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* gccint: (gccint).            Internals of the GNU Compiler Collection.
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   This file documents the internals of the GNU compilers.

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File: gccint.info,  Node: Target Attributes,  Next: Misc,  Prev: Mode Switching,  Up: Target Macros

Defining target-specific uses of `__attribute__'
================================================

   Target-specific attributes may be defined for functions, data and
types.  These are described using the following target hooks; they also
need to be documented in `extend.texi'.

 - Target Hook: const struct attribute_spec * TARGET_ATTRIBUTE_TABLE
     If defined, this target hook points to an array of `struct
     attribute_spec' (defined in `tree.h') specifying the machine
     specific attributes for this target and some of the restrictions
     on the entities to which these attributes are applied and the
     arguments they take.

 - Target Hook: int TARGET_COMP_TYPE_ATTRIBUTES (tree TYPE1, tree TYPE2)
     If defined, this target hook is a function which returns zero if
     the attributes on TYPE1 and TYPE2 are incompatible, one if they
     are compatible, and two if they are nearly compatible (which
     causes a warning to be generated).  If this is not defined,
     machine-specific attributes are supposed always to be compatible.

 - Target Hook: void TARGET_SET_DEFAULT_TYPE_ATTRIBUTES (tree TYPE)
     If defined, this target hook is a function which assigns default
     attributes to newly defined TYPE.

 - Target Hook: tree TARGET_MERGE_TYPE_ATTRIBUTES (tree TYPE1, tree
          TYPE2)
     Define this target hook if the merging of type attributes needs
     special handling.  If defined, the result is a list of the combined
     `TYPE_ATTRIBUTES' of TYPE1 and TYPE2.  It is assumed that
     `comptypes' has already been called and returned 1.  This function
     may call `merge_attributes' to handle machine-independent merging.

 - Target Hook: tree TARGET_MERGE_DECL_ATTRIBUTES (tree OLDDECL, tree
          NEWDECL)
     Define this target hook if the merging of decl attributes needs
     special handling.  If defined, the result is a list of the combined
     `DECL_ATTRIBUTES' of OLDDECL and NEWDECL.  NEWDECL is a duplicate
     declaration of OLDDECL.  Examples of when this is needed are when
     one attribute overrides another, or when an attribute is nullified
     by a subsequent definition.  This function may call
     `merge_attributes' to handle machine-independent merging.

     If the only target-specific handling you require is `dllimport' for
     Windows targets, you should define the macro
     `TARGET_DLLIMPORT_DECL_ATTRIBUTES'.  This links in a function
     called `merge_dllimport_decl_attributes' which can then be defined
     as the expansion of `TARGET_MERGE_DECL_ATTRIBUTES'.  This is done
     in `i386/cygwin.h' and `i386/i386.c', for example.

 - Target Hook: void TARGET_INSERT_ATTRIBUTES (tree NODE, tree
          *ATTR_PTR)
     Define this target hook if you want to be able to add attributes
     to a decl when it is being created.  This is normally useful for
     back ends which wish to implement a pragma by using the attributes
     which correspond to the pragma's effect.  The NODE argument is the
     decl which is being created.  The ATTR_PTR argument is a pointer
     to the attribute list for this decl.  The list itself should not
     be modified, since it may be shared with other decls, but
     attributes may be chained on the head of the list and `*ATTR_PTR'
     modified to point to the new attributes, or a copy of the list may
     be made if further changes are needed.

 - Target Hook: bool TARGET_FUNCTION_ATTRIBUTE_INLINABLE_P (tree FNDECL)
     This target hook returns `true' if it is ok to inline FNDECL into
     the current function, despite its having target-specific
     attributes, `false' otherwise.  By default, if a function has a
     target specific attribute attached to it, it will not be inlined.


File: gccint.info,  Node: Misc,  Prev: Target Attributes,  Up: Target Macros

Miscellaneous Parameters
========================

   Here are several miscellaneous parameters.

`PREDICATE_CODES'
     Define this if you have defined special-purpose predicates in the
     file `MACHINE.c'.  This macro is called within an initializer of an
     array of structures.  The first field in the structure is the name
     of a predicate and the second field is an array of rtl codes.  For
     each predicate, list all rtl codes that can be in expressions
     matched by the predicate.  The list should have a trailing comma.
     Here is an example of two entries in the list for a typical RISC
     machine:

          #define PREDICATE_CODES \
            {"gen_reg_rtx_operand", {SUBREG, REG}},  \
            {"reg_or_short_cint_operand", {SUBREG, REG, CONST_INT}},

     Defining this macro does not affect the generated code (however,
     incorrect definitions that omit an rtl code that may be matched by
     the predicate can cause the compiler to malfunction).  Instead, it
     allows the table built by `genrecog' to be more compact and
     efficient, thus speeding up the compiler.  The most important
     predicates to include in the list specified by this macro are
     those used in the most insn patterns.

     For each predicate function named in `PREDICATE_CODES', a
     declaration will be generated in `insn-codes.h'.

`SPECIAL_MODE_PREDICATES'
     Define this if you have special predicates that know special things
     about modes.  Genrecog will warn about certain forms of
     `match_operand' without a mode; if the operand predicate is listed
     in `SPECIAL_MODE_PREDICATES', the warning will be suppressed.

     Here is an example from the IA-32 port (`ext_register_operand'
     specially checks for `HImode' or `SImode' in preparation for a
     byte extraction from `%ah' etc.).

          #define SPECIAL_MODE_PREDICATES \
            "ext_register_operand",

`CASE_VECTOR_MODE'
     An alias for a machine mode name.  This is the machine mode that
     elements of a jump-table should have.

