Question

According to the Maven GWT Plugin docs, the default optimization level is -1. It is not clear to which optimization level this corresponds. So, does -1 mean that the code is optimized at some level between 0 and 9? Or is it something else entirely?

Answer 1

GWT has 9 different optimization tricks from level 1 to 9 apart from the regular compiler level stuff. However we can opt out of these optimizations by setting it to 0. This is not recommended as commented in the code.

You can probably pick up some more information from GWT source code in ArgHandlerOptimize.java

 @Override
  public String getPurpose() {
    return "Sets the optimization level used by the compiler.  0=none 9=maximum.";
  }

From comments section of ArgHandlerOptimize.java

 * Set the optimization level from the command line.  For now, level 1 is the same
 * as draft compile, and level 9 is the same as the default (maximium optimization).
 * 
 * TODO(zundel): In theory, a level 0 should be possible, where all optimizers 
 * are eliminated for the fastest possible compile.  In practice, code generation 
 * depends on some optimizers being run.