[runtime] fix tbm detection

Author: gnzlbg

src/x86/runtime.rs

@@ -210,9 +210,6 @@ fn detect_features() -> usize {
     if test_bit(proc_info_ecx, 20) {
         value = set_bit(value, __Feature::sse4_2 as u32);
     }
-    if test_bit(proc_info_ecx, 21) {
-        value = set_bit(value, __Feature::tbm as u32);
-    }
     if test_bit(proc_info_ecx, 23) {
         value = set_bit(value, __Feature::popcnt as u32);
     }
@@ -257,9 +254,40 @@ fn detect_features() -> usize {
         }
     }
 
+    if test_bit(proc_info_ecx, 21) && is_amd() {
+        value = set_bit(value, __Feature::tbm as u32);
+    }
+
     value
 }
 
+/// Is this an AMD CPU?
+#[inline(never)]
+fn is_amd() -> bool {
+    let ebx: u32;
+    let edx: u32;
+    let ecx: u32;
+    // EAX = 0: Basic Information. The vendor ID is stored in 12 u8 ascii
+    // chars, returned in EBX, EDX, and ECX (in that order):
+    unsafe {
+        asm!("cpuid"
+             : "={ebx}"(ebx), "={ecx}"(ecx), "={edx}"(edx)
+             : "{eax}"(0x0000_0000_u32), "{ecx}"(0 as u32)
+             : : "volatile" );
+    }
+    let ebx: [u8; 4] = unsafe { ::std::mem::transmute(ebx) };
+    let edx: [u8; 4] = unsafe { ::std::mem::transmute(edx) };
+    let ecx: [u8; 4] = unsafe { ::std::mem::transmute(ecx) };
+    #[cfg_attr(rustfmt, rustfmt_skip)]
+    let vendor_id = [
+        ebx[0], ebx[1], ebx[2], ebx[3],
+        ecx[0], ecx[1], ecx[2], ecx[3],
+        edx[0], edx[1], edx[2], edx[3],
+    ];
+    let vendor_id_amd = b"AuthenticAMD";
+    vendor_id == *vendor_id_amd
+}
+
 /// This global variable is a bitset used to cache the features supported by
 /// the CPU.
 static FEATURES: AtomicUsize = AtomicUsize::new(::std::usize::MAX);