Get rid of a variable shift on the tcmalloc alloc fast path.

tcmalloc/sizemap.h

@@ -141,6 +141,19 @@ class SizeMap {
       return true;
     } else if (s <= kMaxSize) {
       idx = ((s + 127 + 120 * 128) >> 7);
+
+      // TODO(b/64294063): Add a dummy statement to keep the tail of the fast
+      // and the slow paths from being deduplicated, turning the shifts into
+      // a variable-width shift (shr reg, cl on x86); these are especially
+      // bad on Skylake, where they require three µops that often run in
+      // three successive cycles. (If we are compiling with BMI2,
+      // the compiler will use the much cheaper SHRX instruction, and
+      // the problem is more modest.)
+      //
+      // See SLOW_PATH_BARRIER() in tcmalloc.cc for more information
+      // about this technique.
+      asm volatile("");
+
       return true;
     }
     return false;