Introduce musl-math-sys for bindings to musl math symbols

This crate builds math symbols from a musl checkout and provides a Rust
interface. The intent is that we will be able to compare our
implementations against musl on more than just linux (which are the only
currently the only targets we run `*-musl` targets against for
comparison).

Musl libc can't compile on anything other than Linux; however, the
routines in `src/math` are cross platform enough to build on MacOS and
windows-gnu with only minor adjustments. We take advantage of this and
build only needed files using `cc`.

The build script also performs remapping (via defines) so that e.g.
`cos` gets defined as `musl_cos`. This gives us more certainty that we
are actually testing against the intended symbol; without it, it is easy
to unknowingly link to system libraries or even Rust's `libm` itself and
wind up with an ineffective test. There is also a small procedure to
verify remapping worked correctly by checking symbols in object files.

Author: tgross35

.gitignore

@@ -1,8 +1,9 @@
-**/*.rs.bk
+**.bk
 .#*
 /bin
 /math/src
 /math/target
 /target
-/tests
 Cargo.lock
+musl/
+**.tar.gz

Cargo.toml

@@ -24,10 +24,12 @@ unstable = []
 force-soft-floats = []
 
 [workspace]
+resolver = "2"
 members = [
   "crates/compiler-builtins-smoke-test",
   "crates/libm-bench",
   "crates/libm-test",
+  "crates/musl-math-sys",
 ]
 default-members = [
   ".",

crates/musl-math-sys/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "musl-math-sys"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+
+[dev-dependencies]
+libm = { path = "../../" }
+
+[build-dependencies]
+cc = "1.1.24"

