build: only enable simd on targets which feature SSE

While odd, it is possible for x86_64 to not have SSE enabled. In that
case, we want to avoid enabling the explicit SIMD options, as it will
result in a compilation failure. We address this by checking the CPU
target features at compile time. If SSE isn't in them, then we don't
compile the SIMD stuff.

We don't need to do this for SSE4.2 or AVX since they are selected at
runtime. For SSE2, the code assumes it exists for x86_64.

Fixes #57, Closes #77

build.rs

@@ -5,12 +5,19 @@ fn main() {
     enable_libc();
 }
 
-// This adds various simd cfgs if this compiler supports it.
+// This adds various simd cfgs if this compiler and target support it.
 //
 // This can be disabled with RUSTFLAGS="--cfg memchr_disable_auto_simd", but
 // this is generally only intended for testing.
+//
+// On targets which don't feature SSE2, this is disabled, as LLVM wouln't know
+// how to work with SSE2 operands. Enabling SSE4.2 and AVX on SSE2-only targets
+// is not a problem. In that case, the fastest option will be chosen at
+// runtime.
 fn enable_simd_optimizations() {
-    if is_env_set("CARGO_CFG_MEMCHR_DISABLE_AUTO_SIMD") {
+    if is_env_set("CARGO_CFG_MEMCHR_DISABLE_AUTO_SIMD")
+        || !target_has_feature("sse2")
+    {
         return;
     }
     println!("cargo:rustc-cfg=memchr_runtime_simd");
@@ -59,3 +66,9 @@ fn is_feature_set(name: &str) -> bool {
 fn is_env_set(name: &str) -> bool {
     env::var_os(name).is_some()
 }
+
+fn target_has_feature(feature: &str) -> bool {
+    env::var("CARGO_CFG_TARGET_FEATURE")
+        .map(|features| features.contains(feature))
+        .unwrap_or(false)
+}