Few cents about my commits

RoboVM compiler: out of memory -- huge class case


RoboVM compiler is quite memory hungry by its nature: creates and manipulates a bunch of strings, have to keep parsed classes structures while compiling etc. Also there is a bug opened #150. Usually its enough to give JVM bigger heap (-Xmx4g) to make everyone happy.
There was a report on gitter channel about OOM that happens on a single file that can’t be fixed by increasing heap size. File just contained about 8K static final string fields.

To reproduce the case dummy file with 8000 strings was generated:

public class DBkeys
    static public final String T0001 = "";
    static public final String T0002 = "";
    // lot of more these 
    static public final String T8000 = "";

Compilation using default heap settings just produce Out of memory exception but once JVM granted plenty of ram (-Xmx32g) it ends up with exception:

java.lang.OutOfMemoryError: Required array length too large

RoboVM went beyond of JVM limits trying to allocate byte array with size longer than JVM supported (Integer.MAX_VALUE - 2 gig). In other words – giving all memory you have to JVM heap will not solve this issue.

Root case

OOM happens when RoboVM compiles java code into LLVM IR code. This constant string only class doesn’t fit in MAX possible 2 GB java string. Let’s see what is in there.

Long story short: to access field RoboVM calculate offset of the field from start of the object. Class/Object structure is defined in ClassType structure. This structure is being generated while compiling class.
Compiler insert offset calculation code in form of offset = {raw structure}, filed#no. Each time. For such big class (with 8000 fields) this structure will be 170000 chars long. Compiler uses it three times per fields.

Lets do a math:

8000 fields * 170000 * 3 = bytes.

So 4Gb just to store only structure without actual code.


Typedef it once, use alias:

%ClassType = type { very-very-very long struct}
offset = %ClassType, filed#no

Much better. Result: .ll file is only 16Mb (vs 4Gb+ before the fix)

Fix was delivered as PR485.