crates/musl-math-sys/build.rs

@@ -0,0 +1,328 @@
+use std::collections::BTreeMap;
+use std::path::{Path, PathBuf};
+use std::process::{Command, Stdio};
+use std::{env, fs, str};
+
+/// Static library that will be built
+const LIB_NAME: &str = "musl_math_prefixed";
+
+/// Files that have more than one symbol. Map of file names to the symbols defined in that file.
+const MULTIPLE_SYMBOLS: &[(&str, &[&str])] = &[
+    ("__invtrigl", &["__invtrigl", "__invtrigl_R", "__pio2_hi", "__pio2_lo"]),
+    ("__polevll", &["__polevll", "__p1evll"]),
+    ("erf", &["erf", "erfc"]),
+    ("erff", &["erff", "erfcf"]),
+    ("erfl", &["erfl", "erfcl"]),
+    ("exp10", &["exp10", "pow10"]),
+    ("exp10f", &["exp10f", "pow10f"]),
+    ("exp10l", &["exp10l", "pow10l"]),
+    ("exp2f_data", &["exp2f_data", "__exp2f_data"]),
+    ("exp_data", &["exp_data", "__exp_data"]),
+    ("j0", &["j0", "y0"]),
+    ("j0f", &["j0f", "y0f"]),
+    ("j1", &["j1", "y1"]),
+    ("j1f", &["j1f", "y1f"]),
+    ("jn", &["jn", "yn"]),
+    ("jnf", &["jnf", "ynf"]),
+    ("lgamma", &["lgamma", "__lgamma_r"]),
+    ("remainder", &["remainder", "drem"]),
+    ("remainderf", &["remainderf", "dremf"]),
+    ("lgammaf", &["lgammaf", "lgammaf_r", "__lgammaf_r"]),
+    ("lgammal", &["lgammal", "lgammal_r", "__lgammal_r"]),
+    ("log2_data", &["log2_data", "__log2_data"]),
+    ("log2f_data", &["log2f_data", "__log2f_data"]),
+    ("log_data", &["log_data", "__log_data"]),
+    ("logf_data", &["logf_data", "__logf_data"]),
+    ("pow_data", &["pow_data", "__pow_log_data"]),
+    ("powf_data", &["powf_data", "__powf_log2_data"]),
+    ("signgam", &["signgam", "__signgam"]),
+    ("sqrt_data", &["sqrt_data", "__rsqrt_tab"]),
+];
+
+fn main() {
+    let cfg = Config::from_env();
+
+    if cfg.target_env == "msvc"
+        || cfg.target_family == "wasm"
+        || cfg.target_features.iter().any(|f| f == "thumb-mode")
+    {
+        println!(
+            "cargo::warning=Musl doesn't compile with the current \
+            target {}; skipping build",
+            &cfg.target_string
+        );
+        return;
+    }
+
+    build_musl_math(&cfg);
+}
+
+#[allow(dead_code)]
+#[derive(Debug)]
+struct Config {
+    manifest_dir: PathBuf,
+    out_dir: PathBuf,
+    musl_dir: PathBuf,
+    musl_arch: String,
+    target_arch: String,
+    target_env: String,
+    target_family: String,
+    target_os: String,
+    target_string: String,
+    target_vendor: String,
+    target_features: Vec<String>,
+}
+
+impl Config {
+    fn from_env() -> Self {
+        let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
+        let target_features = env::var("CARGO_CFG_TARGET_FEATURE")
+            .map(|feats| feats.split(',').map(ToOwned::to_owned).collect())
+            .unwrap_or_default();
+
+        // Default to the `{workspace_root}/musl` if not specified
+        let musl_dir = env::var("MUSL_SOURCE_DIR")
+            .map(PathBuf::from)
+            .unwrap_or_else(|_| manifest_dir.parent().unwrap().parent().unwrap().join("musl"));
+
+        let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
+        let musl_arch = if target_arch == "x86" { "i386".to_owned() } else { target_arch.clone() };
+
+        println!("cargo::rerun-if-changed={}/c_patches", manifest_dir.display());
+        println!("cargo::rerun-if-env-changed=MUSL_SOURCE_DIR");
+        println!("cargo::rerun-if-changed={}", musl_dir.display());
+
+        Self {
+            manifest_dir,
+            out_dir: PathBuf::from(env::var("OUT_DIR").unwrap()),
+            musl_dir,
+            musl_arch,
+            target_arch,
+            target_env: env::var("CARGO_CFG_TARGET_ENV").unwrap(),
+            target_family: env::var("CARGO_CFG_TARGET_FAMILY").unwrap(),
+            target_os: env::var("CARGO_CFG_TARGET_OS").unwrap(),
+            target_string: env::var("TARGET").unwrap(),
+            target_vendor: env::var("CARGO_CFG_TARGET_VENDOR").unwrap(),
+            target_features,
+        }
+    }
+}
+
+/// Build musl math symbols to a static library
+fn build_musl_math(cfg: &Config) {
+    let musl_dir = &cfg.musl_dir;
+    assert!(
+        musl_dir.exists(),
+        "musl source is missing. it can be downloaded with ./ci/download-musl.sh"
+    );
+
+    let math = musl_dir.join("src/math");
+    let arch_dir = musl_dir.join("arch").join(&cfg.musl_arch);
+    let source_map = find_math_source(&math, cfg);
+    let out_path = cfg.out_dir.join(format!("lib{LIB_NAME}.a"));
+
+    // Run configuration steps. Usually done as part of the musl `Makefile`.
+    let obj_include = cfg.out_dir.join("musl_obj/include");
+    fs::create_dir_all(&obj_include).unwrap();
+    fs::create_dir_all(&obj_include.join("bits")).unwrap();
+    let sed_stat = Command::new("sed")
+        .arg("-f")
+        .arg(musl_dir.join("tools/mkalltypes.sed"))
+        .arg(arch_dir.join("bits/alltypes.h.in"))
+        .arg(musl_dir.join("include/alltypes.h.in"))
+        .stderr(Stdio::inherit())
+        .output()
+        .unwrap();
+    assert!(sed_stat.status.success(), "sed command failed: {:?}", sed_stat.status);
+
+    fs::write(obj_include.join("bits/alltypes.h"), sed_stat.stdout).unwrap();
+
+    let mut cbuild = cc::Build::new();
+    cbuild
+        .extra_warnings(false)
+        .warnings(false)
+        .flag_if_supported("-Wno-bitwise-op-parentheses")
+        .flag_if_supported("-Wno-literal-range")
+        .flag_if_supported("-Wno-parentheses")
+        .flag_if_supported("-Wno-shift-count-overflow")
+        .flag_if_supported("-Wno-shift-op-parentheses")
+        .flag_if_supported("-Wno-unused-but-set-variable")
+        .flag_if_supported("-std=c99")
+        .flag_if_supported("-ffreestanding")
+        .flag_if_supported("-nostdinc")
+        .define("_ALL_SOURCE", "1")
+        .opt_level(3)
+        .define(
+            "ROOT_INCLUDE_FEATURES",
+            Some(musl_dir.join("include/features.h").to_str().unwrap()),
