gcc/ada/targparm.ads

   1 ------------------------------------------------------------------------------
   2 --                                                                          --
   3 --                        GNAT RUN-TIME COMPONENTS                          --
   4 --                                                                          --
   5 --                             T A R G P A R M                              --
   6 --                                                                          --
   7 --                                 S p e c                                  --
   8 --                                                                          --
   9 --          Copyright (C) 1999-2001 Free Software Foundation, Inc.          --
  10 --                                                                          --
  11 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
  12 -- terms of the  GNU General Public License as published  by the Free Soft- --
  13 -- ware  Foundation;  either version 2,  or (at your option) any later ver- --
  14 -- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
  15 -- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
  16 -- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License --
  17 -- for  more details.  You should have  received  a copy of the GNU General --
  18 -- Public License  distributed with GNAT;  see file COPYING.  If not, write --
  19 -- to  the Free Software Foundation,  59 Temple Place - Suite 330,  Boston, --
  20 -- MA 02111-1307, USA.                                                      --
  21 --                                                                          --
  22 -- GNAT was originally developed  by the GNAT team at  New York University. --
  23 -- Extensive contributions were provided by Ada Core Technologies Inc.      --
  24 --                                                                          --
  25 ------------------------------------------------------------------------------
  26
  27 --  This package obtains parameters from the target runtime version of
  28 --  System, to indicate parameters relevant to the target environment.
  29
  30 --  Conceptually, these parameters could be obtained using rtsfind, but
  31 --  we do not do this for three reasons:
  32
  33 --    1. Compiling System for every compilation wastes time
  34 --    2. This compilation impedes debugging by adding extra compile steps
  35 --    3. There are recursion problems coming from compiling System itself
  36 --        or any of its children.
  37
  38 --  For all these reasons, we read in the source of System, and then scan
  39 --  it at the text level to extract the parameter values.
  40
  41 --  Note however, that later on, when the ali file is written, we make sure
  42 --  that the System file is at least parsed, so that the checksum is properly
  43 --  computed and set in the ali file. This partially negates points 1 and 2
  44 --  above although just parsing is quick and does not impact debugging much.
  45
  46 package Targparm is
  47
  48    --  The following parameters correspond to the variables defined in the
  49    --  private part of System (without the terminating _On_Target). Note
  50    --  that it is required that all parameters defined here be specified
  51    --  in the target specific version of system.ads (there are no defaults).
  52
  53    --  All these parameters should be regarded as read only by all clients
  54    --  of the package. The only way they get modified is by calling the
  55    --  Get_Target_Parameters routine which reads the values from System.
  56
  57    -------------------------------
  58    -- Backend Arithmetic Checks --
  59    -------------------------------
  60
  61    --  Divide and overflow checks are either done in the front end or
  62    --  back end. The front end will generate checks when required unless
  63    --  the corresponding parameter here is set to indicate that the back
  64    --  end will generate the required checks (or that the checks are
  65    --  automatically performed by the hardware in an appropriate form).
  66
  67    Backend_Divide_Checks_On_Target : Boolean;
  68    --  Set True if the back end generates divide checks, or if the hardware
  69    --  checks automatically. Set False if the front end must generate the
  70    --  required tests using explicit expanded code.
  71
  72    Backend_Overflow_Checks_On_Target : Boolean;
  73    --  Set True if the back end generates arithmetic overflow checks, or if
  74    --  the hardware checks automatically. Set False if the front end must
  75    --  generate the required tests using explicit expanded code.
  76
  77    -----------------------------------
  78    -- Control of Exception Handling --
  79    -----------------------------------
  80
  81    --  GNAT provides two methods of implementing exceptions:
  82
  83    --    Longjmp/Setjmp (-gnatL)
  84
  85    --      This approach uses longjmp/setjmp to handle exceptions. It
  86    --      uses less storage, and can often propagate exceptions faster,
  87    --      at the expense of (sometimes considerable) overhead in setting
  88    --      up an exception handler. This approach is available on all
  89    --      targets, and is the default where it is the only approach.
  90
  91    --    Zero Cost (-gnatZ)
  92
  93    --      This approach uses separate exception tables. These use extra
  94    --      storage, and exception propagation can be quite slow, but there
  95    --      is no overhead in setting up an exception handler (it is to this
  96    --      latter operation that the phrase zero-cost refers). This approach
  97    --      is only available on some targets, and is the default where it is
  98    --      available.
  99
 100    ZCX_By_Default_On_Target : Boolean;
 101    --  Indicates if zero cost exceptions are active by default.
