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diff --git a/chapter18/README.md b/chapter18/README.md index 606ac9a8..b9a51753 100644 --- a/chapter18/README.md +++ b/chapter18/README.md @@ -1,17 +1,61 @@ # Chapter 18: Functions -Some thinking on function syntax: -Function Types: +I hardly know where to begin! So many things changed here, mostly as indirect +consequences of supporting functions. + +Highlights of the changes: + +- Function types, and a rework of the type lattice diagrams. +- A distiguished `null` which can be used for both functions and struct pointers. +- Functions themselves can be parsed and evaluated; recursive functions are supported. +- Function calls are allowed, call sites can link and functions inlined. +- The top-level is now a call to function `main`, and this changed most tests. +- A top-level compile driver, which enforces optimization steps. +- A local scheduler. +- A new evaluator which relies on at least code motion, both global and local. +- A graphic animated viewer for peepholes. + +Functions are *not* closures in this chapter; that brings about far more +changes and subtle complexities than sensible for such a large chapter. + + + +## Function Types + ``` { argtype, argtype, ... -> rettype } ``` -Leading `{` character denotes the start of a function or function type. +A leading `{` character denotes the start of a function or function type, then +a list of argument types, and arrow `->` and the return type. + +Functions themselves are normal variables and assigned the same way: + +`functiontype fcn = function-typed-expr` + +and function typed variables can be inferred, so `val` and `var` on the left +hand side is fine. -Functions are normal variables and assigned the same way: +`val sq = { int x -> x*x; }` -```functiontype var = function-typed-expr``` +Internally, functions have a `TypeTuple` for the arguments, a return `Type`, +and a *function index* spelled as a `fidx` in the code. Function indices are +unique small dense integers, one per unique function. They will be mapped +1-to-1 to the final code address of the generated function code. + +Function pointers can refer to more than one function with the same signature, +and the `TypeFunPtr` tracks this as a bit set with the same tuple for signature +and return. + +Function types can be `null` just like references. + +Functions cannot return `void`, but they can return `null` and have their +return value ignored. A syntactic sugar for this may be added in the future. + + + +## Functions Functions themselves using the same syntax with as types, but filled in: @@ -47,64 +91,112 @@ optional. This allows any section of code to be "thunked" by wrapping it in Called: `just5() // Returns 5` +Functions are always anonymous. When finally assigned to a variable, they will +pick up the variable name for debugging and display purposes, but this has no +semantics meaning. Here is a function variable referring to more than one +anonymous function; the resulting function either doubles or squares: -Functions can be declared inside `struct`s, and will take a hidden `self` -argument to allow access to the struct members - basically the function becomes -a method. - -Also new this chapter, fields with a leading underscore `_` are private fields, -and only accessible to 'self'. - -```cpp -struct String { - u8[] _chars; // Private, not nullable, final, must be initialized in constructor - - // Functions declared inside of a struct become instance methods - // and have access to the struct members. - val len = { -> _chars#; } - - val at = { idx -> _chars[idx]; } - - // 'self' keyword returns an instance of itself. - val append = { String? str -> - // Appending a null appends nothing - if( !str ) return self; - - // Manual array copy _chars to cs - u8[] cs = new u8[_chars# + str.len()]; - for( int i=0; i<_chars#; i++ ) - cs[i] = _chars[i]; - - // Manual array copy str._chars to cs - for( int i=0; i<str.len(); i++ ) - cs[_chars#+i] = str.at(i); - - // Append returns a new String - return new String { _chars=cs; }; - } - +`val fcn = arg ? { int x -> x+x; } : { int x -> x*x; };` -}; +`fcn(3) // Prints either 6 or 9, depending on arg` +Functions can be recursive: +``` +val fact = { int n -> + n <=1 ? n : n*fact(n-1); +}; +return fact(4); // Returns 4! or 24` ``` -With some syntax sugar from the parser we can produce `Hello, Cliff` -with method calls. +You can early return as normal out of functions: ``` -String name = "Cliff"; -String greet = "Hello"; -return greet.append(", ").append(name); +val find = { int[] xs, int x -> + for( int i=0; i<xs#; i++ ) + if( xs[i] == x ) + return i; + return -1; +}; +return find; ``` -There's only a little special syntax going on here with method calls. The -normal field lookup is used for `greet.append`, yielding a function-typed -value. Since it was loaded from the field, the function is **pre-bound** -(curried) to the `self` it was loaded from. +Unlike prior chapters, all the `return`s from a single function are gathered +together into a single `ReturnNode` point, and they must all be of the same +general type. + +When looking at the returned IR, the `StopNode` now reports one return for each +function, including `main`: +`Stop[ return find; return Phi(Region,int,-1); ]` + + +### Calls + +Calls have the usual syntax: `fcn(3)`. Internally a `CallNode` takes in +Control, Memory, all the normal arguments, and a hidden last argument which is +the function pointer. For calls to named functions this last argument will be +a `ConstantNode` of the named function type, but in general it can be the +result of any function-typed expression. + +After a `Call` is a `CallEndNode`, internally abbreviated as `cend`. The +`CallEnd` will take the `Call` as an input, and also every *linked* function: +functions the call-site *knows* it will call. This will be expanded later to +be a conservative approximation to the *Call Graph*, with each `CallEnd` +*linked* to ever function it *may* call; if a function is not linked it can not +be called from here. This requires a global analysis (fast, cheap, +incremental, and global), not in this chapter. So for the moment we only +link exact constant functions. + +If a call site is linked to a single function, and that single function is only +called by this one call site (its function pointer is only used here, obvious +from GVN) then the function inlines in the IR. + +`CallEnd`s are followed by projections for Control, Memory and the return value. + + + +## CodeGen - The Compile Driver + +There is now a top-level compile driver that enforces a phase ordering, and +allows multiple compilations; the Fuzzer uses this to compare various generated +programs. The phases are: + +- Parse - Convert program text to Simple IR +- Opto - General optimizations; for now iterate peepholes. +- TypeCheck - Error checking after all types have propagated. This mostly + reports on failed null checks. +- Schedule - Global Code Motion scheduler. After this phase, all nodes belong + in some basic block, with the normal suspect CFGNodes being basic blocks. +- LocalSched - a local scheduler. Its completely naive, except it enforces + some required rules: Phis appear at block heads, branchs at block exits. + There's room in the algorithm for a much more sophisticated list scheduler. + The `Eval2` evaluator requires this information. +- RegAlloc - Not implemented (yet). + +Several globals moved from the Parser to CodeGen, and probably several others +ought to move here. + +There is a very nice `toString()` here; hovering over the `code` variable in +the debug winow will pretty-print the IR "as if" globally scheduled, and the +code becomes very readable. + + + +## Eval2 - A new Evaluator + +There is now a second evaluator that uses the scheduling information to +evaluate in a very straightforward way. Essentially the normal IR nodes are +treated like a special "machine instruction set" with infinite registers, and a +globally correct schedule. This evaluator supports functions and calls (and +recursive calls). + -`{int->u8} atCliff = name.at; // The "at" for Cliff; function does not take a String anymore` +## Graph Visualizer -`var atHello = greet.at;` +Run as `make view` or `java JSViewer`, type your program in the text box, click +outside the box and then use the arrow keys to view IR generator both forwards +and backwards. -`atCliff[0]; // Returns 'C'` +Nodes are color coded according to type, same as the lattice diagrams. Nodes +are shaped according to node function as well. At the bottom are `ScopeNode`s, +which only exist for the Parser but are actual Nodes and have `use->def` edges +into the IR. -`atHello[0]; // Returns 'H'` |