+        )
+        // Our overrides are in this directory
+        .include(cfg.manifest_dir.join("c_patches"))
+        .include(musl_dir.join("arch").join(&cfg.musl_arch))
+        .include(musl_dir.join("arch/generic"))
+        .include(musl_dir.join("src/include"))
+        .include(musl_dir.join("src/internal"))
+        .include(obj_include)
+        .include(musl_dir.join("include"))
+        .file(cfg.manifest_dir.join("c_patches/alias.c"));
+
+    for (sym_name, src_file) in source_map {
+        // Build the source file
+        cbuild.file(src_file);
+
+        // Trickery! Redefine the symbol names to have the prefix `musl_`, which allows us to
+        // differentiate these symbols from whatever we provide.
+        if let Some((_names, syms)) =
+            MULTIPLE_SYMBOLS.iter().find(|(name, _syms)| *name == sym_name)
+        {
+            // Handle the occasional file that defines multiple symbols
+            for sym in *syms {
+                cbuild.define(sym, Some(format!("musl_{sym}").as_str()));
+            }
+        } else {
+            // If the file doesn't define multiple symbols, the file name will be the symbol
+            cbuild.define(&sym_name, Some(format!("musl_{sym_name}").as_str()));
+        }
+    }
+
+    if cfg!(windows) {
+        // On Windows we don't have a good way to check symbols, so skip that step.
+        cbuild.compile(LIB_NAME);
+        return;
+    }
+
+    let objfiles = cbuild.compile_intermediates();
+
+    // We create the archive ourselves with relocations rather than letting `cc` do it so we can
+    // encourage it to resolve symbols now. This should help avoid accidentally linking the wrong
+    // thing.
+    let stat = cbuild
+        .get_compiler()
+        .to_command()
+        .arg("-r")
+        .arg("-o")
+        .arg(&out_path)
+        .args(objfiles)
+        .status()
+        .unwrap();
+    assert!(stat.success());
+
+    println!("cargo::rustc-link-lib={LIB_NAME}");
+    println!("cargo::rustc-link-search=native={}", cfg.out_dir.display());
+
+    validate_archive_symbols(&out_path);
+}
+
+/// Build a map of `name -> path`. `name` is typically the symbol name, but this doesn't account
+/// for files that provide multiple symbols.
+fn find_math_source(math_root: &Path, cfg: &Config) -> BTreeMap<String, PathBuf> {
+    let mut map = BTreeMap::new();
+    let mut arch_dir = None;
+
+    // Locate all files and directories
+    for item in fs::read_dir(math_root).unwrap() {
+        let path = item.unwrap().path();
+        let meta = fs::metadata(&path).unwrap();
+
+        if meta.is_dir() {
+            // Make note of the arch-specific directory if it exists
+            if path.file_name().unwrap() == cfg.target_arch.as_str() {
+                arch_dir = Some(path);
+            }
+            continue;
+        }
+
+        // Skip non-source files
+        if path.extension().is_some_and(|ext| ext == "h") {
+            continue;
+        }
+
+        let sym_name = path.file_stem().unwrap();
+        map.insert(sym_name.to_str().unwrap().to_owned(), path.to_owned());
+    }
+
+    // If arch-specific versions are available, build those instead.
+    if let Some(arch_dir) = arch_dir {
+        for item in fs::read_dir(arch_dir).unwrap() {
+            let path = item.unwrap().path();
+            let sym_name = path.file_stem().unwrap();
+
+            if path.extension().unwrap() == "s" {
+                // FIXME: we never build assembly versions since we have no good way to
+                // rename the symbol (our options are probably preprocessor or objcopy).
+                continue;
+            }
+            map.insert(sym_name.to_str().unwrap().to_owned(), path);
+        }
+    }
+
+    map
+}
+
+/// Make sure we don't have something like a loose unprefixed `_cos` called somewhere, which could
+/// wind up linking to system libraries rather than the built musl library.
+fn validate_archive_symbols(out_path: &Path) {
+    const ALLOWED_UNDEF_PFX: &[&str] = &[
+        // PIC and arch-specific
+        ".TOC",
+        "_GLOBAL_OFFSET_TABLE_",
+        "__x86.get_pc_thunk",
+        // gcc/compiler-rt/compiler-builtins symbols
+        "__add",
+        "__aeabi_",
+        "__div",
+        "__eq",
+        "__extend",
+        "__fix",
+        "__float",
+        "__gcc_",
+        "__ge",
+        "__gt",
+        "__le",
+        "__lshr",
+        "__lt",
+        "__mul",
+        "__ne",
+        "__stack_chk_fail",
+        "__stack_chk_guard",
+        "__sub",
+        "__trunc",
+        "__undef",
+        // string routines
+        "__bzero",
+        "bzero",
+        // FPENV interfaces
+        "feclearexcept",
+        "fegetround",
+        "feraiseexcept",
+        "fesetround",
+        "fetestexcept",
+    ];
+
+    // List global undefined symbols
+    let out =
+        Command::new("nm").arg("-guj").arg(out_path).stderr(Stdio::inherit()).output().unwrap();
+
+    let undef = str::from_utf8(&out.stdout).unwrap();
+    let mut undef = undef.lines().collect::<Vec<_>>();
+    undef.retain(|sym| {
+        // Account for file formats that add a leading `_`
+        !ALLOWED_UNDEF_PFX.iter().any(|pfx| sym.starts_with(pfx) || sym[1..].starts_with(pfx))
+    });
+
+    assert!(undef.is_empty(), "found disallowed undefined symbols: {undef:#?}");
+
+    // Find any symbols that are missing the `_musl_` prefix`
+    let out =
+        Command::new("nm").arg("-gUj").arg(out_path).stderr(Stdio::inherit()).output().unwrap();
+
+    let defined = str::from_utf8(&out.stdout).unwrap();
+    let mut defined = defined.lines().collect::<Vec<_>>();
+    defined.retain(|sym| {
+        !(sym.starts_with("_musl_")
+            || sym.starts_with("musl_")
+            || sym.starts_with("__x86.get_pc_thunk"))
+    });
+
+    assert!(defined.is_empty(), "found unprefixed symbols: {defined:#?}");
+}