Request for review (S): 7102044 G1: VM crashes with assert(old_end != new_end) failed: don't call this otherwise

Vladimir Kozlov vladimir.kozlov at
Wed Oct 26 08:44:54 PDT 2011

Hi Bengt,

I thought VM should throw OOM exception in such cases (as your example) since you can't allocate such arrays in 32-bit 
VM. Does the size check for OOM happened after heap resize calculation?

Is SIZE_MAX defined on all platforms?

You still have problem with short[] and char[] since 2*max_jint+header_size will overflow. Also max_array_length() puts 
unneeded limitation on double[] and long[] max length in 64 but VM, it should be max_jint. I think we can do something 
like next:

     const size_t max_words_per_size_t = align_size_down((SIZE_MAX/HeapWordSize - header_size(type)), MinObjAlignment);
     const size_t max_elements_per_size_t = HeapWordSize * max_words_per_size_t / type2aelembytes(type);
     if ((size_t)max_jint < max_elements_per_size_t);
       return max_jint ;
     return (int32_t)max_elements_per_size_t;

check_overflow() should use julong type for expressions instead of size_t to catch overflow:

+bool arrayOopDesc::check_overflow(BasicType type) {
+  julong length = (julong)max_array_length(type);
+  julong bytes_per_element = (julong)type2aelembytes(type);
+  julong bytes = length * bytes_per_element + (julong)header_size_in_bytes();
+  return (julong)(size_t)bytes == bytes;

Rename check_overflow() to check_max_length_overflow() or something.

I don't think we need incorrect old_max_array_length() method and corresponding checks for LP64.

Initialization and execution of additional JPRT tests will increase jobs time. I think you could add InternalVMTests to 
existing client and server tests:

<    $(PRODUCT_HOME)/bin/java $(JAVA_OPTIONS) -version
 >    $(PRODUCT_HOME)/bin/java $(JAVA_OPTIONS) -XX:+IgnoreUnrecognizedVMOptions -XX:+ExecuteInternalVMTests -version


On 10/26/11 6:18 AM, Bengt Rutisson wrote:
> Hi all,
> Can I get a couple of reviews to this fairly small change?
> Sorry for the wide distribution, but the bug is reported on GC, I think the actual change (in arrayOop.hpp) is runtime
> responsibility and the method that is being changed is used on a few occasions in C2. So, I figured I'd like everyone to
> know that this is about to go in.
> CR:
> 7102044 G1: VM crashes with assert(old_end != new_end) failed: don't call this otherwise
> Background
> arrayOopDesc::max_array_length() returns the maximum length an array can have based on the type of the array elements.
> This is passed on through the allcocation path as a word size. When we need to expand the heap we have to convert the
> word size to a byte size. The problem is that on 32 bit platforms this conversion may overflow a size_t. Which will
> result in that we call expand() with a much smaller size than required size or even with a size of 0.
> Here is a small reproducer that will trigger the assert mentioned in the CR if we run it with -XX:+UseG1GC:
> public class LargeObj {
> public static void main(String[] args) {
> int[] a = new int[2147483639];
> }
> }
> It is not only G1 that has this problem. ParallelGC has the same issue, it just doesn't have an assert to detect it. It
> can be argued that the GCs should check for overflows, but I think that since arrayOopDesc::max_array_length() will
> catch the problem earlier and solve it for all GCs this is a better place to fix the problem.
> I added some unit tests to test the new implementation. To make sure that these tests are run I had to update some more
> files. The actual change is in arrayOop.hpp. The rest of the files have just been changed to allow the unit tests to run.
> A few questions:
> (1) In arrayOopDesc::max_array_length() I use MIN2 to make sure that we don't overflow a size_t. This is only needed on
> 32 bit platforms. I prefer this general code compared to #ifndef _LP64. Any thoughts?
> (2) I made the unit tests run as JPRT make targets. I guess it can also be done as jtreg tests, but that way it would
> only be executed for PIT etc. Any thoughts on this?
> Thanks,
> Bengt

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