Changeset 3f8ab8f


Ignore:
Timestamp:
Feb 7, 2018, 5:04:29 PM (4 years ago)
Author:
Peter A. Buhr <pabuhr@…>
Branches:
aaron-thesis, arm-eh, cleanup-dtors, deferred_resn, demangler, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, new-env, no_list, persistent-indexer, resolv-new, with_gc
Children:
169d944
Parents:
0723a57 (diff), 77acd07d (diff)
Note: this is a merge changeset, the changes displayed below correspond to the merge itself.
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Message:

Merge branch 'master' of plg2:software/cfa/cfa-cc

Files:
3 edited

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  • doc/papers/general/Paper.tex

    r0723a57 r3f8ab8f  
    12201220p2 = &y;  $\C{// p2 points to y}$
    12211221p3 = &p1;  $\C{// p3 points to p1}$
     1222*p2 = ((*p1 + *p2) * (**p3 - *p1)) / (**p3 - 15);
    12221223\end{cfa}
    12231224
    12241225Unfortunately, the dereference and address-of operators introduce a great deal of syntactic noise when dealing with pointed-to values rather than pointers, as well as the potential for subtle bugs.
    1225 It would be desirable to have the compiler figure out how to elide the dereference operators in a complex expression such as @*p2 = ((*p1 + *p2) * (**p3 - *p1)) / (**p3 - 15);@, for both brevity and clarity.
     1226For both brevity and clarity, it would be desirable to have the compiler figure out how to elide the dereference operators in a complex expression such as the assignment to @*p2@ above.
    12261227However, since C defines a number of forms of \emph{pointer arithmetic}, two similar expressions involving pointers to arithmetic types (\eg @*p1 + x@ and @p1 + x@) may each have well-defined but distinct semantics, introducing the possibility that a user programmer may write one when they mean the other, and precluding any simple algorithm for elision of dereference operators.
    12271228To solve these problems, \CFA introduces reference types @T&@; a @T&@ has exactly the same value as a @T*@, but where the @T*@ takes the address interpretation by default, a @T&@ takes the value interpretation by default, as below:
     
    12481249
    12491250Secondly, unlike the references in \CC which always point to a fixed address, \CFA references are rebindable.
    1250 This allows \CFA references to be default-initialized (to a null pointer), and also to point to different addresses throughout their lifetime.
     1251This allows \CFA references to be default-initialized (\eg to a null pointer), and also to point to different addresses throughout their lifetime.
    12511252This rebinding is accomplished without adding any new syntax to \CFA, but simply by extending the existing semantics of the address-of operator in C.
    12521253In C, the address of a lvalue is always a rvalue, as in general that address is not stored anywhere in memory, and does not itself have an address.
     
    12741275The syntactic motivation for this is clearest when considering overloaded operator-assignment, \eg @int ?+=?(int &, int)@; given @int x, y@, the expected call syntax is @x += y@, not @&x += y@.
    12751276
    1276 This initialization of references from lvalues rather than pointers can be considered a ``lvalue-to-reference'' conversion rather than an elision of the address-of operator; similarly, use of a the value pointed to by a reference in an rvalue context can be thought of as a ``reference-to-rvalue'' conversion.
    1277 \CFA includes one more reference conversion, an ``rvalue-to-reference'' conversion, implemented by means of an implicit temporary.
     1277More generally, this initialization of references from lvalues rather than pointers is an instance of a ``lvalue-to-reference'' conversion rather than an elision of the address-of operator; this conversion can actually be used in any context in \CFA an implicit conversion would be allowed.
     1278Similarly, use of a the value pointed to by a reference in an rvalue context can be thought of as a ``reference-to-rvalue'' conversion, and \CFA also includes a qualifier-adding ``reference-to-reference'' conversion, analagous to the @T *@ to @const T *@ conversion in standard C.
     1279The final reference conversion included in \CFA is ``rvalue-to-reference'' conversion, implemented by means of an implicit temporary.
    12781280When an rvalue is used to initialize a reference, it is instead used to initialize a hidden temporary value with the same lexical scope as the reference, and the reference is initialized to the address of this temporary.
    12791281This allows complex values to be succinctly and efficiently passed to functions, without the syntactic overhead of explicit definition of a temporary variable or the runtime cost of pass-by-value.
     