 102
 103    GCC_ZCX_Support_On_Target  : Boolean;
 104    --  Indicates that when ZCX is active the mechanism to be used is the
 105    --  standard GCC ZCX mechanism  (introduced in GCC 3.1)
 106
 107    Front_End_ZCX_Support_On_Target : Boolean;
 108    --  Indicates that when ZCX is active (and GCC_ZCX_Support is not set)
 109    --  the mechanism to be used is the GNAT front end specific ZCX mechanism
 110
 111    ---------------------------------------
 112    -- High_Integrity (No Run Time) Mode --
 113    ---------------------------------------
 114
 115    --  In High_Integrity mode, there is no system run-time, and the flag
 116    --  Opt.No_Run_Time is set so that the language is appropriately
 117    --  restricted to forbid construct that would generate run-time calls.
 118
 119    High_Integrity_Mode_On_Target : Boolean;
 120    --  Indicates that this build is for a high integrity mode version of
 121    --  GNAT, so that no run time is permitted.
 122
 123    -------------------------------
 124    -- Control of Stack Checking --
 125    -------------------------------
 126
 127    --  GNAT provides two methods of implementing exceptions:
 128
 129    --    GCC Probing Mechanism
 130
 131    --      This approach uses the standard GCC mechanism for
 132    --      stack checking. The method assumes that accessing
 133    --      storage immediately beyond the end of the stack
 134    --      will result in a trap that is converted to a storage
 135    --      error by the runtime system. This mechanism has
 136    --      minimal overhead, but requires complex hardware,
 137    --      operating system and run-time support. Probing is
 138    --      the default method where it is available. The stack
 139    --      size for the environment task depends on the operating
 140    --      system and cannot be set in a system-independent way.
 141
 142    --   GNAT Stack-limit Checking
 143
 144    --      This method relies on comparing the stack pointer
 145    --      with per-task stack limits. If the check fails, an
 146    --      exception is explicitly raised. The advantage is
 147    --      that the method requires no extra system dependent
 148    --      runtime support and can be used on systems without
 149    --      memory protection as well, but at the cost of more
 150    --      overhead for doing the check. This method is the
 151    --      default on systems that lack complete support for
 152    --      probing.
 153
 154    Stack_Check_Probes_On_Target : Boolean;
 155    --  Indicates if stack check probes are used, as opposed to the standard
 156    --  target independent comparison method.
 157
 158    Stack_Check_Default_On_Target : Boolean;
 159    --  Indicates if stack checking is on by default
 160
 161    ----------------------------
 162    -- Command Line Arguments --
 163    ----------------------------
 164
 165    --  For most ports of GNAT, command line arguments are supported. The
 166    --  following flag is set to False for targets that do not support
 167    --  command line arguments (notably VxWorks).
 168
 169    Command_Line_Args_On_Target : Boolean;
 170    --  Set False if no command line arguments on target
 171
 172    --  Note: this is prepared for future use, but not yet used, since we
 173    --  do not yet have a way of propagating Targparm params to the binder
 174
 175    -----------------------
 176    -- Main Program Name --
 177    -----------------------
 178
 179    --  When the binder generates the main program to be used to create the
 180    --  executable, the main program name is main by default (to match the
 181    --  usual Unix practice). If this parameter is set to True, then the
 182    --  name is instead by default taken from the actual Ada main program
 183    --  name (just the name of the child if the main program is a child unit).
 184    --  In either case, this value can be overridden using -M name.
 185
 186    Use_Ada_Main_Program_Name_On_Target : Boolean;
 187    --  Set True to use the Ada main program name as the main name
 188
 189    --  Note: this is prepared for future use, but not yet used, since we
 190    --  do not yet have a way of propagating Targparm params to the binder
 191
 192    ----------------------------
 193    -- Support of Long Shifts --
 194    ----------------------------
 195
 196    --  In GNORT mode, we cannot call library routines, and in particular
 197    --  we cannot call routines for long (64-bit) shifts if such routines
 198    --  are required on the target. This comes up in the context of support
 199    --  of packed arrays. We can only represent packed arrays whose length
 200    --  is in the range 33- to 64-bits as modular types if long shifts are
 201    --  done with inline code.
 202
 203    --  For the default version, for now we set long shifts inlined as True
 204    --  This may not be quite accurate, but until we get proper separate
 205    --  System's for each target, it is a safer choice.
 206
 207    Long_Shifts_Inlined_On_Target : Boolean;
 208    --  Indicates if long (double word) shifts are generated using inlined
 209    --  code (and thus are permissible in No_Run_Time mode).
