1. 01 Aug, 2017 1 commit
    • Benedikt Meurer's avatar
      [builtins] Speed-up Object.prototype.toString. · 31800120
      Benedikt Meurer authored
      The @@toStringTag lookup in Object.prototype.toString causes quite a
      lot of overhead and oftentimes dominates the builtin performance. These
      lookups are almost always negative, especially for primitive values,
      and Object.prototype.toString is often used to implement predicates
      (like in Node core or in AngularJS), so having a way to skip the
      negative lookup yields big performance gains.
      
      This CL introduces a "MayHaveInterestingSymbols" bit on every map,
      which says whether instances with this map may have an interesting
      symbol. Currently only @@toStringTag is considered an interesting
      symbol, but we can extend that in the future.
      
      In the Object.prototype.toString we can use the interesting symbols
      bit to do a quick check on the prototype chain to see if there are
      any maps that might have the @@toStringTag, and if not, we can just
      immediately return the result, which is very fast because it's derived
      from the instance type. This also avoids the ToObject conversions for
      primitive values, which is important, since this causes unnecessary
      GC traffic and in for example AngularJS, strings are also often probed
      via the Object.prototype.toString based predicates.
      
      This boosts Speedometer/AngularJS by over 3% and Speedometer overall
      by up to 1%. On the microbenchmark from the similar SpiderMonkey bug
      (https://bugzilla.mozilla.org/show_bug.cgi?id=1369042), we go from
      roughly 450ms to 70ms, which corresponds to a 6.5x improvement.
      
      ```
      function f() {
          var res = "";
          var a = [1, 2, 3];
          var toString = Object.prototype.toString;
          var t = new Date;
          for (var i = 0; i < 5000000; i++)
      	res = toString.call(a);
          print(new Date - t);
          return res;
      }
      f();
      ```
      
      The design document at https://goo.gl/e8CruQ has some additional
      data points.
      
      TBR=ulan@chromium.org
      
      Bug: v8:6654
      Change-Id: I31932cf41ecddad079d294e2c322a852af0ed244
      Reviewed-on: https://chromium-review.googlesource.com/593620
      Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
      Reviewed-by: 's avatarCamillo Bruni <cbruni@chromium.org>
      Reviewed-by: 's avatarJaroslav Sevcik <jarin@chromium.org>
      Cr-Commit-Position: refs/heads/master@{#47034}
      31800120
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