Expose the features computed from LLVM in cfg!

Instead of relying on the features explicitly passed through the
command line, compute them from the LLVM `TargetMachine`.

src/librustc_driver/target_features.rs

@@ -9,79 +9,47 @@
 // except according to those terms.
 
 use syntax::{ast, attr};
+use llvm::LLVMRustHasFeature;
 use rustc::session::Session;
+use rustc_trans::back::write::create_target_machine;
 use syntax::parse::token::InternedString;
 use syntax::parse::token::intern_and_get_ident as intern;
+use libc::c_char;
 
 /// Add `target_feature = "..."` cfgs for a variety of platform
 /// specific features (SSE, NEON etc.).
 ///
-/// This uses a scheme similar to that employed by clang: reimplement
-/// the target feature knowledge. *Theoretically* we could query LLVM
-/// since that has perfect knowledge about what things are enabled in
-/// code-generation, however, it is extremely non-obvious how to do
-/// this successfully. Each platform defines a subclass of a
-/// SubtargetInfo, which knows all this information, but the ways to
-/// query them do not seem to be public.
+/// This is performed by checking whether a whitelisted set of
+/// features is available on the target machine, by querying LLVM.
 pub fn add_configuration(cfg: &mut ast::CrateConfig, sess: &Session) {
-    let tf = InternedString::new("target_feature");
-    macro_rules! fillout {
-        ($($func: ident, $name: expr;)*) => {{
-            $(if $func(sess) {
-                cfg.push(attr::mk_name_value_item_str(tf.clone(), intern($name)))
-            })*
-        }}
-    }
-    fillout! {
-        has_sse, "sse";
-        has_sse2, "sse2";
-        has_sse3, "sse3";
-        has_ssse3, "ssse3";
-        has_sse41, "sse4.1";
-        has_sse42, "sse4.2";
-        has_avx, "avx";
-        has_avx2, "avx2";
-        has_neon, "neon";
-        has_vfp, "vfp";
-    }
-}
+    let target_machine = create_target_machine(sess);
 
+    let arm_whitelist = [
+        "neon\0",
+        "vfp\0",
+    ];
 
-fn features_contain(sess: &Session, s: &str) -> bool {
-    sess.target.target.options.features.contains(s) || sess.opts.cg.target_feature.contains(s)
-}
+    let x86_whitelist = [
+        "avx\0",
+        "avx2\0",
+        "sse\0",
+        "sse2\0",
+        "sse3\0",
+        "sse4.1\0",
+        "sse4.2\0",
+        "ssse3\0",
+    ];
 
-pub fn has_sse(sess: &Session) -> bool {
-    features_contain(sess, "+sse") || has_sse2(sess)
-}
-pub fn has_sse2(sess: &Session) -> bool {
-    // x86-64 requires at least SSE2 support
-    sess.target.target.arch == "x86_64" || features_contain(sess, "+sse2") || has_sse3(sess)
-}
-pub fn has_sse3(sess: &Session) -> bool {
-    features_contain(sess, "+sse3") || has_ssse3(sess)
-}
-pub fn has_ssse3(sess: &Session) -> bool {
-    features_contain(sess, "+ssse3") || has_sse41(sess)
-}
-pub fn has_sse41(sess: &Session) -> bool {
-    features_contain(sess, "+sse4.1") || has_sse42(sess)
-}
-pub fn has_sse42(sess: &Session) -> bool {
-    features_contain(sess, "+sse4.2") || has_avx(sess)
-}
-pub fn has_avx(sess: &Session) -> bool {
-    features_contain(sess, "+avx") || has_avx2(sess)
-}
-pub fn has_avx2(sess: &Session) -> bool {
-    features_contain(sess, "+avx2")
-}
+    let whitelist = match &*sess.target.target.arch {
+        "arm" => &arm_whitelist[..],
+        "x86" | "x86_64" => &x86_whitelist[..],
+        _ => &[][..],
+    };
 
-pub fn has_neon(sess: &Session) -> bool {
-    // AArch64 requires NEON support
-    sess.target.target.arch == "aarch64" || features_contain(sess, "+neon")
-}
-pub fn has_vfp(sess: &Session) -> bool {
-    // AArch64 requires VFP support
-    sess.target.target.arch == "aarch64" || features_contain(sess, "+vfp")
+    let tf = InternedString::new("target_feature");
+    for feat in whitelist {
+        if unsafe { LLVMRustHasFeature(target_machine, feat.as_ptr() as *const c_char) } {
+            cfg.push(attr::mk_name_value_item_str(tf.clone(), intern(feat)))
+        }
+    }
 }