`CASE_VECTOR_SHORTEN_MODE (MIN_OFFSET, MAX_OFFSET, BODY)'
     Optional: return the preferred mode for an `addr_diff_vec' when
     the minimum and maximum offset are known.  If you define this, it
     enables extra code in branch shortening to deal with
     `addr_diff_vec'.  To make this work, you also have to define
     INSN_ALIGN and make the alignment for `addr_diff_vec' explicit.
     The BODY argument is provided so that the offset_unsigned and scale
     flags can be updated.

`CASE_VECTOR_PC_RELATIVE'
     Define this macro to be a C expression to indicate when jump-tables
     should contain relative addresses.  If jump-tables never contain
     relative addresses, then you need not define this macro.

`CASE_DROPS_THROUGH'
     Define this if control falls through a `case' insn when the index
     value is out of range.  This means the specified default-label is
     actually ignored by the `case' insn proper.

`CASE_VALUES_THRESHOLD'
     Define this to be the smallest number of different values for
     which it is best to use a jump-table instead of a tree of
     conditional branches.  The default is four for machines with a
     `casesi' instruction and five otherwise.  This is best for most
     machines.

`WORD_REGISTER_OPERATIONS'
     Define this macro if operations between registers with integral
     mode smaller than a word are always performed on the entire
     register.  Most RISC machines have this property and most CISC
     machines do not.

`LOAD_EXTEND_OP (MODE)'
     Define this macro to be a C expression indicating when insns that
     read memory in MODE, an integral mode narrower than a word, set the
     bits outside of MODE to be either the sign-extension or the
     zero-extension of the data read.  Return `SIGN_EXTEND' for values
     of MODE for which the insn sign-extends, `ZERO_EXTEND' for which
     it zero-extends, and `NIL' for other modes.

     This macro is not called with MODE non-integral or with a width
     greater than or equal to `BITS_PER_WORD', so you may return any
     value in this case.  Do not define this macro if it would always
     return `NIL'.  On machines where this macro is defined, you will
     normally define it as the constant `SIGN_EXTEND' or `ZERO_EXTEND'.

`SHORT_IMMEDIATES_SIGN_EXTEND'
     Define this macro if loading short immediate values into registers
     sign extends.

`FIXUNS_TRUNC_LIKE_FIX_TRUNC'
     Define this macro if the same instructions that convert a floating
     point number to a signed fixed point number also convert validly
     to an unsigned one.

`MOVE_MAX'
     The maximum number of bytes that a single instruction can move
     quickly between memory and registers or between two memory
     locations.

`MAX_MOVE_MAX'
     The maximum number of bytes that a single instruction can move
     quickly between memory and registers or between two memory
     locations.  If this is undefined, the default is `MOVE_MAX'.
     Otherwise, it is the constant value that is the largest value that
     `MOVE_MAX' can have at run-time.

`SHIFT_COUNT_TRUNCATED'
     A C expression that is nonzero if on this machine the number of
     bits actually used for the count of a shift operation is equal to
     the number of bits needed to represent the size of the object
     being shifted.  When this macro is nonzero, the compiler will
     assume that it is safe to omit a sign-extend, zero-extend, and
     certain bitwise `and' instructions that truncates the count of a
     shift operation.  On machines that have instructions that act on
     bit-fields at variable positions, which may include `bit test'
     instructions, a nonzero `SHIFT_COUNT_TRUNCATED' also enables
     deletion of truncations of the values that serve as arguments to
     bit-field instructions.

     If both types of instructions truncate the count (for shifts) and
     position (for bit-field operations), or if no variable-position
     bit-field instructions exist, you should define this macro.

     However, on some machines, such as the 80386 and the 680x0,
     truncation only applies to shift operations and not the (real or
     pretended) bit-field operations.  Define `SHIFT_COUNT_TRUNCATED'
     to be zero on such machines.  Instead, add patterns to the `md'
     file that include the implied truncation of the shift instructions.

     You need not define this macro if it would always have the value
     of zero.

`TRULY_NOOP_TRUNCATION (OUTPREC, INPREC)'
     A C expression which is nonzero if on this machine it is safe to
     "convert" an integer of INPREC bits to one of OUTPREC bits (where
     OUTPREC is smaller than INPREC) by merely operating on it as if it
     had only OUTPREC bits.

     On many machines, this expression can be 1.

     When `TRULY_NOOP_TRUNCATION' returns 1 for a pair of sizes for
     modes for which `MODES_TIEABLE_P' is 0, suboptimal code can result.
     If this is the case, making `TRULY_NOOP_TRUNCATION' return 0 in
     such cases may improve things.

`STORE_FLAG_VALUE'
     A C expression describing the value returned by a comparison
     operator with an integral mode and stored by a store-flag
     instruction (`sCOND') when the condition is true.  This
     description must apply to _all_ the `sCOND' patterns and all the
     comparison operators whose results have a `MODE_INT' mode.

     A value of 1 or -1 means that the instruction implementing the
     comparison operator returns exactly 1 or -1 when the comparison is
     true and 0 when the comparison is false.  Otherwise, the value
     indicates which bits of the result are guaranteed to be 1 when the
     comparison is true.  This value is interpreted in the mode of the
     comparison operation, which is given by the mode of the first
     operand in the `sCOND' pattern.  Either the low bit or the sign
     bit of `STORE_FLAG_VALUE' be on.  Presently, only those bits are
     used by the compiler.

