11.7.2 Available WPO optimizations

The -OW and -Ow command-line options require a comma-separated list of whole-program-optimization options. These are strings, each string denotes an option. The following is a list of available options:

all

This enables all available whole program optimisations.

devirtcalls

Changes virtual method calls into normal (static) method calls when the compiler can determine that a virtual method call will always go to the same static method. This makes such code both smaller and faster. In general, it is mainly an enabling optimisation for other optimisations, because it makes the program easier to analyse due to the fact that it reduces indirect control flow.

There are 2 limitations to this option:

1. The current implementation is context-insensitive. This means that the compiler only looks at the program as a whole and determines for each class type which methods can be devirtualised, rather than that it looks at each call statement and the surrounding code to determine whether or not this call can be devirtualised;
2. The current implementation does not yet devirtualise interface method calls. Not when calling them via an interface instance, nor when calling them via a class instance.

optvmts

This optimisation looks at which class types can be instantiated and which virtual methods can be called in a program, and based on this information it replaces virtual method table (VMT) entries that can never be called with references to FPC_ABSTRACTERROR. This means that such methods, unless they are called directly via an inherited call from a child class/object, can be removed by the linker. It has little or no effect on speed, but can help reducing code size.

This option has 2 limitations:

1. Methods that are published, or getters/setters of published properties, can never be optimized in this way, because they can always be referred to and called via the RTTI (which the compiler cannot detect).
2. Such optimisations are not yet done for virtual class methods.

wsymbolliveness

This parameter does not perform any optimisation by itself. It simply tells the compiler to record which functions/procedures were kept by the linker in the final program. During a subsequent wpo pass, the compiler can then ignore the removed functions/procedures as far as WPO is concerned (e.g., if a particular class type is only constructed in one unused procedure, then ignoring this procedure can improve the effectiveness of the previous two optimisations).

Again, there are some limitations:

1. This optimisation requires that the nm utility is installed on the system. For Linux binaries, objdump will also work. In the future, this information could also be extracted from the internal linker for the platforms that it supports.
2. Collecting information for this optimisation (using -OWsymbolliveness) requires that smart linking is enabled (-XX) and that symbol stripping is disabled (-Xs-). When only using such previously collected information, these limitations do not apply.