Thorough tests for saturatedAdd and mulDiv255Round.

src/core/SkNx.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkNx_DEFINED
 #define SkNx_DEFINED
 
@@ skipping 166 matching lines @@
 public:
     SkNi() {}
     explicit SkNi(T val) : fVal(val) {}
     static SkNi Load(const T vals[1]) { return SkNi(vals[0]); }
 
     void store(T vals[1]) const { vals[0] = fVal; }
 
     SkNi saturatedAdd(const SkNi& o) const {
         SkASSERT((T)(~0) > 0); // TODO: support signed T
         T sum = fVal + o.fVal;
-        return SkNi(sum > fVal ? sum : (T)(~0));
+        return SkNi(sum < fVal ? (T)(~0) : sum);
     }
 
     SkNi operator + (const SkNi& o) const { return SkNi(fVal + o.fVal); }
     SkNi operator - (const SkNi& o) const { return SkNi(fVal - o.fVal); }
     SkNi operator * (const SkNi& o) const { return SkNi(fVal * o.fVal); }
 
     SkNi operator << (int bits) const { return SkNi(fVal << bits); }
     SkNi operator >> (int bits) const { return SkNi(fVal >> bits); }
 
     static SkNi Min(const SkNi& a, const SkNi& b) { return SkNi(SkTMin(a.fVal, b.fVal)); }
@@ skipping 84 matching lines @@
 typedef SkNi<4,  uint16_t> Sk4h;
 typedef SkNi<8,  uint16_t> Sk8h;
 typedef SkNi<16, uint16_t> Sk16h;
 
 typedef SkNi<16, uint8_t> Sk16b;
 
 typedef SkNi<4,  int32_t> Sk4i;
 typedef SkNi<4, uint32_t> Sk4u;
 
 #endif//SkNx_DEFINED