        #[cfg(target_arch = "aarch64")]
         #[path = "os/aarch64.rs"]
         mod aarch64;
-        #[path = "os/openbsd/aarch64.rs"]
+        #[path = "os/openbsd/mod.rs"]
         mod os;
     }
     all(target_os = "windows", any(target_arch = "aarch64", target_arch = "arm64ec")) => {

library/std_detect/src/detect/os/openbsd/auxvec.rs

@@ -0,0 +1,58 @@
+//! Parses ELF auxiliary vectors.
+#![cfg_attr(
+    any(
+        target_arch = "aarch64",
+        target_arch = "powerpc64",
+        target_arch = "riscv64"
+    ),
+    allow(dead_code)
+)]
+
+/// Cache HWCAP bitfields of the ELF Auxiliary Vector.
+///
+/// If an entry cannot be read all the bits in the bitfield are set to zero.
+/// This should be interpreted as all the features being disabled.
+#[derive(Debug, Copy, Clone)]
+pub(crate) struct AuxVec {
+    pub hwcap: usize,
+    pub hwcap2: usize,
+}
+
+/// ELF Auxiliary Vector
+///
+/// The auxiliary vector is a memory region in a running ELF program's stack
+/// composed of (key: usize, value: usize) pairs.
+///
+/// The keys used in the aux vector are platform dependent. For OpenBSD, they are
+/// defined in [machine/elf.h][elfh]. The hardware capabilities of a given CPU
+/// can be queried with the  `AT_HWCAP` and `AT_HWCAP2` keys.
+///
+/// Note that run-time feature detection is not invoked for features that can
+/// be detected at compile-time.
+///
+/// [elf.h]: https://github.com/openbsd/src/blob/master/sys/arch/arm64/include/elf.h
+/// [elf.h]: https://github.com/openbsd/src/blob/master/sys/arch/powerpc64/include/elf.h
+pub(crate) fn auxv() -> Result<AuxVec, ()> {
+    let hwcap = archauxv(libc::AT_HWCAP);
+    let hwcap2 = archauxv(libc::AT_HWCAP2);
+    // Zero could indicate that no features were detected, but it's also used to
+    // indicate an error. In particular, on many platforms AT_HWCAP2 will be
+    // legitimately zero, since it contains the most recent feature flags.
+    if hwcap != 0 || hwcap2 != 0 {
+        return Ok(AuxVec { hwcap, hwcap2 });
+    }
+    Err(())
+}
+
+/// Tries to read the `key` from the auxiliary vector.
+fn archauxv(key: libc::c_int) -> usize {
+    const OUT_LEN: libc::c_int = core::mem::size_of::<libc::c_ulong>() as libc::c_int;
+    let mut out: libc::c_ulong = 0;
+    unsafe {
+        let res =
+            libc::elf_aux_info(key, &mut out as *mut libc::c_ulong as *mut libc::c_void, OUT_LEN);
+        // If elf_aux_info fails, `out` will be left at zero (which is the proper default value).
+        debug_assert!(res == 0 || out == 0);
+    }
+    out as usize
+}

library/std_detect/src/detect/os/openbsd/mod.rs

@@ -0,0 +1,21 @@
+//! Run-time feature detection on OpenBSD
+
+mod auxvec;
+
+cfg_select! {
+    target_arch = "aarch64" => {
+        mod aarch64;
+        pub(crate) use self::aarch64::detect_features;
+    }
+    target_arch = "powerpc64" => {
+        mod powerpc;
+        pub(crate) use self::powerpc::detect_features;
+    }
+    _ => {
+        use crate::detect::cache;
+        /// Performs run-time feature detection.
+        pub(crate) fn detect_features() -> cache::Initializer {
+            cache::Initializer::default()
+        }
+    }
+}

library/std_detect/src/detect/os/openbsd/powerpc.rs

@@ -0,0 +1,21 @@
+//! Run-time feature detection for PowerPC on OpenBSD.
+
+use super::auxvec;
+use crate::detect::{Feature, cache};
+
+pub(crate) fn detect_features() -> cache::Initializer {
+    let mut value = cache::Initializer::default();
+    let enable_feature = |value: &mut cache::Initializer, f, enable| {
+        if enable {
+            value.set(f as u32);
+        }
+    };
+
+    if let Ok(auxv) = auxvec::auxv() {
+        enable_feature(&mut value, Feature::altivec, auxv.hwcap & 0x10000000 != 0);
+        enable_feature(&mut value, Feature::vsx, auxv.hwcap & 0x00000080 != 0);
+        enable_feature(&mut value, Feature::power8, auxv.hwcap2 & 0x80000000 != 0);
+        return value;
+    }
+    value
+}

library/std_detect/tests/cpu-detection.rs

@@ -320,8 +320,11 @@ fn powerpc_linux() {
 }
 
 #[test]
-#[cfg(all(target_arch = "powerpc64", any(target_os = "linux", target_os = "freebsd"),))]
-fn powerpc64_linux_or_freebsd() {
+#[cfg(all(
+    target_arch = "powerpc64",
+    any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"),
+))]
+fn powerpc64_linux_or_bsd() {
     println!("altivec: {}", is_powerpc64_feature_detected!("altivec"));
     println!("vsx: {}", is_powerpc64_feature_detected!("vsx"));
     println!("power8: {}", is_powerpc64_feature_detected!("power8"));