Subtle Semantic Changes After Optimization
Some optimizations can alter subtle semantics of the script, causing the script to behave differently when run with or without optimization.
Typically, this involves some form of error that may arise in the original, unoptimized script but is optimized away by the script optimizer.
Needless to say, it is usually a _Very Bad Idea™_ to depend on a script failing with a runtime error
or such kind of subtleties.
If it turns out to be necessary (why? I would never guess), turn script optimization off by setting
the optimization level to [`OptimizationLevel::None`].
Disappearing Runtime Errors
For example:
if true { // condition always true
123.456; // eliminated
hello; // eliminated, EVEN THOUGH the variable doesn't exist!
foo(42) // promoted up-level
}
foo(42) // <- the above optimizes to this
If the original script were evaluated instead, it would have been an error –
the variable hello
does not exist, so the script would have been terminated at that point
with a runtime error.
In fact, any errors inside a statement that has been eliminated will silently disappear.
print("start!");
if my_decision { /* do nothing... */ } // eliminated due to no effect
print("end!");
// The above optimizes to:
print("start!");
print("end!");
In the script above, if my_decision
holds anything other than a boolean value,
the script should have been terminated due to a type error.
However, after optimization, the entire if
statement is removed (because an access to
my_decision
produces no side-effects), thus the script silently runs to completion without errors.
Eliminated Useless Work
Another example is more subtle – that of an empty loop body.
// ... say, the 'Engine' is limited to no more than 10,000 operations...
// The following should fail because it exceeds the operations limit:
for n in 0..42000 {
// empty loop
}
// The above is optimized away because the loop body is empty
// and the iterations simply do nothing.
()
Normally, and empty loop body inside a for
statement with a pure iterator does nothing and can
be safely eliminated.
Thus the script now runs silently to completion without errors.
Without optimization, the script may fail by exceeding the maximum number of operations allowed.