Changeset 2ff78aae for doc/theses/fangren_yu_COOP_S20/Report.tex
- Timestamp:
- Sep 29, 2020, 1:50:22 PM (4 years ago)
- Branches:
- ADT, arm-eh, ast-experimental, enum, forall-pointer-decay, jacob/cs343-translation, master, new-ast-unique-expr, pthread-emulation, qualifiedEnum
- Children:
- 08e8851
- Parents:
- 6cc913e
- File:
-
- 1 edited
Legend:
- Unmodified
- Added
- Removed
-
doc/theses/fangren_yu_COOP_S20/Report.tex
r6cc913e r2ff78aae 112 112 \begin{itemize} 113 113 \item 114 type declaration: @struct@, @union@, @typedef@ or type parameter \TODO{(see Appendix A.3)}114 type declaration: @struct@, @union@, @typedef@ or type parameter (see Appendix A.1) 115 115 \item 116 116 variable declaration … … 374 374 375 375 \subsubsection{Source: \lstinline{AST/SymbolTable.hpp}} 376 377 \TODO{Add something here}378 379 380 376 \subsubsection{Source: \lstinline{SymTab/Indexer.h}} 381 377 … … 530 526 Each pair of compatible branch expression types produce a possible interpretation, and the cost is defined as the sum of the expression costs plus the sum of conversion costs to the common type. 531 527 532 \TODO{Write a specification for expression costs.} 528 \subsection{Conversion and Application Cost} 529 There were some unclear parts in the previous documentation of cost system, as described in the Moss thesis \cite{Moss19}, section 4.1.2. Some clarification are presented in this section. 530 531 \begin{enumerate} 532 \item 533 Conversion to a type denoted by parameter may incur additional cost if the match is not exact. For example, if a function is declared to accept @(T, T)@ and receives @(int, long)@, @T@ is deducted @long@ and an additional widening conversion cost is added for @int@ to @T@. 534 535 \item 536 The specialization level of a function is the sum of the least depth of an appearance of type parameter (counting pointers, references and parameterized types), plus the number of assertions. A higher specialization level is favored if conversion cost of arguments are equal. 537 538 \item 539 Coercion of pointer types is only allowed in explicit cast expressions; the only allowed implicit pointer casts are adding qualifiers to the base type and cast to @void*@, and those counts as safe conversions. Note that implicit cast from @void*@ to other pointer types is no longer valid, as opposed to standard C. 540 541 \end{enumerate} 533 542 534 543 … … 547 556 At the call site, implicit parameters are automatically inserted by the compiler. 548 557 549 \TODO{Explain how recursive assertion satisfaction and polymorphic recursion work.} 550 558 Implementation of implicit parameters is discussed in Appendix A.3. 551 559 552 560 \section{Tests} … … 589 597 It is suggested to run performance tests with optimization (@g++@ flag @-O3@). 590 598 599 \section{Appendix} 600 601 \subsection{Kinds of Type Parameters} 602 The type parameters in a @forall@ clause has three different kinds: 603 \begin{enumerate} 604 \item 605 @dtype@: any data type (built-in or user defined). There is also a difference between opaque types (incomplete types, those with only a forward declaration) and concrete types. Only concrete types can be directly used as a variable type. \CFA provides the @otype@ shorthand to require a type parameter as concrete, which also implicitly asserts the existence of its constructor and destructor\footnote{\CFA implements the same automatic resource management (RAII) semantics as \CC.}. 606 \item 607 @ftype@: any function type. Since @ftype@ does not provide any information about parameter types of a function, it is rarely used. The main purpose of introducing @ftype@ is to disallow a function to match a pointer overload, since variables and functions can have the same names. 608 \item 609 @ttype@: tuple (variadic) type. @ttype@ parameter may only appear as type of the last parameter in a function, and it provides a type-safe way to implement variadic functions. Note however, that it has certain restrictions, as described in the implementation section below. 610 611 \end{enumerate} 612 613 \subsection{GNU C Nested Functions} 614 615 \CFA is designed to be mostly compatible with GNU C, an extension to ISO C99 and C11 standards. The \CFA compiler also implements some language features by GCC extensions, most notably nested functions. 