diff --git a/docs/function-inlining.md b/docs/function-inlining.md index 1b38fb9..9367913 100644 --- a/docs/function-inlining.md +++ b/docs/function-inlining.md @@ -6,9 +6,9 @@ Luau's bytecode compiler is smart about inlining functions. We would rather focu ## Motivation -**context**: function inlining replaces function calls with the function's definition to reduce overhead costs. +**context**: function inlining replaces function calls with the function's implementation to reduce overhead costs. -Whether the goal is to improve instruction locality or perform context specific optimizations, function inlining can be a great resource for developers. At `-02` or higher, the compiler is able to automatically decide if functions ought to be inlined. In the currrent state of the language, exposing this to users directly could do more harm than good. Not only do these use cases only cater to a tiny subset of the language's users, it exposes our bytecode compiler to FILL when the larger subset who may not have a working understanding of the compiler adopts it. For Luau, this would happen at around the 5000th recursive call. +Whether the goal is to improve instruction locality or perform context specific optimizations, function inlining can be a great resource for developers. At `-02` or higher, the compiler is able to automatically decide if functions ought to be inlined. In the currrent state of the language, exposing this to users directly could do more harm than good. Not only do these use cases only cater to a tiny subset of the language's users, it exposes our bytecode compiler to stack overflow errors when the larger subset who may not have a working understanding of the compiler adopts it. For Luau, this would happen at around the 5000th recursive call. ```luau local function sum(lst: {number}) @@ -23,10 +23,9 @@ end local s = sum({1, 2, 3}) ``` -Consider this "inlined" recursive call, it attempts to sum up numbers in a list but it never terminates as `sum` is recursively called on `lst` which does not shrink. If Luau attempted to inline such a call, it would unroll this and one or two things could happen. If we did have an unroll limit then we would attempt our unrolling up until that point and end up making the program larger than it needs to be or thrashing our stack. The stack will grow beyond the system's stack limit and could potentially eat up resources meant for other processes. - +Consider this "inlined" recursive call, it attempts to sum up numbers in a list but it never terminates as `sum` is recursively called on `lst` which does not shrink. If Luau attempted to inline such a call, it would unroll this up until the 5000th recursive call and end up thrashing our stack. The stack will essentially grow beyond the system's stack limit and could potentially eat up resources meant for other processes. -More specifically, due to limited number of cases where the compiler supports inlining, exposing this feature would not be entirely useful. This section further explores common cases were `@inline` would not apply. +More specifically, due to limited number of cases where the compiler can even consider inlining, exposing this feature would not be entirely useful. This section further explores common developer patterns that may adopt `@inline` in ways the compiler cannot handle. 1. Exported a `table` ```luau @@ -52,17 +51,7 @@ end return table.freeze(mod) ``` -Consider this example which exports a mutable table `mod`. Module `B` imports this table and calls it within another function, but because the compiler decides what implementation of this method should be called at runtime through the metatables, it makes the VM unable to guarantee that `A.foo()` has not been mutated somewhere between the boundaries of both modules making `mod.foo` not suitable for an inline. Even with a frozen table being exported by module `B`, its function foo could be mutated before its return leading us back to the same issue. - - - - - - - - - - +This example exports a mutable table `mod` in module `A`. Module `B` imports this table and calls it within another function, but because the compiler tries to decide what implementation of this method should be called at runtime through the metatables, it is unable to guarantee that `A.foo()` has not been mutated somewhere between the boundaries of both modules making `mod.foo` not suitable for an inline. Even with a frozen table being exported by module `B`, its function foo could also be mutated before its return leading us back to the same issue. 2. "OOP" code ```luau @@ -79,7 +68,7 @@ function Account:deposit(bal) end ``` -Even OOP style code, this mutable resolution problem persists. In this code sample, we define a base `Account` class with a deposit method. However, if an instance of this class were made, say `DavesAccount`, it could go on to overload the deposit method making this another case that is not suitable for inlining. +With Object Oriented Programming style code, this mutable resolution problem persists. In this code sample, we define a base `Account` class with a deposit method. However, if an instance of this object were made, say `DavesAccount`, it could go on to overload the deposit method making this another case not suitable for inlining. 3. Metamethods @@ -96,14 +85,13 @@ function Vec3:__add(rhs) return Vec3.new(self.x + rhs.x, self.y + rhs.y, self.z + rhs.z) end ``` -In this example, we declare `Vec3` with an inlined `__add` (`+`) operator overload. To make this inlining work, we would need to do one of two things, (1) confirm the `rhs` and `lhs` share the same shape (i.e, they both have`x`, `y` and `z`) or (2) ensure the `rhs` has a similar operator overload. Due to dynamic dispatch, we cannot make either guarantee making this case unsuitable for overloads. - -## Design -None (yet). +In this example, we declare `Vec3` with an inlined `__add` (`+`) operator overload. To make this inlining work, we would need to do one of two things, (1) confirm the `rhs` and `lhs` share the same shape (i.e, they both have`x`, `y` and `z` keys) or (2) ensure the `rhs` has the same operator overload, giving the compiler additional confidence. However, due to dynamic dispatch, we cannot make either guarantee making this case unsuitable for inlines. ## Drawbacks -None (yet). +The major drawback here is that this prevents developers the opportunity to fine-tune performance-critical optimizations, forcing them to rely on the compiler for fine-grained optimizations, even if their use case may call for it. Doing this assures us that developers have no control over raising the maximum cost of Luau programs, especially in the case where developers would chose to riddle their code base with inlines to make the code "faster". ## Alternatives -Automatic function inlining in Luau is very efficient. We suggest opening github issues with code samples of legitimate cases that we can use to further improve Luau's automatic (function inlining and unrolling) optimizations. +Automatic function inlining in Luau is already very efficient. We suggest opening Github issues with code samples of legitimate cases that we can use to further improve the compiler's automatic (function inlining and unrolling) optimizations. [File a Github issue](https://github.com/luau-lang/luau/issues/new?assignees=&labels=bug&projects=&template=bug_report.md&title=Inlining%20Support) + +