    12841286One of the strengths of C is the control over memory management it gives programmers, allowing resource release to be more consistent and precisely timed than is possible with garbage-collected memory management.
    12851287However, this manual approach to memory management is often verbose, and it is useful to manage resources other than memory (\eg file handles) using the same mechanism as memory.
    1286 \CC is well-known for an approach to manual memory management that addresses both these issues, Resource Allocation Is Initialization (RAII), implemented by means of special \emph{constructor} and \emph{destructor} functions; we have implemented a similar feature in \CFA.
    1287 
    1288 \TODO{Fill out section. Mention field-constructors and at-equal escape hatch to C-style initialization. Probably pull some text from Rob's thesis for first draft.}
    1289 
     1288\CC is well-known for an approach to manual memory management that addresses both these issues, Resource Aquisition Is Initialization (RAII), implemented by means of special \emph{constructor} and \emph{destructor} functions; we have implemented a similar feature in \CFA.
     1289While RAII is a common feature of object-oriented programming languages, its inclusion in \CFA does not violate the design principle that \CFA retain the same procedural paradigm as C.
     1290In particular, \CFA does not implement class-based encapsulation: neither the constructor nor any other function has privileged access to the implementation details of a type, except through the translation-unit-scope method of opaque structs provided by C.
     1291
     1292In \CFA, a constructor is a function named @?{}@, while a destructor is a function named @^?{}@; like other \CFA operators, these names represent the syntax used to call the constructor or destructor, \eg @S s = { ... };@ or @^(s){};@.
     1293Every constructor and destructor must have a return type of @void@, and its first parameter must have a reference type whose base type is the type of the object the function constructs or destructs.
     1294This first parameter is informally called the @this@ parameter, as in many object-oriented languages, though a programmer may give it an arbitrary name.
     1295Destructors must have exactly one parameter, while constructors allow passing of zero or more additional arguments along with the @this@ parameter.
     1296
     1297\begin{cfa}
     1298struct Array {
     1299        int * data;
     1300        int len;
     1301};
     1302
     1303void ?{}( Array& arr ) {
     1304        arr.len = 10;
     1305        arr.data = calloc( arr.len, sizeof(int) );
     1306}
     1307
     1308void ^?{}( Array& arr ) {
     1309        free( arr.data );
     1310}
     1311
     1312{
     1313        Array x;
     1314        `?{}(x);`       $\C{// implicitly compiler-generated}$
     1315        // ... use x
     1316        `^?{}(x);`      $\C{// implicitly compiler-generated}$
     1317}
     1318\end{cfa}
     1319
     1320In the example above, a \emph{default constructor} (\ie one with no parameters besides the @this@ parameter) and destructor are defined for the @Array@ struct, a dynamic array of @int@.
     1321@Array@ is an example of a \emph{managed type} in \CFA, a type with a non-trivial constructor or destructor, or with a field of a managed type.
     1322As in the example, all instances of managed types are implicitly constructed upon allocation, and destructed upon deallocation; this ensures proper initialization and cleanup of resources contained in managed types, in this case the @data@ array on the heap.
     1323The exact details of the placement of these implicit constructor and destructor calls are omitted here for brevity, the interested reader should consult \cite{Schluntz17}.
     1324
     1325Constructor calls are intended to seamlessly integrate with existing C initialization syntax, providing a simple and familiar syntax to veteran C programmers and allowing constructor calls to be inserted into legacy C code with minimal code changes.
     1326As such, \CFA also provides syntax for \emph{copy initialization} and \emph{initialization parameters}:
     1327
     1328\begin{cfa}
     1329void ?{}( Array& arr, Array other );
     1330
     1331void ?{}( Array& arr, int size, int fill );
     1332
     1333Array y = { 20, 0xDEADBEEF }, z = y;
     1334\end{cfa}
     1335
     1336Copy constructors have exactly two parameters, the second of which has the same type as the base type of the @this@ parameter; appropriate care is taken in the implementation to avoid recursive calls to the copy constructor when initializing this second parameter.
     1337Other constructor calls look just like C initializers, except rather than using field-by-field initialization (as in C), an initialization which matches a defined constructor will call the constructor instead.
     1338
     1339In addition to initialization syntax, \CFA provides two ways to explicitly call constructors and destructors.
     1340Explicit calls to constructors double as a placement syntax, useful for construction of member fields in user-defined constructors and reuse of large storage allocations.
     1341While the existing function-call syntax works for explicit calls to constructors and destructors, \CFA also provides a more concise \emph{operator syntax} for both:
     1342
     1343\begin{cfa}
     1344Array a, b;
     1345(a){};                                  $\C{// default construct}$
     1346(b){ a };                               $\C{// copy construct}$
     1347^(a){};                                 $\C{// destruct}$
     1348(a){ 5, 0xFFFFFFFF };   $\C{// explicit constructor call}$
     1349\end{cfa}
     1350
     1351To provide a uniform type interface for @otype@ polymorphism, the \CFA compiler automatically generates a default constructor, copy constructor, assignment operator, and destructor for all types.
     1352These default functions can be overridden by user-generated versions of them.
     1353For compatibility with the standard behaviour of C, the default constructor and destructor for all basic, pointer, and reference types do nothing, while the copy constructor and assignment operator are bitwise copies; if default zero-initialization is desired, the default constructors can be overridden.
     1354For user-generated types, the four functions are also automatically generated.
     1355@enum@ types are handled the same as their underlying integral type, and unions are also bitwise copied and no-op initialized and destructed.
     1356For compatibility with C, a copy constructor from the first union member type is also defined.
     1357For @struct@ types, each of the four functions are implicitly defined to call their corresponding functions on each member of the struct.
     1358To better simulate the behaviour of C initializers, a set of \emph{field constructors} is also generated for structures.
     1359A constructor is generated for each non-empty prefix of a structure's member-list which copy-constructs the members passed as parameters and default-constructs the remaining members.
     1360To allow users to limit the set of constructors available for a type, when a user declares any constructor or destructor, the corresponding generated function and all field constructors for that type are hidden from expression resolution; similarly, the generated default constructor is hidden upon declaration of any constructor.
     1361These semantics closely mirror the rule for implicit declaration of constructors in \CC\cite[p.~186]{ANSI98:C++}.
     1362
     1363In rare situations user programmers may not wish to have constructors and destructors called; in these cases, \CFA provides an ``escape hatch'' to not call them.
     1364If a variable is initialized using the syntax \lstinline|S x @= {}| it will be an \emph{unmanaged object}, and will not have constructors or destructors called.
     1365Any C initializer can be the right-hand side of an \lstinline|@=| initializer, \eg  \lstinline|Array a @= { 0, 0x0 }|, with the usual C initialization semantics.
     1366In addition to the expressive power, \lstinline|@=| provides a simple path for migrating legacy C code to \CFA, by providing a mechanism to incrementally convert initializers; the \CFA design team decided to introduce a new syntax for this escape hatch because we believe that our RAII implementation will handle the vast majority of code in a desirable way, and we wished to maintain familiar syntax for this common case.
    12901367
    12911368\subsection{Default Parameters}
  • src/libcfa/bits/defs.h

