Updated documentation, x86 feature detection testing, and removed LLVM 9 exclusive features

Updated the added documentation in llvm_util.rs to note which copies of LLVM need to be inspected.
Removed avx512bf16 and avx512vp2intersect because they are unsupported before LLVM 9 with the build with external LLVM 8 being supported
Re-introduced detection testing previously removed for un-requestable features tsc and mmx

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -139,6 +139,11 @@ pub fn time_trace_profiler_finish(file_name: &str) {
 // array, leading to crashes.
 // To find a list of LLVM's names, check llvm-project/llvm/include/llvm/Support/*TargetParser.def
 // where the * matches the architecture's name
+// Beware to not use the llvm github project for this, but check the git submodule
+// found in src/llvm-project
+// Though note that Rust can also be build with an external precompiled version of LLVM
+// which might lead to failures if the oldest tested / supported LLVM version
+// doesn't yet support the relevant intrinsics
 pub fn to_llvm_feature<'a>(sess: &Session, s: &'a str) -> &'a str {
     let arch = if sess.target.arch == "x86_64" { "x86" } else { &*sess.target.arch };
     match (arch, s) {

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -55,7 +55,7 @@ const X86_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     ("aes", None),
     ("avx", None),
     ("avx2", None),
-    ("avx512bf16", Some(sym::avx512_target_feature)),
+    //("avx512bf16", Some(sym::avx512_target_feature)), // this seems to be unsupported by the supported build with external LLVM 8, LLVM 9 should be sufficient though
     ("avx512bitalg", Some(sym::avx512_target_feature)),
     ("avx512bw", Some(sym::avx512_target_feature)),
     ("avx512cd", Some(sym::avx512_target_feature)),
@@ -70,7 +70,7 @@ const X86_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     ("avx512vbmi2", Some(sym::avx512_target_feature)),
     ("avx512vl", Some(sym::avx512_target_feature)),
     ("avx512vnni", Some(sym::avx512_target_feature)),
-    ("avx512vp2intersect", Some(sym::avx512_target_feature)),
+    //("avx512vp2intersect", Some(sym::avx512_target_feature)), // this seems to be unsupported by the supported build with external LLVM 8, LLVM 9 should be sufficient though
     ("avx512vpclmulqdq", Some(sym::avx512_target_feature)),
     ("avx512vpopcntdq", Some(sym::avx512_target_feature)),
     ("bmi1", None),

library/std/tests/run-time-detect.rs

@@ -54,6 +54,13 @@ fn powerpc64_linux() {
 #[test]
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 fn x86_all() {
+    // the below is the set of features we can test at runtime, but don't actually
+    // use to gate anything and are thus not part of the X86_ALLOWED_FEATURES list
+
+    println!("abm: {:?}", is_x86_feature_detected!("abm")); // this is a synonym for lzcnt but we test it anyways
+    println!("mmx: {:?}", is_x86_feature_detected!("mmx"));
+    println!("tsc: {:?}", is_x86_feature_detected!("tsc"));
+
     // the below is in alphabetical order and matches
     // the order of X86_ALLOWED_FEATURES in rustc_codegen_ssa's target_features.rs