inline.h: Comments fixed and added

(intended to be squashed before merge)

Author: t-a-k

inline.h

@@ -3416,8 +3416,8 @@ S_lossless_NV_to_IV(const NV nv, IV *ivp)
 
 /* Define IV_*_OVERFLOW_IS_EXPENSIVE below to nonzero value
  * if strict overflow checks are too expensive
- * (for example, for CPUs that has no hardware overflow detection flag).
- * If these macro has nonzero value, or overflow-checking compiler intrinsics
+ * (for example, for CPUs that have no hardware overflow detection flags).
+ * If these macros have nonzero value, or overflow-checking compiler intrinsics
  * are not available, good-old heuristics (with some false positives)
  * will be used.  */
 #  ifndef IV_ADD_SUB_OVERFLOW_IS_EXPENSIVE
@@ -3466,6 +3466,10 @@ S_iv_add_may_overflow (IV il, IV ir, IV *const result)
 PERL_STATIC_INLINE bool
 S_uv_add_overflow (UV auv, UV buv, UV *const result)
 {
+    /* (auv + buv) < auv means that the addition wrapped around,
+       i.e. overflowed.  Note that unsigned integer overflow is well-defined
+       in standard C to wrap around, in constrast to signed integer overflow
+       whose behaviour is undefined.  */
     return (*result = auv + buv) < auv;
 }
 #  endif