Autodetect CPU target features from Cargo

Cargo.toml

@@ -24,7 +24,7 @@ cc = "1"
 glob = "0.3"
 
 [features]
-default = ["std", "parallel", "neon"]
+default = ["std", "parallel"]
 std = []
 parallel = ["cc/parallel"]
 neon = []                      # ARM NEON SIMD (will crash on ARM CPUs without it)

README.md

@@ -4,7 +4,7 @@
 
 libwebp is built with the [`cc`](//lib.rs/cc) crate. It needs a C compiler, but `cmake` is not used.
 
-Set `TARGET_CPU` env var to `native` or your desired CPU architecture to optimize the C code for it.
+Set `RUSTFLAGS="-Ctarget-cpu=native"` or your desired CPU architecture to optimize the C code for it.
 
 ## Usage
 
@@ -15,19 +15,16 @@ Add the following to the `Cargo.toml` in your project:
 libwebp-sys = "0.9"
 ```
 
-or to require newer CPUs with SIMD support:
-
-```toml
-[dependencies]
-libwebp-sys = { version = "0.9", features = ["avx2", "sse41", "neon"] }
-```
-
 or to require `no_std` support:
 
 ```toml
-libwebp-sys = { version = "0.9", default-features = false, features = ["parallel", "neon"] }
+libwebp-sys = { version = "0.9", default-features = false, features = ["parallel"] }
 ```
 
+The `neon`, `sse41` and `avx2` feature flags can be set to force support for Neon, SSE 4.1 and AVX2
+respectively, but this is usually unnecessary as it can be set through
+`-Ctarget-feature` (e.g. `RUSTFLAGS="-Ctarget-feature=avx2"`) as well.
+
 ## Examples
 
 ### Encode

build.rs

@@ -28,10 +28,8 @@ fn main() {
         cc.file(manifest_dir.join(f));
     }
 
-    for f in glob::glob("vendor/sharpyuv/**/*.c")
-        .expect("glob vender/src failed")
-        .flatten()
-    {
+    for f in glob::glob("vendor/sharpyuv/**/*.c").expect("glob vendor/src failed") {
+        let f = f.expect("glob iteration vendor/src failed");
         sharpyuv_build.file(manifest_dir.join(f));
     }
     sharpyuv_build.compile("sharpyuv");
@@ -48,25 +46,28 @@ fn setup_build(build: &mut cc::Build, include_dir: &PathBuf) {
         build.flag("-D_CRT_SECURE_NO_WARNINGS");
     }
 
-    if let Ok(target_cpu) = env::var("TARGET_CPU") {
-        build.flag_if_supported(&format!("-march={}", target_cpu));
-    }
+    let target_arch = env::var("CARGO_CFG_TARGET_ARCH").expect("CARGO_CFG_TARGET_ARCH");
+    let target_features = env::var("CARGO_CFG_TARGET_FEATURE").expect("CARGO_CFG_TARGET_FEATURE");
+    let has_feature = |f: &str| target_features.split(',').any(|feature| feature == f);
+
+    let target_cpu = env::var("TARGET_CPU").ok();
+    let target_cpu = target_cpu.as_deref().unwrap_or(&*target_arch);
+    build.flag_if_supported(format!("-march={}", target_cpu));
 
-    let target = env::var("CARGO_CFG_TARGET_ARCH").expect("CARGO_CFG_TARGET_ARCH");
-    match target.as_str() {
+    match target_arch.as_str() {
         "x86_64" | "i686" => {
             build.define("WEBP_HAVE_SSE2", Some("1"));
-            if cfg!(feature = "sse41") {
+            if cfg!(feature = "sse41") || has_feature("sse4.1") {
                 build.define("WEBP_HAVE_SSE41", Some("1"));
                 build.flag_if_supported("-msse4.1");
             }
-            if cfg!(feature = "avx2") {
+            if cfg!(feature = "avx2") || has_feature("avx2") {
                 build.define("WEBP_HAVE_AVX2", Some("1"));
                 build.flag_if_supported("-mavx2");
             }
         }
         "aarch64" => {
-            if cfg!(feature = "neon") {
+            if cfg!(feature = "neon") || has_feature("neon") {
                 build.define("WEBP_HAVE_NEON", Some("1"));
             }