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21.2 Run-time Target Specification

Here are run-time target specifications.

Define this to be a string constant containing `-D' options to define the predefined macros that identify this machine and system. These macros will be predefined unless the `-ansi' option (or a `-std' option for strict ISO C conformance) is specified.

In addition, a parallel set of macros are predefined, whose names are made by appending `__' at the beginning and at the end. These `__' macros are permitted by the ISO standard, so they are predefined regardless of whether `-ansi' or a `-std' option is specified.

For example, on the Sun, one can use the following value:

"-Dmc68000 -Dsun -Dunix"

The result is to define the macros __mc68000__, __sun__ and __unix__ unconditionally, and the macros mc68000, sun and unix provided `-ansi' is not specified.

extern int target_flags;
This declaration should be present.

This series of macros is to allow compiler command arguments to enable or disable the use of optional features of the target machine. For example, one machine description serves both the 68000 and the 68020; a command argument tells the compiler whether it should use 68020-only instructions or not. This command argument works by means of a macro TARGET_68020 that tests a bit in target_flags.

Define a macro TARGET_featurename for each such option. Its definition should test a bit in target_flags. It is recommended that a helper macro TARGET_MASK_featurename is defined for each bit-value to test, and used in TARGET_featurename and TARGET_SWITCHES. For example:

#define TARGET_MASK_68020 1
#define TARGET_68020 (target_flags & TARGET_MASK_68020)

One place where these macros are used is in the condition-expressions of instruction patterns. Note how TARGET_68020 appears frequently in the 68000 machine description file, `m68k.md'. Another place they are used is in the definitions of the other macros in the `machine.h' file.

This macro defines names of command options to set and clear bits in target_flags. Its definition is an initializer with a subgrouping for each command option.

Each subgrouping contains a string constant, that defines the option name, a number, which contains the bits to set in target_flags, and a second string which is the description displayed by `--help'. If the number is negative then the bits specified by the number are cleared instead of being set. If the description string is present but empty, then no help information will be displayed for that option, but it will not count as an undocumented option. The actual option name is made by appending `-m' to the specified name. Non-empty description strings should be marked with N_(...) for xgettext. In addition to the description for `--help', more detailed documentation for each option should be added to `invoke.texi'.

One of the subgroupings should have a null string. The number in this grouping is the default value for target_flags. Any target options act starting with that value.

Here is an example which defines `-m68000' and `-m68020' with opposite meanings, and picks the latter as the default:

  { { "68020", TARGET_MASK_68020, "" },      \
    { "68000", -TARGET_MASK_68020, \
      N_("Compile for the 68000") }, \
    { "", TARGET_MASK_68020, "" }}

This macro is similar to TARGET_SWITCHES but defines names of command options that have values. Its definition is an initializer with a subgrouping for each command option.

Each subgrouping contains a string constant, that defines the fixed part of the option name, the address of a variable, and a description string (which should again be marked with N_(...)). The variable, type char *, is set to the variable part of the given option if the fixed part matches. The actual option name is made by appending `-m' to the specified name. Again, each option should also be documented in `invoke.texi'.

Here is an example which defines `-mshort-data-number'. If the given option is `-mshort-data-512', the variable m88k_short_data will be set to the string "512".

extern char *m88k_short_data;
 { { "short-data-", &m88k_short_data, \
     N_("Specify the size of the short data section") } }

This macro is a C statement to print on stderr a string describing the particular machine description choice. Every machine description should define TARGET_VERSION. For example:

  fprintf (stderr, " (68k, Motorola syntax)");
  fprintf (stderr, " (68k, MIT syntax)");

Sometimes certain combinations of command options do not make sense on a particular target machine. You can define a macro OVERRIDE_OPTIONS to take account of this. This macro, if defined, is executed once just after all the command options have been parsed.

Don't use this macro to turn on various extra optimizations for `-O'. That is what OPTIMIZATION_OPTIONS is for.

Some machines may desire to change what optimizations are performed for various optimization levels. This macro, if defined, is executed once just after the optimization level is determined and before the remainder of the command options have been parsed. Values set in this macro are used as the default values for the other command line options.

level is the optimization level specified; 2 if `-O2' is specified, 1 if `-O' is specified, and 0 if neither is specified.

size is non-zero if `-Os' is specified and zero otherwise.

You should not use this macro to change options that are not machine-specific. These should uniformly selected by the same optimization level on all supported machines. Use this macro to enable machine-specific optimizations.

Do not examine write_symbols in this macro! The debugging options are not supposed to alter the generated code.

Define this macro if debugging can be performed even without a frame pointer. If this macro is defined, GCC will turn on the `-fomit-frame-pointer' option whenever `-O' is specified.

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This document was generated by Charlie & on June, 17 2001 using texi2html