Add a second multi-thread method

method 1 grouping: [parse + reconstruction] // [filtering + output]
method 2 grouping: [parse] // [reconstruction+filtering + output]

Depending on some heuristics (see VP8ThreadMethod()), we
can pick one of the other when -mt flag (or option.use_threads)
is selected.

Conservatively, we always use method #2 for now until the heuristic
is refined (so, timing should be the same the before this patch)

+ replace 'use_threads' by 'mt_method'
+ define MIN_WIDTH_FOR_THREADS constant
+ fix comment alignment

Change-Id: I11a756dea9070d6e21b1a9481d357a1e8aa0663e

src/dec/frame.c

@@ -181,7 +181,9 @@ static int FinishRow(VP8Decoder* const dec, VP8Io* const io) {
   const int is_first_row = (mb_y == 0);
   const int is_last_row = (mb_y >= dec->br_mb_y_ - 1);
 
+  if (dec->mt_method_ == 2) {
     ReconstructRow(dec, ctx);
+  }
 
   if (ctx->filter_row_) {
     FilterRow(dec);
@@ -263,10 +265,11 @@ int VP8ProcessRow(VP8Decoder* const dec, VP8Io* const io) {
   const int filter_row =
       (dec->filter_type_ > 0) &&
       (dec->mb_y_ >= dec->tl_mb_y_) && (dec->mb_y_ <= dec->br_mb_y_);
-  if (!dec->use_threads_) {
+  if (dec->mt_method_ == 0) {
     // ctx->id_ and ctx->f_info_ are already set
     ctx->mb_y_ = dec->mb_y_;
     ctx->filter_row_ = filter_row;
+    ReconstructRow(dec, ctx);
     ok = FinishRow(dec, io);
   } else {
     WebPWorker* const worker = &dec->worker_;
@@ -278,17 +281,20 @@ int VP8ProcessRow(VP8Decoder* const dec, VP8Io* const io) {
       ctx->id_ = dec->cache_id_;
       ctx->mb_y_ = dec->mb_y_;
       ctx->filter_row_ = filter_row;
-      {
+      if (dec->mt_method_ == 2) {  // swap macroblock data
         VP8MBData* const tmp = ctx->mb_data_;
         ctx->mb_data_ = dec->mb_data_;
         dec->mb_data_ = tmp;
+      } else {
+        // perform reconstruction directly in main thread
+        ReconstructRow(dec, ctx);
       }
-      if (filter_row) {    // just swap filter info
+      if (filter_row) {            // swap filter info
         VP8FInfo* const tmp = ctx->f_info_;
         ctx->f_info_ = dec->f_info_;
         dec->f_info_ = tmp;
       }
-      WebPWorkerLaunch(worker);
+      WebPWorkerLaunch(worker);    // (reconstruct)+filter in parallel
       if (++dec->cache_id_ == dec->num_caches_) {
         dec->cache_id_ = 0;
       }
@@ -357,7 +363,7 @@ VP8StatusCode VP8EnterCritical(VP8Decoder* const dec, VP8Io* const io) {
 
 int VP8ExitCritical(VP8Decoder* const dec, VP8Io* const io) {
   int ok = 1;
-  if (dec->use_threads_) {
+  if (dec->mt_method_ > 0) {
     ok = WebPWorkerSync(&dec->worker_);
   }
 
@@ -397,7 +403,7 @@ int VP8ExitCritical(VP8Decoder* const dec, VP8Io* const io) {
 // Initialize multi/single-thread worker
 static int InitThreadContext(VP8Decoder* const dec) {
   dec->cache_id_ = 0;
-  if (dec->use_threads_) {
+  if (dec->mt_method_ > 0) {
     WebPWorker* const worker = &dec->worker_;
     if (!WebPWorkerReset(worker)) {
       return VP8SetError(dec, VP8_STATUS_OUT_OF_MEMORY,
@@ -414,6 +420,28 @@ static int InitThreadContext(VP8Decoder* const dec) {
   return 1;
 }
 
