start, specify CPU target

decnumber-sys/Cargo.toml

@@ -20,3 +20,4 @@ version-sync = "0.9"
 
 [build-dependencies]
 cc = "1.0.50"
+rustflags = "0.1.4"

decnumber-sys/build.rs

@@ -7,15 +7,42 @@ use std::env;
 use std::fs;
 use std::path::{Path, PathBuf};
 
+/// Extracts the `target-cpu` from `CARGO_ENCODED_RUSTFLAGS`.
+fn target_cpu() -> Option<String> {
+    rustflags::from_env().find_map(|flag| match flag {
+        rustflags::Flag::Codegen { opt, value } if opt == "target-cpu" => value,
+        _ => None,
+    })
+}
+
 fn main() {
+    let target = env::var("TARGET").unwrap();
     let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
 
     let declitend = if cfg!(target_endian = "little") {
         "1"
     } else {
         "0"
     };
-    cc::Build::new()
+    let mut build = cc::Build::new();
+
+    if let Some(target_cpu) = target_cpu() {
+        if target.contains("x86_64") {
+            build.flag_if_supported(&format!("-march={target_cpu}"));
+
+            // The x86-64-vX targets are generic and do not support tuning.
+            if !target_cpu.starts_with("x86-64-v") {
+                build.flag_if_supported(&format!("-mtune={target_cpu}"));
+            }
+        } else if target.contains("aarch64") {
+            // The -mcpu argument is deprecated for x86 but supported for AArch64.
+            //
+            // See: https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html
+            build.flag_if_supported(&format!("-mcpu={target_cpu}"));
+        }
+    }
+
+    build
         .define("DECLITEND", Some(declitend))
         .file("decnumber/decimal128.c")
         .file("decnumber/decimal64.c")