Fix ASAN error for wbmp swizzles

src/codec/SkSwizzler.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkSwizzler.h"
@@ skipping 25 matching lines @@
     for (i = 0; i < bytesInRow; i++) {
         U8CPU currByte = src[i];
         for (int j = 0; j < 8; j++) {
             dst[j] = ((currByte >> (7 - j)) & 1) ? GRAYSCALE_WHITE : GRAYSCALE_BLACK;
         }
         dst += 8;
     }
 
     // Finish the remaining bits
     width &= 7;
-    U8CPU currByte = src[i];

[msarett, 2015/08/05 20:39:08]

This reads out of bounds memory when width is a multiple of 8.  It's unused, but
ASAN complains anyway.

-    for (int j = 0; j < width; j++) {
-        dst[j] = ((currByte >> 7) & 1) ? GRAYSCALE_WHITE : GRAYSCALE_BLACK;
-        currByte <<= 1;
+    if (width > 0) {
+        U8CPU currByte = src[i];
+        for (int j = 0; j < width; j++) {
+            dst[j] = ((currByte >> 7) & 1) ? GRAYSCALE_WHITE : GRAYSCALE_BLACK;
+            currByte <<= 1;
+        }
     }
     return SkSwizzler::kOpaque_ResultAlpha;
 }
 
 #undef GRAYSCALE_BLACK
 #undef GRAYSCALE_WHITE
 
 static SkSwizzler::ResultAlpha swizzle_bit_to_index(
         void* SK_RESTRICT dstRow, const uint8_t* SK_RESTRICT src, int width,
         int /*bitsPerPixel*/, const SkPMColor* /*ctable*/) {
     uint8_t* SK_RESTRICT dst = (uint8_t*) dstRow;
 
     // Determine how many full bytes are in the row
     int bytesInRow = width >> 3;
     int i;
     for (i = 0; i < bytesInRow; i++) {
         U8CPU currByte = src[i];
         for (int j = 0; j < 8; j++) {
             dst[j] = (currByte >> (7 - j)) & 1;
         }
         dst += 8;
     }
 
     // Finish the remaining bits
     width &= 7;
-    U8CPU currByte = src[i];
-    for (int j = 0; j < width; j++) {
-        dst[j] = ((currByte >> 7) & 1);
-        currByte <<= 1;
+    if (width > 0) {
+        U8CPU currByte = src[i];
+        for (int j = 0; j < width; j++) {
+            dst[j] = ((currByte >> 7) & 1);
+            currByte <<= 1;
+        }
     }
     return SkSwizzler::kOpaque_ResultAlpha;
 }
 
 static SkSwizzler::ResultAlpha swizzle_bit_to_n32(
         void* SK_RESTRICT dstRow, const uint8_t* SK_RESTRICT src, int width,
         int /*bitsPerPixel*/, const SkPMColor* /*ctable*/) {
     SkPMColor* SK_RESTRICT dst = (SkPMColor*) dstRow;
 
     // Determine how many full bytes are in the row
     int bytesInRow = width >> 3;
     int i;
     for (i = 0; i < bytesInRow; i++) {
         U8CPU currByte = src[i];
         for (int j = 0; j < 8; j++) {
             dst[j] = ((currByte >> (7 - j)) & 1) ? SK_ColorWHITE : SK_ColorBLACK;
         }
         dst += 8;
     }
 
     // Finish the remaining bits
     width &= 7;
-    U8CPU currByte = src[i];
-    for (int j = 0; j < width; j++) {
-        dst[j] = ((currByte >> 7) & 1) ? SK_ColorWHITE : SK_ColorBLACK;
-        currByte <<= 1;
+    if (width > 0) {
+        U8CPU currByte = src[i];
+        for (int j = 0; j < width; j++) {
+            dst[j] = ((currByte >> 7) & 1) ? SK_ColorWHITE : SK_ColorBLACK;
+            currByte <<= 1;
+        }
     }
     return SkSwizzler::kOpaque_ResultAlpha;
 }
 
 // kIndex1, kIndex2, kIndex4
 
 static SkSwizzler::ResultAlpha swizzle_small_index_to_index(
         void* SK_RESTRICT dstRow, const uint8_t* SK_RESTRICT src, int width,
         int bitsPerPixel, const SkPMColor ctable[]) {
 
@@ skipping 465 matching lines @@
             // bits of SK_ColorBLACK are identical to the 565 representation
             // for black.
             memset(dstStartRow, (uint16_t) colorOrIndex, bytesToFill);
             break;
         default:
             SkCodecPrintf("Error: Unsupported dst color type for fill().  Doing nothing.\n");
             SkASSERT(false);
             break;
     }
 }