source: doc/proposals/closure.md @ f739788

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Last change on this file since f739788 was fef8293, checked in by Aaron Moss <a3moss@…>, 5 years ago

Add draft of closure proposal

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1## Light-weight Closures for Cforall ##
2
3A core capability of the Cforall type system is the ability to use
4monomorphic specializations of polymorphic functions seamlessly and
5invisibly to the user programmer, primarily in type assertions:
6
7    forall(otype T | { T ?+?(T, T); })
8    T double( T x ) { return x + x; }
9
10    forall(otype R | { R double(R); })
11    R quadruple( R y ) { return double( double( y ) ); }
12
13    void fred() {
14        float magic = quadruple( 10.5f );
15    }
16
17In the example above, `R` and `T` are both bound to `float`, and the
18`double` type assertion on `quadruple` is satisfied by monomorphizing the
19polymorphic `double` function to `float` (using the builtin `float` addition
20operator). The existing implementation uses GCC nested functions to
21implement this monomorphization, as in the following (much simplified)
22generated code:
23
24    void double(size_t _sizeof_T, 
25                void (*_assign_T)(void*, void*),
26                void (*_add_T)(void*, void*, void*),
27                void *_rtn,
28                void *x ) {
29        _add_T( _rtn, x, x );
30    }
31
32    void quadruple(size_t _sizeof_R, 
33                   void (*_assign_R)(void*, void*),
34                   void (*_double_R)(void*, void*),
35                   void *_rtn,
36                   void *y ) {
37        void *_tmp0 = alloca(_sizeof_R);
38        _double_R( _rtn, (_double_R( _tmp0, y ), _tmp0) );
39     }
40
41     void fred() {
42         // nested thunk to adapt double() to _double_R()
43         void _thunk0( void *_rtn, void *x ) {
44             double( sizeof(float), _builtin_assign_float,
45                     _builtin_add_float,
46                     _rtn, x );
47         }
48
49         float magic;
50         float _tmp1 = 10.5f;
51         quadruple( sizeof(float), _builtin_assign_float, _thunk0,
52                    &magic, &_tmp1 );
53     }
54
55Now, in the example above, `_thunk0` could be hoisted to static scope, as
56`sizeof(float)`, `_builtin_assign_float`, and `builtin_add_float` all exist
57in static scope. In general, however, these parameters which are used to
58monomorphize polymorphic functions could be local to the calling scope (e.g.
59if `fred()` was a polymorphic function itself, or had a local overload of
60`float` addition).
61
62The crux of the issue is that these monomorphization thunks need a lexical
63link to their creation context, but C's standard calling convention provides
64no way to include such a lexical link. GCC fixes this for nested functions
65by placing an executable trampoline on the stack to modify the calling
66convention; this trampoline has the standard calling convention, and calls
67the nested function after setting up the lexical link. This prevents the
68stack from being marked as non-executable, opening a variety of potential
69security vulnerabilities. More to the point of this proposal, it also means
70that the thunk exists on the stack, and may go out of scope before it is
71used if it is copied somewhere else (for instance, to the root of a new
72stack in a coroutine).
73
74The standard solution to this sort of problem is a *closure*; this proposal
75describes how to integrate a restricted sort of closure into Cforall that
76would be sufficiently powerful to monomorphize polymorphic functions.
77
78Monomorphization parameters in the current implementation fall into four
79categories:
80  1. Size/alignment of types; a single unsigned integer
81  2. Field offsets for generic types; a fixed length array of unsigned
82  3. Functions used to satisfy type assertions: a single function pointer
83  4. Variables used to satisfy type assertions: a single void pointer
84
85The gist of this proposal is to develop a copyable closure object (similar 
86in concept to `std::function` in C++) that can encapsulate a function
87pointer and an arbitrary list of these monomorphization parameters and
88provide a function call operator that passes the appropriate parameters to
89the underlying function. In (very-)pseudo-Cforall, it might look something
90like the following:
91
92    forall(ttype Rtns, ttype Args) struct Fn {
93        [assertion...] closed;
94        forall(closed) Rtns (*f)(Args...);
95    };
96
97    forall(ttype Rtns, ttype Args, [assertion...] Closed)
98    void ?{}( Fn(Rtns, Args) *this,
99              forall(Closed) Rtns (*f)(Args), Closed closed ) {
100        this->closed = closed;
101        this->f = f;
102    }
103    // ^ function pointers convert implicitly to Fn, as they have an empty
104    // assertion list here
105
106    forall(ttype Rtns, ttype Args)
107    Rtns ?() ( Fn(Rtns, Args) fn, Args args ) {
108        return fn.f( fn.closed, args );
109    }
110
111Using this `Fn` closure internally (even if it was never exposed to user
112programmers), the top example would codegen something like this, with `Fn` 
113substituted for the implicit function pointers:
114
115    void double(size_t _sizeof_T, 
116                Fn(void, [void*, void*]) _assign_T,
117                Fn(void, [void*, void*, void*]) _add_T,
118                void *_rtn,
119                void *x ) {
120        _add_T( _rtn, x, x );
121    }
122
123    void quadruple(size_t _sizeof_R, 
124                   Fn(void, [void*, void*]) _assign_R,
125                   Fn(void, [void*, void*]) _double_R,
126                   void *_rtn,
127                   void *y ) {
128        void *_tmp0 = alloca(_sizeof_R);
129        _double_R( _rtn, (_double_R( _tmp0, y ), _tmp0) );
130     }
131
132     void fred() {
133         // closure wrapper for static function
134         Fn(void, [void*, void*]) _thunk0 = { _builtin_assign_float };
135         // nested closure to adapt double() to _double_R()
136         Fn(void, [void*, void*]) _thunk1 = { double,
137             [sizeof(float), _builtin_assign_float, _builtin_add_float] };
138           
139         float magic;
140         float _tmp1 = 10.5f;
141         quadruple( sizeof(float), _thunk0, _thunk1,
142                    &magic, &_tmp1 );
143     }
144
145The main challenge with this approach is that the `Fn` closure is
146(necessarily) variable in size, as it can close over an arbitrary (but fixed
147at construction time) number of parameters. This will make memory management
148for it somewhat challenging, and writing the code in the translator to
149implement the function call operator passing a variable number of type
150assertions should also be non-trivial, but tractable.
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