# Performance

Logtalk is implemented as a trans-compiler to Prolog. When compiling predicates, it preserves in the generated Prolog code all cases of first-argument indexing and tail-recursion. In practice, this mean that if you know how to write efficient Prolog predicates, you already know how to write efficient Logtalk predicates.

The Logtalk compiler adds an hidden execution-context argument to all entity predicates. In the common case where a predicate makes no calls to the execution-context predicates and message-sending control constructs and is neither a meta-predicate nor a coinductive predicate, the execution-context argument is simply passed between goals. In this case, with most backend Prolog VMs, the cost of this extra argument is null or negligible. When the execution-context argument needs to be accessed (e.g. to fetch the value of self for a ::/1 call) there may be a small inherent overhead due to the implicit unifications.

## Local predicate calls

Local calls to object (or category) predicates have zero overhead in terms of number of inferences, as expected, compared with local Prolog calls.

## Calls to imported or inherited predicates

Assuming the optimize flag is turned on and a static predicate, ^^/1 calls have zero overhead in terms of number of inferences.

## Calls to module predicates

Local calls from an object (or category) to a module predicate have zero overhead (assuming both the module and the predicate are bound at compile time).

## Messages

For `::/1-2` calls Logtalk implements static binding and dynamic binding. For dynamic binding, a caching mechanism is used by the runtime. It’s useful to measure the performance overhead in number of inferences compared with plain Prolog and Prolog modules. The results for Logtalk 3.17.0 and later versions are:

- Static binding: 0
- Dynamic binding (object bound at compile time): 1
- Dynamic binding (object bound at runtime time): 2

Static binding is the common case with libraries and most application code; it requires compiling code with the optimize flag turned on. Dynamic binding numbers are after the first call (i.e. after the generalization of the query is cached). All numbers with the events flag set to `deny` (setting this flag to `allow` adds an overhead of 5 inferences to the results above).

The dynamic binding caches assume the used backend Prolog compiler does indexing of dynamic predicates. This is a common feature of modern Prolog systems but the actual details vary from system to system and may have an impact on dynamic binding performance.

Note that messages to self (::/1 calls) always use dynamic binding as the object that receives the message is only know at runtime.

## Inlining

When the optimize flag is turned on, the Logtalk compiler performs inlining of predicate calls whenever possible. This includes calls to built-in predicates such as `once/1`, `ignore/1`, and `phrase/2-3` but also calls to Prolog predicates that are either built-in, foreign, or defined in a module (including `user`). Inlining notably allows wrapping module or foreign predicates using an object without introducing any overhead. In the specific case of the execution-context predicates, calls are inlined independently of the `optimize` flag value.

## Generated code simplification and optimizations

When the optimize flag is turned on, the Logtalk compiler simplifies and optimizes generated clauses (including those resulting from the compilation of grammar rules), by flattening conjunctions, folding left unifications (e.g. generated as a by-product of the compilation of grammar rules), and removing redundant calls to `true/0`.

## Other considerations

One aspect of performance, that affects both Logtalk and Prolog code, is the characteristics of the Prolog VM. The Logtalk distribution includes two examples, bench and benchmarks, to help evaluate performance with specific backend Prolog systems. A table with results for a subset of the supported systems is also available in the Logtalk website.