tm.texi

GUN开源阻止下的编译器GCC

@enumerate
@item
Any prefixes specified by the user with @samp{-B}.

@item
The environment variable @code{GCC_EXEC_PREFIX}, if any.

@item
The directories specified by the environment variable @code{COMPILER_PATH}.

@item
The macro @code{STANDARD_EXEC_PREFIX}.

@item
@file{/usr/lib/gcc/}.

@item
The macro @code{MD_EXEC_PREFIX}, if any.
@end enumerate

Here is the order of prefixes tried for startfiles:

@enumerate
@item
Any prefixes specified by the user with @samp{-B}.

@item
The environment variable @code{GCC_EXEC_PREFIX}, if any.

@item
The directories specified by the environment variable @code{LIBRARY_PATH}
(native only, cross compilers do not use this).

@item
The macro @code{STANDARD_EXEC_PREFIX}.

@item
@file{/usr/lib/gcc/}.

@item
The macro @code{MD_EXEC_PREFIX}, if any.

@item
The macro @code{MD_STARTFILE_PREFIX}, if any.

@item
The macro @code{STANDARD_STARTFILE_PREFIX}.

@item
@file{/lib/}.

@item
@file{/usr/lib/}.
@end enumerate

@node Run-time Target
@section Run-time Target Specification
@cindex run-time target specification
@cindex predefined macros
@cindex target specifications

@c prevent bad page break with this line
Here are run-time target specifications.

@table @code
@findex CPP_PREDEFINES
@item CPP_PREDEFINES
Define this to be a string constant containing @samp{-D} options to
define the predefined macros that identify this machine and system.
These macros will be predefined unless the @samp{-ansi} option is
specified.

In addition, a parallel set of macros are predefined, whose names are
made by appending @samp{__} at the beginning and at the end.  These
@samp{__} macros are permitted by the ANSI standard, so they are
predefined regardless of whether @samp{-ansi} is specified.

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

@smallexample
"-Dmc68000 -Dsun -Dunix"
@end smallexample

The result is to define the macros @code{__mc68000__}, @code{__sun__}
and @code{__unix__} unconditionally, and the macros @code{mc68000},
@code{sun} and @code{unix} provided @samp{-ansi} is not specified.

@findex extern int target_flags
@item extern int target_flags;
This declaration should be present.

@cindex optional hardware or system features
@cindex features, optional, in system conventions
@item TARGET_@dots{}
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 @code{TARGET_68020} that tests a bit in
@code{target_flags}.

Define a macro @code{TARGET_@var{featurename}} for each such option.
Its definition should test a bit in @code{target_flags}; for example:

@smallexample
#define TARGET_68020 (target_flags & 1)
@end smallexample

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

@findex TARGET_SWITCHES
@item TARGET_SWITCHES
This macro defines names of command options to set and clear
bits in @code{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, and a number, which contains the bits to set in
@code{target_flags}.  A negative number says to clear bits instead;
the negative of the number is which bits to clear.  The actual option
name is made by appending @samp{-m} to the specified name.

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

Here is an example which defines @samp{-m68000} and @samp{-m68020}
with opposite meanings, and picks the latter as the default:

@smallexample
#define TARGET_SWITCHES \
  @{ @{ "68020", 1@},      \
    @{ "68000", -1@},     \
    @{ "", 1@}@}
@end smallexample

@findex TARGET_OPTIONS
@item TARGET_OPTIONS
This macro is similar to @code{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, and the address of a variable.  The variable, type
@code{char *}, is set to the variable part of the given option if the fixed
part matches.  The actual option name is made by appending @samp{-m} to the
specified name. 

Here is an example which defines @samp{-mshort-data-@var{number}}.  If the
given option is @samp{-mshort-data-512}, the variable @code{m88k_short_data}
will be set to the string @code{"512"}. 

@smallexample
extern char *m88k_short_data;
#define TARGET_OPTIONS \
 @{ @{ "short-data-", &m88k_short_data @} @}
@end smallexample

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

@smallexample
#ifdef MOTOROLA
#define TARGET_VERSION \
  fprintf (stderr, " (68k, Motorola syntax)");
#else
#define TARGET_VERSION \
  fprintf (stderr, " (68k, MIT syntax)");
#endif
@end smallexample

@findex OVERRIDE_OPTIONS
@item OVERRIDE_OPTIONS
Sometimes certain combinations of command options do not make sense on
a particular target machine.  You can define a macro
@code{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
@samp{-O}.  That is what @code{OPTIMIZATION_OPTIONS} is for.

@findex OPTIMIZATION_OPTIONS
@item OPTIMIZATION_OPTIONS (@var{level})
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.

@var{level} is the optimization level specified; 2 if @samp{-O2} is
specified, 1 if @samp{-O} is specified, and 0 if neither is specified.

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
machbine-specific optimizations.

