[OpenJDK 2D-Dev]  Review Request: 8080847 Copying of overlapping memory should be improved in java2d
james.graham at oracle.com
Tue May 26 10:43:19 UTC 2015
What crash in memcpy? The issue you pointed to is about dealing with
overlapping memory. memcpy does not crash on overlapping memory copies,
it just duplicates data oddly in a way that most uses probably don't want.
Also, the fix you gave only fixed the problem for the horizontal
direction, if the drawImage call were directed at 0,1 then we'd still
get all scan lines duplicated from the first...
On 5/25/2015 12:32 PM, Sergey Bylokhov wrote:
> Hi, Jim.
> Actually there is a difference in support: it works but result is a
> little bit wrong, and it does not work and crashes. This fix is not a
> general solution for the incorrect result of the blit+aliasing, but for
> the possible crash of memcpy especially after some improvements like in
> glibc. I think this still an issue.
>> I don't recall if we ever promised that this case would work without
>> aliasing issues. I know that we went out of our way in the copyArea
>> method to prevent the aliasing issue, doing the blits piecemeal so
>> that they don't interfere with each other. Further, while it may be
>> easy enough to just call memmove to have the libraray do this for us
>> in the IsoBlit case, other cases that don't fall into the IsoBlit
>> macro will not be similarly protected. In particular, if you specify
>> an alpha value, you will not get this protection (at least not without
>> a huge amount of work to overhaul the entire DrawImage pipeline).
>> I would say that this would be OK if we planned to make this promise
>> about drawImage across all image formats and composition modes, but
>> that would be a far more complicated fix. Until then, we should not
>> open this can of worms by modifying this one specific Blit case...
>> On 5/25/2015 5:35 AM, Sergey Bylokhov wrote:
>>> Please review the fix forjdk9.
>>> I found this issue during code review of another task, related to
>>> The sample code below will call the IsomorphicCopy method which call
>>> memcpy on the overlapping memory(this is the simplest example)
>>> BufferedImage img = new BufferedImage(100, 100,
>>> Graphics2D g = img.createGraphics();
>>> g.drawImage(img, 0, 0, null);
>>> "The memcpy() function copies n bytes from memory area src to memory
>>> area dest. The memory areas must not overlap. Use memmove(3) if the
>>> memory areas do overlap"
>>> I can confirm this bug using valgrind and a program above:
>>> valgrind --smc-check=all --tool=memcheck --leak-check=full -v
>>> ==60975== Source and destination overlap in memcpy(0xe1b8b4d8,
>>> 0xe1b8b4d8, 400)
>>> ==60975== at 0x4C2F71C: memcpy@@GLIBC_2.14 (in
>>> ==60975== by 0x1E0F504D: AnyIntIsomorphicCopy (in
>>> ==60975== by 0x1E0F5DE8: Java_sun_java2d_loops_Blit_Blit (in
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8080847
>>> Webrev can be found at:
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