     If `STORE_FLAG_VALUE' is neither 1 or -1, the compiler will
     generate code that depends only on the specified bits.  It can also
     replace comparison operators with equivalent operations if they
     cause the required bits to be set, even if the remaining bits are
     undefined.  For example, on a machine whose comparison operators
     return an `SImode' value and where `STORE_FLAG_VALUE' is defined as
     `0x80000000', saying that just the sign bit is relevant, the
     expression

          (ne:SI (and:SI X (const_int POWER-OF-2)) (const_int 0))

     can be converted to

          (ashift:SI X (const_int N))

     where N is the appropriate shift count to move the bit being
     tested into the sign bit.

     There is no way to describe a machine that always sets the
     low-order bit for a true value, but does not guarantee the value
     of any other bits, but we do not know of any machine that has such
     an instruction.  If you are trying to port GCC to such a machine,
     include an instruction to perform a logical-and of the result with
     1 in the pattern for the comparison operators and let us know at
     <gcc@gcc.gnu.org>.

     Often, a machine will have multiple instructions that obtain a
     value from a comparison (or the condition codes).  Here are rules
     to guide the choice of value for `STORE_FLAG_VALUE', and hence the
     instructions to be used:

        * Use the shortest sequence that yields a valid definition for
          `STORE_FLAG_VALUE'.  It is more efficient for the compiler to
          "normalize" the value (convert it to, e.g., 1 or 0) than for
          the comparison operators to do so because there may be
          opportunities to combine the normalization with other
          operations.

        * For equal-length sequences, use a value of 1 or -1, with -1
          being slightly preferred on machines with expensive jumps and
          1 preferred on other machines.

        * As a second choice, choose a value of `0x80000001' if
          instructions exist that set both the sign and low-order bits
          but do not define the others.

        * Otherwise, use a value of `0x80000000'.

     Many machines can produce both the value chosen for
     `STORE_FLAG_VALUE' and its negation in the same number of
     instructions.  On those machines, you should also define a pattern
     for those cases, e.g., one matching

          (set A (neg:M (ne:M B C)))

     Some machines can also perform `and' or `plus' operations on
     condition code values with less instructions than the corresponding
     `sCOND' insn followed by `and' or `plus'.  On those machines,
     define the appropriate patterns.  Use the names `incscc' and
     `decscc', respectively, for the patterns which perform `plus' or
     `minus' operations on condition code values.  See `rs6000.md' for
     some examples.  The GNU Superoptizer can be used to find such
     instruction sequences on other machines.

     You need not define `STORE_FLAG_VALUE' if the machine has no
     store-flag instructions.

`FLOAT_STORE_FLAG_VALUE (MODE)'
     A C expression that gives a nonzero `REAL_VALUE_TYPE' value that is
     returned when comparison operators with floating-point results are
     true.  Define this macro on machine that have comparison
     operations that return floating-point values.  If there are no
     such operations, do not define this macro.

`Pmode'
     An alias for the machine mode for pointers.  On most machines,
     define this to be the integer mode corresponding to the width of a
     hardware pointer; `SImode' on 32-bit machine or `DImode' on 64-bit
     machines.  On some machines you must define this to be one of the
     partial integer modes, such as `PSImode'.

     The width of `Pmode' must be at least as large as the value of
     `POINTER_SIZE'.  If it is not equal, you must define the macro
     `POINTERS_EXTEND_UNSIGNED' to specify how pointers are extended to
     `Pmode'.

`FUNCTION_MODE'
     An alias for the machine mode used for memory references to
     functions being called, in `call' RTL expressions.  On most
     machines this should be `QImode'.

`INTEGRATE_THRESHOLD (DECL)'
     A C expression for the maximum number of instructions above which
     the function DECL should not be inlined.  DECL is a
     `FUNCTION_DECL' node.

     The default definition of this macro is 64 plus 8 times the number
     of arguments that the function accepts.  Some people think a larger
     threshold should be used on RISC machines.

`STDC_0_IN_SYSTEM_HEADERS'
     In normal operation, the preprocessor expands `__STDC__' to the
     constant 1, to signify that GCC conforms to ISO Standard C.  On
     some hosts, like Solaris, the system compiler uses a different
     convention, where `__STDC__' is normally 0, but is 1 if the user
     specifies strict conformance to the C Standard.

     Defining `STDC_0_IN_SYSTEM_HEADERS' makes GNU CPP follows the host
     convention when processing system header files, but when
     processing user files `__STDC__' will always expand to 1.

`SCCS_DIRECTIVE'
     Define this if the preprocessor should ignore `#sccs' directives
     and print no error message.

`NO_IMPLICIT_EXTERN_C'
     Define this macro if the system header files support C++ as well
     as C.  This macro inhibits the usual method of using system header
     files in C++, which is to pretend that the file's contents are
     enclosed in `extern "C" {...}'.

`HANDLE_PRAGMA (GETC, UNGETC, NAME)'
     This macro is no longer supported.  You must use
     `REGISTER_TARGET_PRAGMAS' instead.

`REGISTER_TARGET_PRAGMAS (PFILE)'
     Define this macro if you want to implement any target-specific
     pragmas.  If defined, it is a C expression which makes a series of
     calls to `cpp_register_pragma' for each pragma, with PFILE passed
     as the first argument to to these functions.  The macro may also
     do any setup required for the pragmas.

     The primary reason to define this macro is to provide
     compatibility with other compilers for the same target.  In
     general, we discourage definition of target-specific pragmas for
     GCC.

     If the pragma can be implemented by attributes then you should
     consider defining the target hook `TARGET_INSERT_ATTRIBUTES' as
     well.

     Preprocessor macros that appear on pragma lines are not expanded.
     All `#pragma' directives that do not match any registered pragma
     are silently ignored, unless the user specifies
     `-Wunknown-pragmas'.