 210
 211    ----------------------------------------------
 212    -- Boolean-Valued Floating-Point Attributes --
 213    ----------------------------------------------
 214
 215    --  The constants below give the values for representation oriented
 216    --  floating-point attributes that are the same for all float types
 217    --  on the target. These are all boolean values.
 218
 219    --  A value is only True if the target reliably supports the corresponding
 220    --  feature. Reliably here means that support is guaranteed for all
 221    --  possible settings of the relevant compiler switches (like -mieee),
 222    --  since we cannot control the user setting of those switches.
 223
 224    --  The attributes cannot dependent on the current setting of compiler
 225    --  switches, since the values must be static and consistent throughout
 226    --  the partition. We probably should add such consistency checks in future,
 227    --  but for now we don't do this.
 228
 229    AAMP_On_Target : Boolean;
 230    --  Set to True if target is AAMP.
 231
 232    Denorm_On_Target : Boolean;
 233    --  Set to False on targets that do not reliably support denormals.
 234    --  Reliably here means for all settings of the relevant -m flag, so
 235    --  for example, this is False on the Alpha where denormals are not
 236    --  supported unless -mieee is used.
 237
 238    Machine_Rounds_On_Target : Boolean;
 239    --  Set to False for targets where S'Machine_Rounds is False
 240
 241    Machine_Overflows_On_Target : Boolean;
 242    --  Set to True for targets where S'Machine_Overflows is True
 243
 244    Signed_Zeros_On_Target : Boolean;
 245    --  Set to False on targets that do not reliably support signed zeros.
 246
 247    OpenVMS_On_Target : Boolean;
 248    --  Set to True if target is OpenVMS.
 249
 250    -------------------------------------------
 251    -- Boolean-Valued Fixed-Point Attributes --
 252    -------------------------------------------
 253
 254    Fractional_Fixed_Ops_On_Target : Boolean;
 255    --  Set to True for targets that support fixed-by-fixed multiplication
 256    --  and division for fixed-point types with a small value equal to
 257    --  2 ** (-(T'Object_Size - 1)) and whose values have an absolute
 258    --  value less than 1.0.
 259
 260    --------------------------------------------------------------
 261    -- Handling of Unconstrained Values Returned from Functions --
 262    --------------------------------------------------------------
 263
 264    --  Functions that return variable length objects, notably unconstrained
 265    --  arrays are a special case, because there is no simple obvious way of
 266    --  implementing this feature. Furthermore, this capability is not present
 267    --  in C++ or C, so typically the system ABI does not handle this case.
 268
 269    --  GNAT uses two different approaches
 270
 271    --    The Secondary Stack
 272
 273    --      The secondary stack is a special storage pool that is used for
 274    --      this purpose. The called function places the result on the
 275    --      secondary stack, and the caller uses or copies the value from
 276    --      the secondary stack, and pops the secondary stack after the
 277    --      value is consumed. The secondary stack is outside the system
 278    --      ABI, and the important point is that although generally it is
 279    --      handled in a stack like manner corresponding to the subprogram
 280    --      call structure, a return from a function does NOT pop the stack.
 281
 282    --    DSP (Depressed Stack Pointer)
 283
 284    --      Some targets permit the implementation of a function call/return
 285    --      protocol in which the function does not pop the main stack pointer
 286    --      on return, but rather returns with the stack pointer depressed.
 287    --      This is not generally permitted by any ABI, but for at least some
 288    --      targets, the implementation of alloca provides a model for this
 289    --      approach. If return-with-DSP is implemented, then functions that
 290    --      return variable length objects do it by returning with the stack
 291    --      pointer depressed, and the returned object is a pointer to the
 292    --      area within the stack frame of the called procedure that contains
 293    --      the returned value. The caller must then pop the main stack when
 294    --      this value is consumed.
 295
 296    Functions_Return_By_DSP_On_Target : Boolean;
 297    --  Set to True if target permits functions to return with using the
 298    --  DSP (depressed stack pointer) approach.
 299
 300    -----------------
 301    -- Data Layout --
 302    -----------------
 303
 304    --  Normally when using the GCC backend, Gigi and GCC perform much of the
 305    --  data layout using the standard layout capabilities of GCC. If the
 306    --  parameter Backend_Layout is set to False, then the front end must
 307    --  perform all data layout. For further details see the package Layout.
 308
 309    Frontend_Layout_On_Target : Boolean;
 310    --  Set True if front end does layout
 311
 312    -----------------
 313    -- Subprograms --
 314    -----------------
 315
 316    procedure Get_Target_Parameters;
 317    --  Called at the start of execution to read the source of System and
 318    --  obtain and set the values of the above parameters.
 319
 320 end Targparm;