    r0723a57 r3f8ab8f  
    1 // 
     1//
    22// Cforall Version 1.0.0 Copyright (C) 2016 University of Waterloo
    33//
    44// The contents of this file are covered under the licence agreement in the
    55// file "LICENCE" distributed with Cforall.
    6 // 
    7 // defs.h -- 
    8 // 
     6//
     7// defs.h --
     8//
    99// Author           : Thierry Delisle
    1010// Created On       : Thu Nov  9 13:24:10 2017
     
    1212// Last Modified On : Tue Jan  2 09:17:06 2018
    1313// Update Count     : 2
    14 // 
     14//
    1515
    1616#pragma once
     
    3434
    3535#ifdef __cforall
     36#ifndef __NO_ABORT_OVERLOAD
     37void abort ( const char fmt[], ... ) __attribute__ ((__nothrow__, __leaf__, __noreturn__));
     38#endif
    3639extern "C" {
    3740#endif
  • src/libcfa/interpose.c

    r0723a57 r3f8ab8f  
    2828}
    2929
     30#define __NO_ABORT_OVERLOAD // no abort overload avoid ambiguities
    3031#include "bits/debug.h"
    3132#include "bits/defs.h"
    3233#include "bits/signal.h"
    3334#include "startup.h"
     35
     36//=============================================================================================
     37// Interposing helpers
     38//=============================================================================================
    3439
    3540typedef void (*generic_fptr_t)(void);
     
    6974}
    7075
    71 
    72 __typeof__( exit ) libc_exit __attribute__(( noreturn ));
    73 __typeof__( abort ) libc_abort __attribute__(( noreturn ));
    74 
    7576forall(dtype T)
    76 static inline void assign_ptr( T** symbol_ptr, const char * symbol_name, const char * version) {
     77static inline void ptr_from_symbol( T** symbol_ptr, const char * symbol_name, const char * version) {
    7778        union {
    7879                generic_fptr_t gp;
     
    8586}
    8687
    87 #define INIT_REALRTN( x, ver ) assign_ptr( (void**)&libc_##x, #x, ver)
     88#define INTERPOSE_LIBC( x, ver ) ptr_from_symbol( (void**)&__cabi_libc.x, #x, ver)
     89
     90//=============================================================================================
     91// Terminating Signals logic
     92//=============================================================================================
    8893
    8994void sigHandler_segv ( __CFA_SIGPARMS__ );
     