      - Function: void cpp_register_pragma (cpp_reader *PFILE, const
               char *SPACE, const char *NAME, void (*CALLBACK)
               (cpp_reader *))
          Each call to `cpp_register_pragma' establishes one pragma.
          The CALLBACK routine will be called when the preprocessor
          encounters a pragma of the form

               #pragma [SPACE] NAME ...

          SPACE is the case-sensitive namespace of the pragma, or
          `NULL' to put the pragma in the global namespace.  The
          callback routine receives PFILE as its first argument, which
          can be passed on to cpplib's functions if necessary.  You can
          lex tokens after the NAME by calling `c_lex'.  Tokens that
          are not read by the callback will be silently ignored.  The
          end of the line is indicated by a token of type `CPP_EOF'.

          For an example use of this routine, see `c4x.h' and the
          callback routines defined in `c4x-c.c'.

          Note that the use of `c_lex' is specific to the C and C++
          compilers.  It will not work in the Java or Fortran
          compilers, or any other language compilers for that matter.
          Thus if `c_lex' is going to be called from target-specific
          code, it must only be done so when building the C and C++
          compilers.  This can be done by defining the variables
          `c_target_objs' and `cxx_target_objs' in the target entry in
          the `config.gcc' file.  These variables should name the
          target-specific, language-specific object file which contains
          the code that uses `c_lex'.  Note it will also be necessary
          to add a rule to the makefile fragment pointed to by
          `tmake_file' that shows how to build this object file.

`HANDLE_SYSV_PRAGMA'
     Define this macro (to a value of 1) if you want the System V style
     pragmas `#pragma pack(<n>)' and `#pragma weak <name> [=<value>]'
     to be supported by gcc.

     The pack pragma specifies the maximum alignment (in bytes) of
     fields within a structure, in much the same way as the
     `__aligned__' and `__packed__' `__attribute__'s do.  A pack value
     of zero resets the behavior to the default.

     The weak pragma only works if `SUPPORTS_WEAK' and
     `ASM_WEAKEN_LABEL' are defined.  If enabled it allows the creation
     of specifically named weak labels, optionally with a value.

`HANDLE_PRAGMA_PACK_PUSH_POP'
     Define this macro (to a value of 1) if you want to support the
     Win32 style pragmas `#pragma pack(push,N)' and `#pragma
     pack(pop)'.  The `pack(push,N)' pragma specifies the maximum
     alignment (in bytes) of fields within a structure, in much the
     same way as the `__aligned__' and `__packed__' `__attribute__'s
     do.  A pack value of zero resets the behavior to the default.
     Successive invocations of this pragma cause the previous values to
     be stacked, so that invocations of `#pragma pack(pop)' will return
     to the previous value.

`DOLLARS_IN_IDENTIFIERS'
     Define this macro to control use of the character `$' in identifier
     names.  0 means `$' is not allowed by default; 1 means it is
     allowed.  1 is the default; there is no need to define this macro
     in that case.  This macro controls the compiler proper; it does
     not affect the preprocessor.

`NO_DOLLAR_IN_LABEL'
     Define this macro if the assembler does not accept the character
     `$' in label names.  By default constructors and destructors in
     G++ have `$' in the identifiers.  If this macro is defined, `.' is
     used instead.

`NO_DOT_IN_LABEL'
     Define this macro if the assembler does not accept the character
     `.' in label names.  By default constructors and destructors in G++
     have names that use `.'.  If this macro is defined, these names
     are rewritten to avoid `.'.

`DEFAULT_MAIN_RETURN'
     Define this macro if the target system expects every program's
     `main' function to return a standard "success" value by default
     (if no other value is explicitly returned).

     The definition should be a C statement (sans semicolon) to
     generate the appropriate rtl instructions.  It is used only when
     compiling the end of `main'.

`NEED_ATEXIT'
     Define this if the target system lacks the function `atexit' from
     the ISO C standard.  If this macro is defined, a default definition
     will be provided to support C++.  If `ON_EXIT' is not defined, a
     default `exit' function will also be provided.

`ON_EXIT'
     Define this macro if the target has another way to implement atexit
     functionality without replacing `exit'.  For instance, SunOS 4 has
     a similar `on_exit' library function.

     The definition should be a functional macro which can be used just
     like the `atexit' function.

`EXIT_BODY'
     Define this if your `exit' function needs to do something besides
     calling an external function `_cleanup' before terminating with
     `_exit'.  The `EXIT_BODY' macro is only needed if `NEED_ATEXIT' is
     defined and `ON_EXIT' is not defined.

`INSN_SETS_ARE_DELAYED (INSN)'
     Define this macro as a C expression that is nonzero if it is safe
     for the delay slot scheduler to place instructions in the delay
     slot of INSN, even if they appear to use a resource set or
     clobbered in INSN.  INSN is always a `jump_insn' or an `insn'; GCC
     knows that every `call_insn' has this behavior.  On machines where
     some `insn' or `jump_insn' is really a function call and hence has
     this behavior, you should define this macro.

     You need not define this macro if it would always return zero.

`INSN_REFERENCES_ARE_DELAYED (INSN)'
     Define this macro as a C expression that is nonzero if it is safe
     for the delay slot scheduler to place instructions in the delay
     slot of INSN, even if they appear to set or clobber a resource
     referenced in INSN.  INSN is always a `jump_insn' or an `insn'.
     On machines where some `insn' or `jump_insn' is really a function
     call and its operands are registers whose use is actually in the
     subroutine it calls, you should define this macro.  Doing so
     allows the delay slot scheduler to move instructions which copy
     arguments into the argument registers into the delay slot of INSN.