+int VP8GetThreadMethod(const WebPDecoderOptions* const options,
+                       const WebPHeaderStructure* const headers,
+                       int width, int height) {
+  if (options == NULL || options->use_threads == 0) {
+    return 0;
+  }
+  (void)headers;
+  (void)width;
+  (void)height;
+  assert(!headers->is_lossless);
+#if defined(WEBP_USE_THREAD)
+  if (width < MIN_WIDTH_FOR_THREADS) return 0;
+  // TODO(skal): tune the heuristic further
+#if 0
+  if (height < 2 * width) return 2;
+#endif
+  return 2;
+#else   // !WEBP_USE_THREAD
+  return 0;
+#endif
+}
+
 #undef MT_CACHE_LINES
 #undef ST_CACHE_LINES
 
@@ -429,11 +457,11 @@ static int AllocateMemory(VP8Decoder* const dec) {
   const size_t mb_info_size = (mb_w + 1) * sizeof(VP8MB);
   const size_t f_info_size =
       (dec->filter_type_ > 0) ?
-          mb_w * (dec->use_threads_ ? 2 : 1) * sizeof(VP8FInfo)
+          mb_w * (dec->mt_method_ > 0 ? 2 : 1) * sizeof(VP8FInfo)
         : 0;
   const size_t yuv_size = YUV_SIZE * sizeof(*dec->yuv_b_);
   const size_t mb_data_size =
-      (dec->use_threads_ ? 2 : 1) * mb_w * sizeof(*dec->mb_data_);
+      (dec->mt_method_ == 2 ? 2 : 1) * mb_w * sizeof(*dec->mb_data_);
   const size_t cache_height = (16 * num_caches
                             + kFilterExtraRows[dec->filter_type_]) * 3 / 2;
   const size_t cache_size = top_size * cache_height;
@@ -473,7 +501,7 @@ static int AllocateMemory(VP8Decoder* const dec) {
   mem += f_info_size;
   dec->thread_ctx_.id_ = 0;
   dec->thread_ctx_.f_info_ = dec->f_info_;
-  if (dec->use_threads_) {
+  if (dec->mt_method_ > 0) {
     // secondary cache line. The deblocking process need to make use of the
     // filtering strength from previous macroblock row, while the new ones
     // are being decoded in parallel. We'll just swap the pointers.
@@ -487,7 +515,7 @@ static int AllocateMemory(VP8Decoder* const dec) {
 
   dec->mb_data_ = (VP8MBData*)mem;
   dec->thread_ctx_.mb_data_ = (VP8MBData*)mem;
-  if (dec->use_threads_) {
+  if (dec->mt_method_ == 2) {
     dec->thread_ctx_.mb_data_ += mb_w;
   }
   mem += mb_data_size;

src/dec/idec.c

@@ -329,12 +329,6 @@ static VP8StatusCode DecodeWebPHeaders(WebPIDecoder* const idec) {
       return VP8_STATUS_OUT_OF_MEMORY;
     }
     idec->dec_ = dec;
-#ifdef WEBP_USE_THREAD
-    dec->use_threads_ = (idec->params_.options != NULL) &&
-                        (idec->params_.options->use_threads > 0);
-#else
-    dec->use_threads_ = 0;
-#endif
     dec->alpha_data_ = headers.alpha_data;
     dec->alpha_data_size_ = headers.alpha_data_size;
     ChangeState(idec, STATE_VP8_HEADER, headers.offset);
@@ -352,13 +346,14 @@ static VP8StatusCode DecodeWebPHeaders(WebPIDecoder* const idec) {
 static VP8StatusCode DecodeVP8FrameHeader(WebPIDecoder* const idec) {
   const uint8_t* data = idec->mem_.buf_ + idec->mem_.start_;
   const size_t curr_size = MemDataSize(&idec->mem_);
+  int width, height;
   uint32_t bits;
 
   if (curr_size < VP8_FRAME_HEADER_SIZE) {
     // Not enough data bytes to extract VP8 Frame Header.
     return VP8_STATUS_SUSPENDED;
   }
-  if (!VP8GetInfo(data, curr_size, idec->chunk_size_, NULL, NULL)) {
+  if (!VP8GetInfo(data, curr_size, idec->chunk_size_, &width, &height)) {
     return IDecError(idec, VP8_STATUS_BITSTREAM_ERROR);
   }
 
