Avoid dec = -dec overflow when appending most negative signed integers.

src/core/SkString.cpp

 
 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "SkString.h"
@@ skipping 72 matching lines @@
     SkASSERT(p >= buffer);
     char* stop = buffer + sizeof(buffer);
     while (p < stop) {
         *string++ = *p++;
     }
     SkASSERT(string - start <= SkStrAppendU32_MaxSize);
     return string;
 }
 
 char* SkStrAppendS32(char string[], int32_t dec) {
+    uint32_t udec = dec;
     if (dec < 0) {
         *string++ = '-';
-        dec = -dec;
+        udec = ~udec + 1;  // udec = -udec, but silences some warnings that are trying to be helpful
     }
-    return SkStrAppendU32(string, static_cast<uint32_t>(dec));
+    return SkStrAppendU32(string, udec);
 }
 
 char* SkStrAppendU64(char string[], uint64_t dec, int minDigits) {
     SkDEBUGCODE(char* start = string;)
 
     char    buffer[SkStrAppendU64_MaxSize];
     char*   p = buffer + sizeof(buffer);
 
     do {
         *--p = SkToU8('0' + (int32_t) (dec % 10));
         dec /= 10;
         minDigits--;
     } while (dec != 0);
 
     while (minDigits > 0) {
         *--p = '0';
         minDigits--;
     }
 
     SkASSERT(p >= buffer);
     size_t cp_len = buffer + sizeof(buffer) - p;
     memcpy(string, p, cp_len);
     string += cp_len;
 
     SkASSERT(string - start <= SkStrAppendU64_MaxSize);
     return string;
 }
 
 char* SkStrAppendS64(char string[], int64_t dec, int minDigits) {
+    uint64_t udec = dec;
     if (dec < 0) {
         *string++ = '-';
-        dec = -dec;
+        udec = ~udec + 1;  // udec = -udec, but silences some warnings that are trying to be helpful
     }
-    return SkStrAppendU64(string, static_cast<uint64_t>(dec), minDigits);
+    return SkStrAppendU64(string, udec, minDigits);
 }
 
 char* SkStrAppendFloat(char string[], float value) {
     // since floats have at most 8 significant digits, we limit our %g to that.
     static const char gFormat[] = "%.8g";
     // make it 1 larger for the terminating 0
     char buffer[SkStrAppendScalar_MaxSize + 1];
     int len = SNPRINTF(buffer, sizeof(buffer), gFormat, value);
     memcpy(string, buffer, len);
     SkASSERT(len <= SkStrAppendScalar_MaxSize);
@@ skipping 512 matching lines @@
         const size_t len = strcspn(str, delimiters);
         out->push_back().set(str, len);
         str += len;
         // Skip any delimiters.
         str += strspn(str, delimiters);
     }
 }
 
 #undef VSNPRINTF
 #undef SNPRINTF