     You need not define this macro if it would always return zero.

`MACHINE_DEPENDENT_REORG (INSN)'
     In rare cases, correct code generation requires extra machine
     dependent processing between the second jump optimization pass and
     delayed branch scheduling.  On those machines, define this macro
     as a C statement to act on the code starting at INSN.

`MULTIPLE_SYMBOL_SPACES'
     Define this macro if in some cases global symbols from one
     translation unit may not be bound to undefined symbols in another
     translation unit without user intervention.  For instance, under
     Microsoft Windows symbols must be explicitly imported from shared
     libraries (DLLs).

`MD_ASM_CLOBBERS (CLOBBERS)'
     A C statement that adds to CLOBBERS `STRING_CST' trees for any
     hard regs the port wishes to automatically clobber for all asms.

`MAX_INTEGER_COMPUTATION_MODE'
     Define this to the largest integer machine mode which can be used
     for operations other than load, store and copy operations.

     You need only define this macro if the target holds values larger
     than `word_mode' in general purpose registers.  Most targets
     should not define this macro.

`MATH_LIBRARY'
     Define this macro as a C string constant for the linker argument
     to link in the system math library, or `""' if the target does not
     have a separate math library.

     You need only define this macro if the default of `"-lm"' is wrong.

`LIBRARY_PATH_ENV'
     Define this macro as a C string constant for the environment
     variable that specifies where the linker should look for libraries.

     You need only define this macro if the default of `"LIBRARY_PATH"'
     is wrong.

`TARGET_HAS_F_SETLKW'
     Define this macro if the target supports file locking with fcntl /
     F_SETLKW.  Note that this functionality is part of POSIX.
     Defining `TARGET_HAS_F_SETLKW' will enable the test coverage code
     to use file locking when exiting a program, which avoids race
     conditions if the program has forked.

`MAX_CONDITIONAL_EXECUTE'
     A C expression for the maximum number of instructions to execute
     via conditional execution instructions instead of a branch.  A
     value of `BRANCH_COST'+1 is the default if the machine does not
     use cc0, and 1 if it does use cc0.

`IFCVT_MODIFY_TESTS'
     A C expression to modify the tests in `TRUE_EXPR', and
     `FALSE_EXPR' for use in converting insns in `TEST_BB', `THEN_BB',
     `ELSE_BB', and `JOIN_BB' basic blocks to conditional execution.
     Set either `TRUE_EXPR' or `FALSE_EXPR' to a null pointer if the
     tests cannot be converted.

`IFCVT_MODIFY_INSN'
     A C expression to modify the `PATTERN' of an `INSN' that is to be
     converted to conditional execution format.

`IFCVT_MODIFY_FINAL'
     A C expression to perform any final machine dependent
     modifications in converting code to conditional execution in the
     basic blocks `TEST_BB', `THEN_BB', `ELSE_BB', and `JOIN_BB'.

`IFCVT_MODIFY_CANCEL'
     A C expression to cancel any machine dependent modifications in
     converting code to conditional execution in the basic blocks
     `TEST_BB', `THEN_BB', `ELSE_BB', and `JOIN_BB'.

 - Target Hook: void TARGET_INIT_BUILTINS ()
     Define this hook if you have any machine-specific built-in
     functions that need to be defined.  It should be a function that
     performs the necessary setup.

     Machine specific built-in functions can be useful to expand
     special machine instructions that would otherwise not normally be
     generated because they have no equivalent in the source language
     (for example, SIMD vector instructions or prefetch instructions).

     To create a built-in function, call the function `builtin_function'
     which is defined by the language front end.  You can use any type
     nodes set up by `build_common_tree_nodes' and
     `build_common_tree_nodes_2'; only language front ends that use
     those two functions will call `TARGET_INIT_BUILTINS'.

 - Target Hook: rtx TARGET_EXPAND_BUILTIN (tree EXP, rtx TARGET, rtx
          SUBTARGET, enum machine_mode MODE, int IGNORE)
     Expand a call to a machine specific built-in function that was set
     up by `TARGET_INIT_BUILTINS'.  EXP is the expression for the
     function call; the result should go to TARGET if that is
     convenient, and have mode MODE if that is convenient.  SUBTARGET
     may be used as the target for computing one of EXP's operands.
     IGNORE is nonzero if the value is to be ignored.  This function
     should return the result of the call to the built-in function.

`MD_CAN_REDIRECT_BRANCH(BRANCH1, BRANCH2)'
     Take a branch insn in BRANCH1 and another in BRANCH2.  Return true
     if redirecting BRANCH1 to the destination of BRANCH2 is possible.

     On some targets, branches may have a limited range.  Optimizing the
     filling of delay slots can result in branches being redirected,
     and this may in turn cause a branch offset to overflow.

`ALLOCATE_INITIAL_VALUE(HARD_REG)'
     When the initial value of a hard register has been copied in a
     pseudo register, it is often not necessary to actually allocate
     another register to this pseudo register, because the original
     hard register or a stack slot it has been saved into can be used.
     `ALLOCATE_INITIAL_VALUE', if defined, is called at the start of
     register allocation once for each hard register that had its
     initial value copied by using `get_func_hard_reg_initial_val' or
     `get_hard_reg_initial_val'.  Possible values are `NULL_RTX', if
     you don't want to do any special allocation, a `REG' rtx--that
     would typically be the hard register itself, if it is known not to
     be clobbered--or a `MEM'.  If you are returning a `MEM', this is
     only a hint for the allocator; it might decide to use another
     register anyways.  You may use `current_function_leaf_function' in
     the definition of the macro, functions that use `REG_N_SETS', to
     determine if the hard register in question will not be clobbered.