@@ -425,7 +420,9 @@ static VP8StatusCode DecodePartition0(WebPIDecoder* const idec) {
   if (dec->status_ != VP8_STATUS_OK) {
     return IDecError(idec, dec->status_);
   }
-
+  // This change must be done before calling VP8InitFrame()
+  dec->mt_method_ = VP8GetThreadMethod(params->options, NULL,
+                                       io->width, io->height);
   if (!CopyParts0Data(idec)) {
     return IDecError(idec, VP8_STATUS_OUT_OF_MEMORY);
   }

src/dec/vp8.c

@@ -662,8 +662,8 @@ static int ParseFrame(VP8Decoder* const dec, VP8Io* io) {
       return VP8SetError(dec, VP8_STATUS_USER_ABORT, "Output aborted.");
     }
   }
-  if (dec->use_threads_ && !WebPWorkerSync(&dec->worker_)) {
+  if (dec->mt_method_ > 0) {
-    return 0;
+    if (!WebPWorkerSync(&dec->worker_)) return 0;
   }
 
   // Finish
@@ -728,7 +728,7 @@ void VP8Clear(VP8Decoder* const dec) {
   if (dec == NULL) {
     return;
   }
-  if (dec->use_threads_) {
+  if (dec->mt_method_ > 0) {
     WebPWorkerEnd(&dec->worker_);
   }
   ALPHDelete(dec->alph_dec_);

src/dec/vp8i.h

@@ -100,6 +100,9 @@ enum { MB_FEATURE_TREE_PROBS = 3,
 #define U_OFF    (Y_OFF + BPS * 16 + BPS)
 #define V_OFF    (U_OFF + 16)
 
+// minimal width under which lossy multi-threading is always disabled
+#define MIN_WIDTH_FOR_THREADS 512
+
 //------------------------------------------------------------------------------
 // Headers
 
@@ -229,7 +232,8 @@ struct VP8Decoder {
 
   // Worker
   WebPWorker worker_;
-  int use_threads_;    // use multi-thread
+  int mt_method_;      // multi-thread method: 0=off, 1=[parse+recon][filter]
+                       // 2=[parse][recon+filter]
   int cache_id_;       // current cache row
   int num_caches_;     // number of cached rows of 16 pixels (1, 2 or 3)
   VP8ThreadContext thread_ctx_;  // Thread context
@@ -333,6 +337,11 @@ VP8StatusCode VP8EnterCritical(VP8Decoder* const dec, VP8Io* const io);
 // Must always be called in pair with VP8EnterCritical().
 // Returns false in case of error.
 int VP8ExitCritical(VP8Decoder* const dec, VP8Io* const io);
+// Return the multi-threading method to use (0=off), depending
+// on options and bitstream size. Only for lossy decoding.
+int VP8GetThreadMethod(const WebPDecoderOptions* const options,
+                       const WebPHeaderStructure* const headers,
+                       int width, int height);
 // Process the last decoded row (filtering + output)
 int VP8ProcessRow(VP8Decoder* const dec, VP8Io* const io);
 // To be called at the start of a new scanline, to initialize predictors.

src/dec/webp.c

@@ -452,11 +452,6 @@ static VP8StatusCode DecodeInto(const uint8_t* const data, size_t data_size,
     if (dec == NULL) {
       return VP8_STATUS_OUT_OF_MEMORY;
     }
-#ifdef WEBP_USE_THREAD
-    dec->use_threads_ = params->options && (params->options->use_threads > 0);
-#else
-    dec->use_threads_ = 0;
-#endif
     dec->alpha_data_ = headers.alpha_data;
     dec->alpha_data_size_ = headers.alpha_data_size;
 
@@ -468,6 +463,9 @@ static VP8StatusCode DecodeInto(const uint8_t* const data, size_t data_size,
       status = WebPAllocateDecBuffer(io.width, io.height, params->options,
                                      params->output);
       if (status == VP8_STATUS_OK) {  // Decode
+        // This change must be done before calling VP8Decode()
+        dec->mt_method_ = VP8GetThreadMethod(params->options, &headers,
+                                             io.width, io.height);
         if (!VP8Decode(dec, &io)) {
           status = dec->status_;
         }