[Rev 02] RFR: 8238954: Improve performance of tiled snapshot rendering

[Frederic Thevenet]

On Fri, 13 Mar 2020 17:24:02 GMT, Ambarish Rapte <arapte at openjdk.org> wrote:

>> Frederic Thevenet has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Avoid useless width and height calculation
>
> modules/javafx.graphics/src/main/java/com/sun/javafx/tk/quantum/QuantumToolkit.java line 1600:
> 
>> 1599:                                 }
>> 1600:                             }
>> 1601:                         }
> 
> The `if (exactHeightDivFound.get())` and `else` block here look effectively same. Could you please take a re-look or
> explain what am I missing here.

Functionally, both side of the condition actually do the exact same thing, but the side effects of which dimension is
processed in the inner loop sometime have measurable consequences in terms of performances; I've added the following
comment to the code to better explain this:
  // In order to minimize the number of time we have to resize the underlying
  // surface for capturing a tile, choose a dimension that has an exact divider
  // (if any) to be processed in the inner most loop.
  // E.g. looping on width then height in the example bellow requires four
  // surface resizing, whereas the opposite requires only two:
  //
  //       for (w;;)                    for (h;;)
  //           for(h;;)                     for(w;;)
  //       -----------------           -----------------
  //       |       |       |           |       |       |
  //       |   1   |   3   |           |   1   |   2   |
  //    h  |       |       |        h  |       |       |
  //       -----------------           -----------------
  //       |   2   |   4   |           |   3   |   4   |
  //       -----------------           -----------------
  //               w                           w

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PR: https://git.openjdk.java.net/jfx/pull/112