`TARGET_OBJECT_SUFFIX'
     Define this macro to be a C string representing the suffix for
     object files on your target machine.  If you do not define this
     macro, GCC will use `.o' as the suffix for object files.

`TARGET_EXECUTABLE_SUFFIX'
     Define this macro to be a C string representing the suffix to be
     automatically added to executable files on your target machine.
     If you do not define this macro, GCC will use the null string as
     the suffix for executable files.

`COLLECT_EXPORT_LIST'
     If defined, `collect2' will scan the individual object files
     specified on its command line and create an export list for the
     linker.  Define this macro for systems like AIX, where the linker
     discards object files that are not referenced from `main' and uses
     export lists.


 - Target Hook: bool TARGET_CANNOT_MODIFY_JUMPS_P (void)
     This target hook returns `true' past the point in which new jump
     instructions could be created.  On machines that require a
     register for every jump such as the SHmedia ISA of SH5, this point
     would typically be reload, so this target hook should be defined
     to a function such as:

          static bool
          cannot_modify_jumps_past_reload_p ()
          {
            return (reload_completed || reload_in_progress);
          }


File: gccint.info,  Node: Host Config,  Next: Fragments,  Prev: Target Macros,  Up: Top

Host Configuration Headers
**************************

   Host configuration headers contain macro definitions that describe
the machine and system on which the compiler is running.  They are
usually unnecessary.  Most of the things GCC needs to know about the
host system can be deduced by the `configure' script.

   If your host does need a special configuration header, it should be
named `xm-MACHINE.h', where MACHINE is a short mnemonic for the
machine.  Here are some macros which this header can define.

`VMS'
     Define this macro if the host system is VMS.

`FATAL_EXIT_CODE'
     A C expression for the status code to be returned when the compiler
     exits after serious errors.  The default is the system-provided
     macro `EXIT_FAILURE', or `1' if the system doesn't define that
     macro.  Define this macro only if these defaults are incorrect.

`SUCCESS_EXIT_CODE'
     A C expression for the status code to be returned when the compiler
     exits without serious errors.  (Warnings are not serious errors.)
     The default is the system-provided macro `EXIT_SUCCESS', or `0' if
     the system doesn't define that macro.  Define this macro only if
     these defaults are incorrect.

`USE_C_ALLOCA'
     Define this macro if GCC should use the C implementation of
     `alloca' provided by `libiberty.a'.  This only affects how some
     parts of the compiler itself allocate memory.  It does not change
     code generation.

     When GCC is built with a compiler other than itself, the C `alloca'
     is always used.  This is because most other implementations have
     serious bugs.  You should define this macro only on a system where
     no stack-based `alloca' can possibly work.  For instance, if a
     system has a small limit on the size of the stack, GCC's builtin
     `alloca' will not work reliably.

`HAVE_DOS_BASED_FILE_SYSTEM'
     Define this macro if the host file system obeys the semantics
     defined by MS-DOS instead of Unix.  DOS file systems are case
     insensitive, file specifications may begin with a drive letter,
     and both forward slash and backslash (`/' and `\') are directory
     separators.  If you define this macro, you probably need to define
     the next three macros too.

`PATH_SEPARATOR'
     If defined, this macro should expand to a character constant
     specifying the separator for elements of search paths.  The
     default value is a colon (`:').  DOS-based systems usually use
     semicolon (`;').

`DIR_SEPARATOR'
`DIR_SEPARATOR_2'
     If defined, these macros expand to character constants specifying
     separators for directory names within a file specification.  They
     are used somewhat inconsistently throughout the compiler.  If your
     system behaves like Unix (only forward slash separates pathnames),
     define neither of them.  If your system behaves like DOS (both
     forward and backward slash can be used), define `DIR_SEPARATOR' to
     `/' and `DIR_SEPARATOR_2' to `\'.

`HOST_OBJECT_SUFFIX'
     Define this macro to be a C string representing the suffix for
     object files on your host machine.  If you do not define this
     macro, GCC will use `.o' as the suffix for object files.

`HOST_EXECUTABLE_SUFFIX'
     Define this macro to be a C string representing the suffix for
     executable files on your host machine.  If you do not define this
     macro, GCC will use the null string as the suffix for executable
     files.

`HOST_BIT_BUCKET'
     A pathname defined by the host operating system, which can be
     opened as a file and written to, but all the information written
     is discarded.  This is commonly known as a "bit bucket" or "null
     device".  If you do not define this macro, GCC will use
     `/dev/null' as the bit bucket.  If the host does not support a bit
     bucket, define this macro to an invalid filename.

`COLLECT2_HOST_INITIALIZATION'
     If defined, a C statement (sans semicolon) that performs
     host-dependent initialization when `collect2' is being initialized.

`GCC_DRIVER_HOST_INITIALIZATION'
     If defined, a C statement (sans semicolon) that performs
     host-dependent initialization when a compilation driver is being
     initialized.

`UPDATE_PATH_HOST_CANONICALIZE (PATH)'
     If defined, a C statement (sans semicolon) that performs
     host-dependent canonicalization when a path used in a compilation
     driver or preprocessor is canonicalized.  PATH is a malloc-ed path
     to be canonicalized.  If the C statement does canonicalize PATH
     into a different buffer, the old path should be freed and the new
     buffer should have been allocated with malloc.

`DUMPFILE_FORMAT'
     Define this macro to be a C string representing the format to use
     for constructing the index part of debugging dump file names.  The
     resultant string must fit in fifteen bytes.  The full filename
     will be the concatenation of: the prefix of the assembler file
     name, the string resulting from applying this format to an index
     number, and a string unique to each dump file kind, e.g. `rtl'.

