use 64-bit compare-and-swap for versioned pointers (48-bit ptr, 16-bit ver),

instead of 128-bit cmpxchg16. minor speedup, even though the profiler said that cmpxchg16 was the top offender.

use 64-bit compare-and-swap for versioned pointers (48-bit ptr, 16-bit ver),
05f41512 · Nickolai Zeldovich · 412c2695 · 05f41512 · 05f41512
--- a/include/kalloc.hh
+++ b/include/kalloc.hh
 #include "atomic.hh"

 template<class T>
-struct vptr {
-  u128 _a;
-  T ptr() const { return (T) (_a & 0xffffffffffffffffULL); }
+struct vptr64 {
+  typedef u128 __inttype;
+  typedef T __ptrtype;
+  __inttype _a;
+
+  T ptr() const { return (T) iptr(); }
+  u64 iptr() const { return _a & 0xffffffffffffffffULL; }
  u64 v() const { return _a >> 64; }

-  vptr(T p, u64 v) : _a((((u128)v)<<64) | (u64) p) {}
-  vptr(u128 a) : _a(a) {}
+  vptr64(T p, u64 v) : _a((((u128)v)<<64) | (u64) p) {}
+  vptr64(u128 a) : _a(a) {}
 };

 template<class T>
+struct vptr48 {
+  typedef u64 __inttype;
+  typedef T __ptrtype;
+  __inttype _a;
+
+  T ptr() const {
+    u64 i = iptr();
+    if (i & (1ULL << 47))
+      i += 0xffffULL << 48;
+    return (T) i;
+  }
+  u64 iptr() const { return _a & 0xffffffffffffULL; }
+  u16 v() const { return _a >> 48; }
+
+  vptr48(T p, u16 v) : _a((((u64)v)<<48) | (((u64)p) & 0xffffffffffffULL)) {}
+  vptr48(u64 a) : _a(a) {}
+};
+
+template<class VPTR>
 class versioned {
 private:
-  std::atomic<u128> _a;
+  std::atomic<typename VPTR::__inttype> _a;

 public:
-  vptr<T> load() { return vptr<T>(_a.load()); }
-  bool compare_exchange(const vptr<T> &expected, T desired) {
-    vptr<T> n(desired, expected.v());
+  VPTR load() { return VPTR(_a.load()); }
+  bool compare_exchange(const VPTR &expected, typename VPTR::__ptrtype desired) {
+    VPTR n(desired, expected.v());
    return cmpxch(&_a, expected._a, n._a);
  }
 };
@@ -31,7 +54,7 @@ struct kmem {
  char name[MAXNAME];
  u64 size;
  u64 ninit;
-  versioned<run*> freelist;
+  versioned<vptr48<run*>> freelist;
  std::atomic<u64> nfree;
 } __mpalign__;


--- a/kernel/kmalloc.cc
+++ b/kernel/kmalloc.cc
@@ -19,7 +19,7 @@ struct header {
 };

 struct freelist {
-  versioned<header*> buckets[KMMAX+1];
+  versioned<vptr48<header*>> buckets[KMMAX+1];
  char name[MAXNAME];
 };