关闭Dalvik VM的所有优化

Question

So I'm trying to write some low-level code for Android, and my main concern is that I want to avoid ALL optimization by the JIT compiler (or anything else). After doing some research, the best approach seems to be to:

1. write Java bytecode by hand
2. convert it to a dex file using the "dx" command
3. run it on the program using the "dalvikvm" command (via adb shell) with the "-Xverify:none -Xdexopt:none" paramaters specified

My question is: will this in fact avoid ALL optimization? The previous discussion here https://groups.google.com/forum/#!topic/android-platform/Y-pzP9z6xLw makes me unsure, and I can't 100% convince myself by reading the docs.

Any confirmation one way or the other is greatly appreciated.

Answer 1

Some of the instruction rewriting performed by dexopt cannot be disabled. For example, accesses to volatile long fields must be handled differently from access to long fields, and the specialization is handled by replacing the field-get instruction with a different instruction.

The optimizations performed by dexopt take the form of instruction replacement, usually some sort of "quickening" that allows the VM to do a little less work. All such optimizations are performed statically, ahead of time, not dynamically at run time, so you will get consistent behavior. Enabling the dexopt optimizations doesn't introduce unknowns, it just changes from one set of knowns to a different set of knowns.

The biggest source of variation is going to be Dalvik's JIT compiler, which you can disable with -Xint:fast . See this slightly outdated doc for notes on how to configure this system-wide.