Clean up comments in SkSwizzler

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 "SkOpts.h"
@@ skipping 911 matching lines @@
     , fSrcWidth(srcWidth)
     , fDstWidth(dstWidth)
     , fSwizzleWidth(srcWidth)
     , fAllocatedWidth(dstWidth)
     , fSampleX(1)
     , fSrcBPP(srcBPP)
     , fDstBPP(dstBPP)
 {}
 
 int SkSwizzler::onSetSampleX(int sampleX) {
-    SkASSERT(sampleX > 0); // Surely there is an upper limit? Should there be

[msarett, 2016/02/10 13:42:59]

I don't think there is an upper limit.  If they ask for a ridiculously large
sampleSize, they'll get a 1x1 image.

[scroggo, 2016/02/10 16:06:09]

sgtm

-                           // way to report failure?
+    SkASSERT(sampleX > 0);
+
     fSampleX = sampleX;
     fSrcOffsetUnits = (get_start_coord(sampleX) + fSrcOffset) * fSrcBPP;
     fDstOffsetBytes = (fDstOffset / sampleX) * fDstBPP;
     fSwizzleWidth = get_scaled_dimension(fSrcWidth, sampleX);
     fAllocatedWidth = get_scaled_dimension(fDstWidth, sampleX);
 
-    // The optimized swizzler routines do not (yet) support sampling.
+    // The optimized swizzler functions do not support sampling.  Sampled swizzles
+    // are already fast because they skip pixels.  We haven't seen a situation
+    // where speeding up sampling has a significant impact on total decode time.
     if (1 == fSampleX && fFastProc) {
         fActualProc = fFastProc;
     } else {
         fActualProc = fSlowProc;
     }
 
     return fAllocatedWidth;
 }
 
 void SkSwizzler::swizzle(void* dst, const uint8_t* SK_RESTRICT src) {
     SkASSERT(nullptr != dst && nullptr != src);
     fActualProc(SkTAddOffset<void>(dst, fDstOffsetBytes), src, fSwizzleWidth, fSrcBPP,
             fSampleX * fSrcBPP, fSrcOffsetUnits, fColorTable);
 }