int128: optimize and add test cases

For add, the carry only requires checking one of the arguments.
For sub and neg, we can similarly optimize computation of the
carry.

For ge, we can just do lexicographic order.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 17 insertions, 8 deletions

diff --git a/include/qemu/int128.h b/include/qemu/int128.h
index bfe7678..9ed47aa 100644
--- a/include/qemu/int128.h
+++ b/include/qemu/int128.h
@@ -1,6 +1,10 @@
 #ifndef INT128_H
 #define INT128_H
 
+#include <assert.h>
+#include <stdint.h>
+#include <stdbool.h>
+
 typedef struct Int128 Int128;
 
 struct Int128 {
@@ -55,21 +59,26 @@ static inline Int128 int128_rshift(Int128 a, int n)
 
 static inline Int128 int128_add(Int128 a, Int128 b)
 {
-    Int128 r = { a.lo + b.lo, a.hi + b.hi };
-    r.hi += (r.lo < a.lo) || (r.lo < b.lo);
-    return r;
+    uint64_t lo = a.lo + b.lo;
+
+    /* a.lo <= a.lo + b.lo < a.lo + k (k is the base, 2^64).  Hence,
+     * a.lo + b.lo >= k implies 0 <= lo = a.lo + b.lo - k < a.lo.
+     * Similarly, a.lo + b.lo < k implies a.lo <= lo = a.lo + b.lo < k.
+     *
+     * So the carry is lo < a.lo.
+     */
+    return (Int128) { lo, (uint64_t)a.hi + b.hi + (lo < a.lo) };
 }
 
 static inline Int128 int128_neg(Int128 a)
 {
-    a.lo = ~a.lo;
-    a.hi = ~a.hi;
-    return int128_add(a, int128_one());
+    uint64_t lo = -a.lo;
+    return (Int128) { lo, ~(uint64_t)a.hi + !lo };
 }
 
 static inline Int128 int128_sub(Int128 a, Int128 b)
 {
-    return int128_add(a, int128_neg(b));
+    return (Int128){ a.lo - b.lo, a.hi - b.hi - (a.lo < b.lo) };
 }
 
 static inline bool int128_nonneg(Int128 a)
@@ -89,7 +98,7 @@ static inline bool int128_ne(Int128 a, Int128 b)
 
 static inline bool int128_ge(Int128 a, Int128 b)
 {
-    return int128_nonneg(int128_sub(a, b));
+    return a.hi > b.hi || (a.hi == b.hi && a.lo >= b.lo);
 }
 
 static inline bool int128_lt(Int128 a, Int128 b)