    9297void sigHandler_abort( __CFA_SIGPARMS__ );
    9398
     99struct {
     100        __typeof__( exit  ) exit  __attribute__(( noreturn ));
     101        __typeof__( abort ) abort __attribute__(( noreturn ));
     102} __cabi_libc;
     103
    94104extern "C" {
    95105        void __cfaabi_interpose_startup(void)  __attribute__(( constructor( STARTUP_PRIORITY_CORE ) ));
     
    97107                const char *version = NULL;
    98108
    99                 INIT_REALRTN( abort, version );
    100                 INIT_REALRTN( exit, version );
     109                INTERPOSE_LIBC( abort, version );
     110                INTERPOSE_LIBC( exit , version );
    101111
    102112                __cfaabi_sigaction( SIGSEGV, sigHandler_segv , SA_SIGINFO ); // Failure handler
     
    112122//=============================================================================================
    113123
     124// Forward declare abort after the __typeof__ call to avoid ambiguities
     125void abort ( const char fmt[], ... ) __attribute__ ((__nothrow__, __leaf__, __noreturn__));
     126
    114127extern "C" {
    115128        void abort( void ) __attribute__ ((__nothrow__, __leaf__, __noreturn__)) {
     
    117130        }
    118131
     132        void abortf( const char fmt[], ... ) __attribute__ ((__nothrow__, __leaf__, __noreturn__)) {
     133                va_list argp;
     134                va_start( argp, fmt );
     135                abort( fmt, argp );
     136                va_end( argp );
     137        }
     138
    119139        void exit( int __status ) __attribute__ ((__nothrow__, __leaf__, __noreturn__)) {
    120                 libc_exit(__status);
    121         }
    122 }
    123 
    124 void abort( const char fmt[], ... ) __attribute__ ((__nothrow__, __leaf__, __noreturn__)) {
    125         va_list argp;
    126         va_start( argp, fmt );
    127         abortf( fmt, argp );
    128         va_end( argp );
     140                __cabi_libc.exit(__status);
     141        }
    129142}
    130143
     
    137150static int abort_lastframe;
    138151
    139 extern "C" {
    140         void abortf( const char fmt[], ... ) __attribute__ ((__nothrow__, __leaf__, __noreturn__)) {
    141                 void * kernel_data = kernel_abort();                    // must be done here to lock down kernel
    142                 int len;
    143 
    144                 abort_lastframe = kernel_abort_lastframe();
    145                 len = snprintf( abort_text, abort_text_size, "Cforall Runtime error (UNIX pid:%ld) ", (long int)getpid() ); // use UNIX pid (versus getPid)
     152void abort( const char fmt[], ... ) __attribute__ ((__nothrow__, __leaf__, __noreturn__)) {
     153        void * kernel_data = kernel_abort();                    // must be done here to lock down kernel
     154        int len;
     155
     156        abort_lastframe = kernel_abort_lastframe();
     157        len = snprintf( abort_text, abort_text_size, "Cforall Runtime error (UNIX pid:%ld) ", (long int)getpid() ); // use UNIX pid (versus getPid)
     158        __cfaabi_dbg_bits_write( abort_text, len );
     159
     160        if ( fmt ) {
     161                va_list args;
     162                va_start( args, fmt );
     163
     164                len = vsnprintf( abort_text, abort_text_size, fmt, args );
     165                va_end( args );
    146166                __cfaabi_dbg_bits_write( abort_text, len );
    147167
    148                 if ( fmt ) {
    149                         va_list args;
    150                         va_start( args, fmt );
    151 
    152                         len = vsnprintf( abort_text, abort_text_size, fmt, args );
    153                         va_end( args );
    154                         __cfaabi_dbg_bits_write( abort_text, len );
    155 
    156                         if ( fmt[strlen( fmt ) - 1] != '\n' ) {         // add optional newline if missing at the end of the format text
    157                                 __cfaabi_dbg_bits_write( "\n", 1 );
    158                         }
    159                 }
    160 
    161                 kernel_abort_msg( kernel_data, abort_text, abort_text_size );
    162                 libc_abort();
    163         }
     168                if ( fmt[strlen( fmt ) - 1] != '\n' ) {         // add optional newline if missing at the end of the format text
     169                        __cfaabi_dbg_bits_write( "\n", 1 );
     170                }
     171        }
     172
     173        kernel_abort_msg( kernel_data, abort_text, abort_text_size );
     174        __cabi_libc.abort();
    164175}
    165176
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