     If you do not define this macro, GCC will use `.%02d.'.  You should
     define this macro if using the default will create an invalid file
     name.

`SMALL_ARG_MAX'
     Define this macro if the host system has a small limit on the total
     size of an argument vector.  This causes the driver to take more
     care not to pass unnecessary arguments to subprocesses.

   In addition, if `configure' generates an incorrect definition of any
of the macros in `auto-host.h', you can override that definition in a
host configuration header.  If you need to do this, first see if it is
possible to fix `configure'.

   If you need to define only a few of these macros, and they have
simple definitions, consider using the `xm_defines' variable in your
`config.gcc' entry instead of creating a host configuration header.
*Note System Config::.


File: gccint.info,  Node: Fragments,  Next: Collect2,  Prev: Host Config,  Up: Top

Makefile Fragments
******************

   When you configure GCC using the `configure' script, it will
construct the file `Makefile' from the template file `Makefile.in'.
When it does this, it can incorporate makefile fragments from the
`config' directory.  These are used to set Makefile parameters that are
not amenable to being calculated by autoconf.  The list of fragments to
incorporate is set by `config.gcc'; *Note System Config::.

   Fragments are named either `t-TARGET' or `x-HOST', depending on
whether they are relevant to configuring GCC to produce code for a
particular target, or to configuring GCC to run on a particular host.
Here TARGET and HOST are mnemonics which usually have some relationship
to the canonical system name, but no formal connection.

   If these files do not exist, it means nothing needs to be added for a
given target or host.  Most targets need a few `t-TARGET' fragments,
but needing `x-HOST' fragments is rare.

* Menu:

* Target Fragment:: Writing `t-TARGET' files.
* Host Fragment::   Writing `x-HOST' files.


File: gccint.info,  Node: Target Fragment,  Next: Host Fragment,  Up: Fragments

Target Makefile Fragments
=========================

   Target makefile fragments can set these Makefile variables.

`LIBGCC2_CFLAGS'
     Compiler flags to use when compiling `libgcc2.c'.

`LIB2FUNCS_EXTRA'
     A list of source file names to be compiled or assembled and
     inserted into `libgcc.a'.

`Floating Point Emulation'
     To have GCC include software floating point libraries in `libgcc.a'
     define `FPBIT' and `DPBIT' along with a few rules as follows:
          # We want fine grained libraries, so use the new code
          # to build the floating point emulation libraries.
          FPBIT = fp-bit.c
          DPBIT = dp-bit.c
          
          
          fp-bit.c: $(srcdir)/config/fp-bit.c
                  echo '#define FLOAT' > fp-bit.c
                  cat $(srcdir)/config/fp-bit.c >> fp-bit.c
          
          dp-bit.c: $(srcdir)/config/fp-bit.c
                  cat $(srcdir)/config/fp-bit.c > dp-bit.c

     You may need to provide additional #defines at the beginning of
     `fp-bit.c' and `dp-bit.c' to control target endianness and other
     options.

`CRTSTUFF_T_CFLAGS'
     Special flags used when compiling `crtstuff.c'.  *Note
     Initialization::.

`CRTSTUFF_T_CFLAGS_S'
     Special flags used when compiling `crtstuff.c' for shared linking.
     Used if you use `crtbeginS.o' and `crtendS.o' in `EXTRA-PARTS'.
     *Note Initialization::.

`MULTILIB_OPTIONS'
     For some targets, invoking GCC in different ways produces objects
     that can not be linked together.  For example, for some targets GCC
     produces both big and little endian code.  For these targets, you
     must arrange for multiple versions of `libgcc.a' to be compiled,
     one for each set of incompatible options.  When GCC invokes the
     linker, it arranges to link in the right version of `libgcc.a',
     based on the command line options used.

     The `MULTILIB_OPTIONS' macro lists the set of options for which
     special versions of `libgcc.a' must be built.  Write options that
     are mutually incompatible side by side, separated by a slash.
     Write options that may be used together separated by a space.  The
     build procedure will build all combinations of compatible options.

     For example, if you set `MULTILIB_OPTIONS' to `m68000/m68020
     msoft-float', `Makefile' will build special versions of `libgcc.a'
     using the following sets of options:  `-m68000', `-m68020',
     `-msoft-float', `-m68000 -msoft-float', and `-m68020 -msoft-float'.

`MULTILIB_DIRNAMES'
     If `MULTILIB_OPTIONS' is used, this variable specifies the
     directory names that should be used to hold the various libraries.
     Write one element in `MULTILIB_DIRNAMES' for each element in
     `MULTILIB_OPTIONS'.  If `MULTILIB_DIRNAMES' is not used, the
     default value will be `MULTILIB_OPTIONS', with all slashes treated
     as spaces.

     For example, if `MULTILIB_OPTIONS' is set to `m68000/m68020
     msoft-float', then the default value of `MULTILIB_DIRNAMES' is
     `m68000 m68020 msoft-float'.  You may specify a different value if
     you desire a different set of directory names.

`MULTILIB_MATCHES'
     Sometimes the same option may be written in two different ways.
     If an option is listed in `MULTILIB_OPTIONS', GCC needs to know
     about any synonyms.  In that case, set `MULTILIB_MATCHES' to a
     list of items of the form `option=option' to describe all relevant
     synonyms.  For example, `m68000=mc68000 m68020=mc68020'.

