GNU Info
(gcc-295.info)Run-time Target
Run-time Target Specification
=============================
Here are run-time target specifications.
`CPP_PREDEFINES'
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 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 ANSI standard, so they are
predefined regardless of whether `-ansi' 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.
`TARGET_...'
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'; for example:
#define TARGET_68020 (target_flags & 1)
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.
`TARGET_SWITCHES'
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.
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:
#define TARGET_SWITCHES \
{ { "68020", 1, "" }, \
{ "68000", -1, "Compile for the 68000" }, \
{ "", 1, "" }}
`TARGET_OPTIONS'
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. 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.
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;
#define TARGET_OPTIONS \
{ { "short-data-", &m88k_short_data, "Specify the size of the short data section" } }
`TARGET_VERSION'
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:
#ifdef MOTOROLA
#define TARGET_VERSION \
fprintf (stderr, " (68k, Motorola syntax)");
#else
#define TARGET_VERSION \
fprintf (stderr, " (68k, MIT syntax)");
#endif
`OVERRIDE_OPTIONS'
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.
`OPTIMIZATION_OPTIONS (LEVEL, SIZE)'
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.
`CAN_DEBUG_WITHOUT_FP'
Define this macro if debugging can be performed even without a
frame pointer. If this macro is defined, GNU CC will turn on the
`-fomit-frame-pointer' option whenever `-O' is specified.
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