Detect loops overflow for gpu benches.

bench/nanobench.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <ctype.h>
 
 #include "Benchmark.h"
@@ skipping 107 matching lines @@
 static int detect_forever_loops(int loops) {
     // look for a magic run-forever value
     if (loops < 0) {
         loops = SK_MaxS32;
     }
     return loops;
 }
 
 static int clamp_loops(int loops) {
     if (loops < 1) {
-        SkDebugf("ERROR: clamping loops from %d to 1.\n", loops);
+        SkDebugf("ERROR: clamping loops from %d to 1. "
+                 "There's probably something wrong with the bench.\n", loops);
         return 1;
     }
     if (loops > FLAGS_maxLoops) {
         SkDebugf("WARNING: clamping loops from %d to FLAGS_maxLoops, %d.\n", loops, FLAGS_maxLoops);
         return FLAGS_maxLoops;
     }
     return loops;
 }
 
 static bool write_canvas_png(SkCanvas* canvas, const SkString& filename) {
@@ skipping 82 matching lines @@
     gl->makeCurrent();
     // Make sure we're done with whatever came before.
     SK_GL(*gl, Finish());
 
     // First, figure out how many loops it'll take to get a frame up to FLAGS_gpuMs.
     int loops = FLAGS_loops;
     if (kAutoTuneLoops == loops) {
         loops = 1;
         double elapsed = 0;
         do {
+            if (1<<30 == loops) {
+                // We're about to wrap.  Something's wrong with the bench.
+                loops = 0;
+                break;
+            }
             loops *= 2;
             // If the GPU lets frames lag at all, we need to make sure we're timing
             // _this_ round, not still timing last round.  We force this by looping
             // more times than any reasonable GPU will allow frames to lag.
             for (int i = 0; i < FLAGS_gpuFrameLag; i++) {
                 elapsed = time(loops, bench, canvas, gl);
             }
         } while (elapsed < FLAGS_gpuMs);
 
         // We've overshot at least a little.  Scale back linearly.
@@ skipping 497 matching lines @@
 
     return 0;
 }
 
 #if !defined SK_BUILD_FOR_IOS
 int main(int argc, char** argv) {
     SkCommandLineFlags::Parse(argc, argv);
     return nanobench_main();
 }
 #endif