616 617 In ISO C, function definitions are not allowed to be nested. GCC allows nested functions with full lexical scoping. The following example is taken from GCC documentation\footnote{\url{https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html}}: 618 619 \begin{C++} 620 bar (int *array, int offset, int size) 621 { 622 int access (int *array, int index) 623 { return array[index + offset]; } 624 int i; 625 /* ... */ 626 for (i = 0; i < size; i++) 627 /* ... */ access (array, i) /* ... */ 628 } 629 \end{C++} 630 631 GCC nested functions behave identically to \CC lambda functions with default by-reference capture (stack-allocated, lifetime ends upon exiting the block declared in), while also possible to be passed as arguments with standard function pointer types. 632 633 \subsection{Implementation of Parametric Functions} 634 \CFA implements parametric functions using the implicit parameter approach: required assertions are passed to the callee by function pointers; size of a parametric type must also be known if referenced directly (i.e. not as a pointer). 635 636 The implementation is similar to the one from Scala\footnote{\url{https://www.scala-lang.org/files/archive/spec/2.13/07-implicits.html}}, with some notable differences in resolution: 637 \begin{enumerate} 638 \item 639 All types are function declarations are candidates of implicit parameters. 640 \item 641 The parameter (assertion) name must match the actual declarations. 642 \item 643 Currently, assertions are all functions. Note that since \CFA has variable overloading, implicit value parameters might also be supported in the future. 644 \end{enumerate} 645 646 For example, the \CFA function declaration 647 648 \begin{cfa} 649 forall(otype T | {int foo(T, int);}) 650 int bar(T); 651 \end{cfa} 652 653 after implicit parameter expansion, has the actual signature\footnote{\textbf{otype} also requires the type to have constructor and destructor, which are the first two function pointers preceding the one for \textbf{foo}.} 654 655 \begin{C++} 656 int bar(T, size_t, void (*)(T&), void (*)(T&), int (*)(T, int)); 657 \end{C++} 658 659 The implicit parameter approach has an apparent issue: when the satisfying declaration is also parametric, it may require its own implicit parameters too. That also causes the supplied implicit parameter to have a different \textbf{actual} type than the \textbf{nominal} type, so it cannot be passed directly. Therefore, a wrapper with matching actual type must be created, and here it is where GCC nested function is used internally by the compiler. 660 661 Consider the following program: 662 \begin{cfa} 663 int assertion(int); 664 665 forall (otype T | {int assertion(T);}) 666 void foo(T); 667 668 forall (otype T | {void foo(T);}) 669 void bar(T t) { 670 foo(t); 671 } 672 \end{cfa} 673 674 \CFA compiler translates the program to non-parametric form\footnote{In the final code output, T needs to be replaced by an opaque type, and arguments must be accessed by a frame pointer offset table, due to the unknown sizes. The presented code here is simplified for better understanding.} 675 676 \begin{C++} 677 // ctor, dtor and size arguments are omitted 678 void foo(T, int (*)(T)); 679 680 void bar(T t, void (*foo)(T)) { 681 foo(t); 682 } 683 \end{C++} 684 685 However, when @bar(1)@ is called, @foo@ cannot be directly provided as an argument: 686 687 \begin{C++} 688 bar(1, foo); // WRONG: foo has different actual type 689 \end{C++} 690 691 and an additional step is required: 692 693 \begin{C++} 694 { 695 void _foo_wrapper(int t) { 696 foo(t, assertion); 697 } 698 bar(1, _foo_wrapper); 699 } 700 \end{C++} 701 702 Nested assertions and implicit parameter creation may continue indefinitely. This is a limitation of implicit parameter implementation. In particular, polymorphic variadic recursion must be structural (i.e. number of arguments decreases in any possible recursive calls), otherwise code generation gets into an infinite loop. \CFA compiler sets a limit on assertion depth and reports an error if assertion resolution does not terminate within the limit. 591 703 592 704 \bibliographystyle{plain}
Note: See TracChangeset
for help on using the changeset viewer.