`MULTILIB_EXCEPTIONS'
     Sometimes when there are multiple sets of `MULTILIB_OPTIONS' being
     specified, there are combinations that should not be built.  In
     that case, set `MULTILIB_EXCEPTIONS' to be all of the switch
     exceptions in shell case syntax that should not be built.

     For example, in the PowerPC embedded ABI support, it is not
     desirable to build libraries compiled with the `-mcall-aix' option
     and either of the `-fleading-underscore' or `-mlittle' options at
     the same time.  Therefore `MULTILIB_EXCEPTIONS' is set to
          *mcall-aix/*fleading-underscore* *mlittle/*mcall-aix*

`MULTILIB_EXTRA_OPTS'
     Sometimes it is desirable that when building multiple versions of
     `libgcc.a' certain options should always be passed on to the
     compiler.  In that case, set `MULTILIB_EXTRA_OPTS' to be the list
     of options to be used for all builds.


File: gccint.info,  Node: Host Fragment,  Prev: Target Fragment,  Up: Fragments

Host Makefile Fragments
=======================

   The use of `x-HOST' fragments is discouraged.  You should do so only
if there is no other mechanism to get the behavior desired.  Host
fragments should never forcibly override variables set by the configure
script, as they may have been adjusted by the user.

   Variables provided for host fragments to set include:

`X_CFLAGS'
`X_CPPFLAGS'
     These are extra flags to pass to the C compiler and preprocessor,
     respectively.  They are used both when building GCC, and when
     compiling things with the just-built GCC.

`XCFLAGS'
     These are extra flags to use when building the compiler.  They are
     not used when compiling `libgcc.a'.  However, they _are_ used when
     recompiling the compiler with itself in later stages of a
     bootstrap.

`BOOT_LDFLAGS'
     Flags to be passed to the linker when recompiling the compiler with
     itself in later stages of a bootstrap.  You might need to use this
     if, for instance, one of the front ends needs more text space than
     the linker provides by default.

`EXTRA_PROGRAMS'
     A list of additional programs required to use the compiler on this
     host, which should be compiled with GCC and installed alongside
     the front ends.  If you set this variable, you must also provide
     rules to build the extra programs.



File: gccint.info,  Node: Collect2,  Next: Header Dirs,  Prev: Fragments,  Up: Top

`collect2'
**********

   GNU CC uses a utility called `collect2' on nearly all systems to
arrange to call various initialization functions at start time.

   The program `collect2' works by linking the program once and looking
through the linker output file for symbols with particular names
indicating they are constructor functions.  If it finds any, it creates
a new temporary `.c' file containing a table of them, compiles it, and
links the program a second time including that file.

   The actual calls to the constructors are carried out by a subroutine
called `__main', which is called (automatically) at the beginning of
the body of `main' (provided `main' was compiled with GNU CC).  Calling
`__main' is necessary, even when compiling C code, to allow linking C
and C++ object code together.  (If you use `-nostdlib', you get an
unresolved reference to `__main', since it's defined in the standard
GCC library.  Include `-lgcc' at the end of your compiler command line
to resolve this reference.)

   The program `collect2' is installed as `ld' in the directory where
the passes of the compiler are installed.  When `collect2' needs to
find the _real_ `ld', it tries the following file names:

   * `real-ld' in the directories listed in the compiler's search
     directories.

   * `real-ld' in the directories listed in the environment variable
     `PATH'.

   * The file specified in the `REAL_LD_FILE_NAME' configuration macro,
     if specified.

   * `ld' in the compiler's search directories, except that `collect2'
     will not execute itself recursively.

   * `ld' in `PATH'.

   "The compiler's search directories" means all the directories where
`gcc' searches for passes of the compiler.  This includes directories
that you specify with `-B'.

   Cross-compilers search a little differently:

   * `real-ld' in the compiler's search directories.

   * `TARGET-real-ld' in `PATH'.

   * The file specified in the `REAL_LD_FILE_NAME' configuration macro,
     if specified.

   * `ld' in the compiler's search directories.

   * `TARGET-ld' in `PATH'.

   `collect2' explicitly avoids running `ld' using the file name under
which `collect2' itself was invoked.  In fact, it remembers up a list
of such names--in case one copy of `collect2' finds another copy (or
version) of `collect2' installed as `ld' in a second place in the
search path.

   `collect2' searches for the utilities `nm' and `strip' using the
same algorithm as above for `ld'.


File: gccint.info,  Node: Header Dirs,  Next: Funding,  Prev: Collect2,  Up: Top

Standard Header File Directories
********************************

   `GCC_INCLUDE_DIR' means the same thing for native and cross.  It is
where GNU CC stores its private include files, and also where GNU CC
stores the fixed include files.  A cross compiled GNU CC runs
`fixincludes' on the header files in `$(tooldir)/include'.  (If the
cross compilation header files need to be fixed, they must be installed
before GNU CC is built.  If the cross compilation header files are
already suitable for ISO C and GNU CC, nothing special need be done).

   `GPLUSPLUS_INCLUDE_DIR' means the same thing for native and cross.
It is where `g++' looks first for header files.  The C++ library
installs only target independent header files in that directory.

   `LOCAL_INCLUDE_DIR' is used only by native compilers.  GNU CC
doesn't install anything there.  It is normally `/usr/local/include'.
This is where local additions to a packaged system should place header
files.

   `CROSS_INCLUDE_DIR' is used only by cross compilers.  GNU CC doesn't
install anything there.

   `TOOL_INCLUDE_DIR' is used for both native and cross compilers.  It
is the place for other packages to install header files that GNU CC will
use.  For a cross-compiler, this is the equivalent of `/usr/include'.
When you build a cross-compiler, `fixincludes' processes any header
files in this directory.