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

@findex CAN_DEBUG_WITHOUT_FP
@item 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
@samp{-fomit-frame-pointer} option whenever @samp{-O} is specified.
@end table

@node Storage Layout
@section Storage Layout
@cindex storage layout

Note that the definitions of the macros in this table which are sizes or
alignments measured in bits do not need to be constant.  They can be C
expressions that refer to static variables, such as the @code{target_flags}.
@xref{Run-time Target}.

@table @code
@findex BITS_BIG_ENDIAN
@item BITS_BIG_ENDIAN
Define this macro to have the value 1 if the most significant bit in a
byte has the lowest number; otherwise define it to have the value zero.
This means that bit-field instructions count from the most significant
bit.  If the machine has no bit-field instructions, then this must still
be defined, but it doesn't matter which value it is defined to.  This
macro need not be a constant.

This macro does not affect the way structure fields are packed into
bytes or words; that is controlled by @code{BYTES_BIG_ENDIAN}.

@findex BYTES_BIG_ENDIAN
@item BYTES_BIG_ENDIAN
Define this macro to have the value 1 if the most significant byte in a
word has the lowest number.  This macro need not be a constant.

@findex WORDS_BIG_ENDIAN
@item WORDS_BIG_ENDIAN
Define this macro to have the value 1 if, in a multiword object, the
most significant word has the lowest number.  This applies to both
memory locations and registers; GNU CC fundamentally assumes that the
order of words in memory is the same as the order in registers.  This
macro need not be a constant.

@findex LIBGCC2_WORDS_BIG_ENDIAN
@item LIBGCC2_WORDS_BIG_ENDIAN
Define this macro if WORDS_BIG_ENDIAN is not constant.  This must be a
constant value with the same meaning as WORDS_BIG_ENDIAN, which will be
used only when compiling libgcc2.c.  Typically the value will be set
based on preprocessor defines.

@findex FLOAT_WORDS_BIG_ENDIAN
@item FLOAT_WORDS_BIG_ENDIAN
Define this macro to have the value 1 if @code{DFmode}, @code{XFmode} or
@code{TFmode} floating point numbers are stored in memory with the word
containing the sign bit at the lowest address; otherwise define it to
have the value 0.  This macro need not be a constant.

You need not define this macro if the ordering is the same as for
multi-word integers.

@findex BITS_PER_UNIT
@item BITS_PER_UNIT
Define this macro to be the number of bits in an addressable storage
unit (byte); normally 8.

@findex BITS_PER_WORD
@item BITS_PER_WORD
Number of bits in a word; normally 32.

@findex MAX_BITS_PER_WORD
@item MAX_BITS_PER_WORD
Maximum number of bits in a word.  If this is undefined, the default is
@code{BITS_PER_WORD}.  Otherwise, it is the constant value that is the
largest value that @code{BITS_PER_WORD} can have at run-time.

@findex UNITS_PER_WORD
@item UNITS_PER_WORD
Number of storage units in a word; normally 4.

@findex MIN_UNITS_PER_WORD
@item MIN_UNITS_PER_WORD
Minimum number of units in a word.  If this is undefined, the default is
@code{UNITS_PER_WORD}.  Otherwise, it is the constant value that is the
smallest value that @code{UNITS_PER_WORD} can have at run-time.

@findex POINTER_SIZE
@item POINTER_SIZE
Width of a pointer, in bits.  You must specify a value no wider than the
width of @code{Pmode}.  If it is not equal to the width of @code{Pmode},
you must define @code{POINTERS_EXTEND_UNSIGNED}.

@findex POINTERS_EXTEND_UNSIGNED
@item POINTERS_EXTEND_UNSIGNED
A C expression whose value is nonzero if pointers that need to be 
extended from being @code{POINTER_SIZE} bits wide to @code{Pmode}
are sign-extended and zero if they are zero-extended.

You need not define this macro if the @code{POINTER_SIZE} is equal
to the width of @code{Pmode}.

@findex PROMOTE_MODE
@item PROMOTE_MODE (@var{m}, @var{unsignedp}, @var{type})
A macro to update @var{m} and @var{unsignedp} when an object whose type
is @var{type} and which has the specified mode and signedness is to be
stored in a register.  This macro is only called when @var{type} is a
scalar type.

On most RISC machines, which only have operations that operate on a full
register, define this macro to set @var{m} to @code{word_mode} if
@var{m} is an integer mode narrower than @code{BITS_PER_WORD}.  In most
cases, only integer modes should be widened because wider-precision
floating-point operations are usually more expensive than their narrower
counterparts.

For most machines, the macro definition does not change @var{unsignedp}.
However, some machines, have instructions that preferentially handle
either signed or unsigned quantities of certain modes.  For example, on
the DEC Alpha, 32-bit loads from memory and 32-bit add instructions
sign-extend the result to 64 bits.  On such machines, set
@var{unsignedp} according to which kind of extension is more efficient.